G21C-0654
Explicit Application of the No-Net-Rotation Condition Using ITRF2005 Velocity Field
In order to apply directly the No Net Rotation Condition on ITRF2005, we aim at computing regularly spaced velocity field and its covariance matrix. As the sites of the ITRF2005 are scattered, we interpolate the ITRF2005 horizontal velocity field to the nodes of a regular grid. The method takes into account the spatial correlation among the velocities, allows to interpolate velocities to regular spaced points and provides as well an estimation of the covariance matrix of the interpolated velocities. Moreover, the retained approach is valid all over the earth and not only locally. We use a least-squares collocation method on the sphere. A mean rigid body rotation is removed from the velocities prior to interpolation. The interpolation is applied over the angular velocity field, rather than over the horizontal velocity field itself. This allows: (1) to preserve the spherical motion characteristic of the velocity field and (2) to define a covariance model which is independent from the coordinate system and then remain valid on large areas. A covariogram is computed for plates that provide enough sites such as Eurasia and North America to define an isotropic covariance function, giving the necessary spatial correlation between velocities. This method is applied to the interpolation of the ITRF2005 horizontal velocity field on every plate where it's possible. Then, the No Net Rotation Condition is applied over the ITRF2005 interpolated horizontal velocity field. Pertinent intermediate results of this study will be presented. Estimated rotation rate parameters between our NNR model and ITRF2005, as well as other existing model will be discussed.
G21C-0655
Postglacial slip rate increase on the Teton normal fault, northern Basin and Range Province, caused by melting of the Yellowstone ice cap and deglaciation of the Teton Range?
Along the eastern front of the Teton Range, Wyoming, prominent fault scarps offset Pinedale deposits by up to 30 m and document that multiple earthquakes ruptured the range-bounding Teton normal fault after the last glacial period. Paleoseismological data suggest that ~70 per cent of the postglacial slip on the southern Teton fault accumulated during or shortly after deglaciation, before 8 ka (Byrd et al., JGR, 1994). Here we use a three- dimensional finite-element model to show that melting of the Yellowstone ice cap and the valley glaciers in the Teton Range may have caused the postglacial slip rate increase on the Teton fault (Hampel et al., Geology, in press). During deglaciation, slip on our model fault accelerates by a factor of ~6 with respect to the long-term rate. Our model further shows that the impact of the melting Yellowstone ice cap on fault slip increases along-strike of the fault from south to north and is everywhere larger than the effect of the former valley glaciers in the Teton Range. The results demonstrate that postglacial slip on faults in glaciated regions may not be uniform through time. Rather, a significant fraction of slip may have accumulated within a few thousand years after the last glaciation. We hypothesize that the rebound caused by the Yellowstone ice cap has also triggered clusters of earthquakes on other normal faults in the surrounding Basin and Range Province.
G21C-0656
Dislocation Modeling and Comparison With GPS Data to Assess Possible Elastic Strain Accumulation in the Central Lesser Antilles: New Constraints From the NSF REU Site in Dominica Between 2001 and 2007
The Caribbean, North and South American plates are converging at a rate of 2 cm/yr in the central region of the Lesser Antilles arc. Here we report high-precision GPS data in concert with forward modeling of a simplified subduction zone geometry to assess strain accumulation for the Lesser Antilles trench. We are able to constrain both vertical and horizontal surface deformation from campaign and continuous GPS observations from 28 geodetic benchmarks located in Guadeloupe, Dominica and Aves Island. Precise station positions were estimated with GIPSY-OASIS II using an absolute point positioning strategy and final, precise orbits, clocks, earth orientation parameters, and x-files. All position estimates were updated to ITRF05 and a revised Caribbean Euler pole was used to place our observations in a CAR-fixed frame. Surface displacements for each site were estimated over 2-7 years. CAR-fixed velocities are projected onto a 500 kilometer transect from the LA trench to Aves Island and compared to calculated displacements for 88 different subduction models. Finite dislocations within an elastic half-space with variable parameters such as angle of the subducting slab, the downdip extent of the locked zone, and percentage of plate interface locking were investigated. Other parameters, such as trench length and slip remained constant. Using a chi-squared, best-fit statistical criterion, the GPS data constrain the subduction interface to a 75 kilometer downdip extent, a 10° dip angle, and near 50% locking. This implies that the subduction zone offshore Dominica is in an interseismic state, thus accumulating strain and causing small westward and upward displacement of the Lesser Antilles relative to the stable Caribbean interior.
G21C-0657
A Revised Caribbean Plate Motion Model: GPS Geodetic Results From the Dominica NSF- REU Site
Velocities from sixteen campaign GPS sites on the Caribbean island of Dominica are analyzed in combination with fifteen existing Caribbean GPS sites to further constrain Caribbean plate motion. High precision GPS geodesy was used to determine the site positions of 16 sites in Dominica between 2000 and 2007. All observations were obtained using dual-frequency, code-phase receivers and geodetic-quality antennae, primarily choke rings. Generally, three consecutive 24 hour observation days were acquired for each site at every epoch. Absolute point positions were obtained using GIPSY-OASIS II along with final, precise orbits, clocks, earth orientation parameters, and x-files from JPL. All site velocities are calculated relative to ITRF05 and legacy site velocities from elsewhere in the eastern and western stable Caribbean were transformed from ITRF00 to ITRF05 before inversion. The addition of Dominican GPS data from the 16 new sites resulted in no statistically significant (the 95% confidence level) change in the Caribbean Euler pole as recently published by DeMets et al., 2007. Our calculated pole is 35.929°N, 102.536° E, and rotating at a rate of .2610 degrees/m.yr. The updated rotation model verifies the previously published pole and supports the conclusion that within current error bounds, Dominica is part of the stable Caribbean plate, with residual motions on the order of only a few mm/yr.
G21C-0658
Present-day microplates motion in the central Mediterranean
Motion of microplates in diffuse plate boundaries often accommodates convergence between major plates. In this work we present a kinematic model for the central mediterranean sector of the Eurasia-Nubia plate boundary, derived from GPS and earthquake slip vectors, which describe the crustal motion as resulting from the interactions of two rigid microplates: Adria and Apulia-Ionia. The styles of deformation predicted by the kinematic model along the boundaries of the proposed microplates are consistent with geological observations and present significant implications for the characterization of seismically active regions in the Eurasia-Nubia plate boundary. Microplates rotation rates are succesfully reproduced by a simple slat model in which the edges of the microplates are constrained to move with the adjacent plate or microplate, suggesting a significant coupling between crustal blocks and the primary role of edge-transmitted forces as driving factor for crustal motion. These results are compared with surface wave tomographic models and other seismological observables which suggest that the rigid microplates correspond to regions of high shear wave velocities in the upper mantle and high integrated lithospheric strength. The present-day plate boundary configuration derives from the inherited Mesozoic African margin, from the rapid Neogene evolution of the Tyrrhenian subduction zone and the final fragmentation of the Adriatic promontory.
G21C-0659
Data Analysis of Permanent GPS Sites (RING) in Italy
The RING (Rete Integrata Nazionale GPS) GPS network is the result of a scientific project started by the Istituto Nazionale di Geofisica e Vulcanologia (INGV) in 2004 with the aim of increasing the number of continuous GPS stations (CGPS) in Italy in order to improve the knowledge of the geodynamics and tectonic processes acting in this area which is characterized by a complex set of independent or partially independent crustal blocks, within the slowly converging African and Eurasian plates. At present RING is a dense CGPS network of more than 110 stations covering the Italian area and integrating GPS receivers with broad-band seismometers and accelerometers in real time connection with three acquisition centers. In this work we describe the data analysis strategy of the whole GPS network (consisting of RING sites, other public Italian CGPS and some IGS sites for a total amount of about 300 stations) and some results in terms of position time series and velocities. The processing is performed adopting a distributed session approach, with more than 10 clusters, sharing common stations, each of them consisting of about 40 stations. Daily loosely constrained solutions are routinely produced for each cluster using the Bernese and Gamit softwares and then transformed into the ITRF05 reference frame. In the next months these solutions will be freely available as daily SINEX files on the public RING website http://ring.gm.ingv.it and subsequently other derived geodetic products (full time series, velocity field, etc.) will also be available.
G21C-0660
GPS Observations and Analysis from Macaronesia and Western Africa: the Canary GNSS Center (CGC), an effort to promote Space-Geodetic Data and Techniques.
Space Geodetic techniques are of great importance for the study of The Macaronesia and Western Africa. Composed of 5 archipelagos (Azores, Madeira, Savage Islands, Canary Islands, and Cape Verde) and sitting less than 60 miles from the African coast at its closest point. The Macaronesia groups of islands undergo so far understudied loading effects, relative movements, volcanic hot spot' deformations, etc which using GNSS techniques we are trying to study and uncover. In the Canary Islands the newly established Canary GNSS Center (CGC) is an open association between companies, universities and institutions to promote the installation, maintenance, data dissemination and quality control of GNSS stations in the Macaronesia and Western Africa. Several important research lines using space geodetic techniques will be discussed herein. An initial study of the baselines for the GPS stations in our region has been carried out and is presented which points out certain effects, this study underscores the lack of consistent data, and articulates the needs for more Continuous Operating Reference Stations (CORS) in our region. The CGC hopes to officially participate in the AFREF soon, the association is promoting the free publication of data from all operating regional stations that may already exist and which have been kept private up to now. The CGC are involved in contacting the owners of these stations to make as much past and present data as possible available through our ftp server and website. The center makes available a free centralized site for the dissemination of all our region's current data and will start producing relevant quality and statistical information about all the stored data soon. The CGC promotes station maintenance following the International GNSS Service (IGS) standards. Through its members' activities the CGC wants to show the benefits to public and private institutions in the Canary Islands and beyond of establishing more CORS in the Macaronesia (especially in the Canary Islands and Cape Verde) and Western Africa (initially in Mauritania, Senegal and Gambia) to support mapping activities and topography, differential correction networks, in-depth scientific studies, and to promote the AFREF project as an essential development for our continent. http://www.canarygnsscenter.org
G21C-0661
Rupture parameters of the 21 May 2003, Mw 6.8, Zemmouri (Northern Algeria) earthquake deduced from InSAR
We study the surface deformation associated with the 21 May, 2003 (Mw=6.8) Zemmouri (Algeria) earthquake, the strongest seismic event felt in the region since 1716. Global and NEIC focal mechanism solutions of the mainshock indicate reverse faulting with a ~N60E trending rupture dipping ~ 45° SE. The earthquake epicenter relocated along the shoreline caused an average 0.50 m coastal uplift along ~50-km-long coastline. We mapped the coseismic surface displacement field caused by the earthquake using the ENVISAT ASAR (IS2) and RADARSAT standard beam (ST5) data. We were able to obtain coseismic interferograms from both the ascending and descending orbits of ENVISAT satellite. The RADARSAT data proved useful only in the descending mode. While the two RADARSAT interferograms cover the entire area of coastal uplift, ENVISAT data cover only the western half of the epicentral zone. Although the InSAR coherence in the epicenter area is poor, deformation fringes are observed along the coast in different patches. In the Boumerdes area, the maximum deformation is indicated by the high gradient of fringes visible in all interferograms in agreement with field measurements (tape, DGPS and leveling). To constrain the earthquake rupture parameters, we model the interferograms and uplift measurements using elastic dislocations on triangular fault patches in an elastic and homogeneous half space. We invert the coseismic slip using a curved surface constructed from triangular elements using Poly3Dinv program that uses a damped least square minimization. The fault rupture is 65-km-long and dips ~40° to the south with a smooth change in strike north of Boumerdes from N 60°-65° to N 95°-105°.
G21C-0662
Active Tectonics of Western Turkmenistan; Implications for the Onset of South Caspian Subduction
The Kopeh Dagh and Balkan mountain ranges of West Turkmenistan are actively deforming as a result of Arabia- Eurasia collision. We combine observations of the geomorphology made from satellite and topographic data, with historical and recent seismicity to identify major active faults, and how they contribute to regional shortening. Between 55--57.5°E, partitioned (north-vergent) thrust and right-lateral strike-slip fault segments, comprising the Ashkabad fault zone, accommodate regional shortening and the westward-extrusion of the NW Kopeh Dagh-South Caspian block, relative to Central Iran and Eurasia. Reconstruction of displaced geology indicates 35~km total right-lateral motion across the Ashkabad fault zone. The Balkan region lies along-strike of the Ashkabad fault zone, west of 55°E. Fault plane solutions indicate shortening is partitioned onto the Balkan thrust and right-lateral Kum-Dagh fault zones. Thrust earthquakes are relatively deep (30--45~km) and lie along a north-dipping plane which extends 40±5~km north beneath the Balkan anticline. Receiver function data from Turkmenbashi and Nebit Dagh indicate these earthquakes occur in the base of the crust, and may therefore be related to bending of the NW Kopeh Dagh-South Caspian lithosphere as it is overthrust by Eurasia. Movement on a north-dipping blind thrust fault is consistent with the broad asymmetric (south-vergent) fold structure of the Balkan range. Recent uplift is also indicated by extensional faults which displace Quaternary geomorphology along the range crest. South of the Balkan range, right-lateral shear occurs across the Kum-Dagh fault zone which is expressed as a series of right-stepping anticlines (affecting Pliocene Red Series and younger sediments), forming important traps for hydrocarbons. An important structural change occurs near 55°E. To the west, Eurasia overthrusts the NW Kopeh Dagh- South Caspian block, while to the east the polarity of thrusting changes and the Kopeh Dagh overthrusts Eurasia. This unstable geometry has resulted in the right-lateral Kum-Dagh and Ashkabad fault zones becoming offset by 40±5~km, and also explains the presence of thrust earthquakes in the South Caspian lithosphere extending 40±5~km north beneath the Balkan range. Resolving estimates for South Caspian-Eurasia motion (from published GPS velocities) onto the Balkan thrust fault indicates a NNE-shortening rate of 4--6~mm/yr, and a slip-rate of 5--7~mm/yr for the right-lateral Kum-Dagh fault zone. Similar estimates using GPS velocities from the Iranian Kopeh Dagh indicate 2--4~mm/yr right-lateral motion and 1--3~mm/yr NNE-shortening across the Ashkabad fault zone. If the Kum-Dagh-Ashkabad right-lateral shear zone becomes offset at the rate of shortening across the Balkan thrust, 40±5~km offset would be accommodated in 6--11~Ma. Similarly, 8--15~Ma is required to accommodate 35~km right-lateral motion on the Ashkabad fault. These estimates suggest the present-day kinematics of the Kopeh Dagh and the onset of South Caspian subduction beneath Eurasia may be older than previously thought (>5.5~Ma). Furthermore, the westward increase in slip-rate between the right-lateral Ashkabad and Kum-Dagh fault zones requires E-W extension across the NW Kopeh Dagh. This is probably accommodated on distributed left-lateral fault systems which cross-cut the range west of 57°E, and may account for the westward decrease in elevation of the range.
G21C-0663
Current Arabian Plate Motion From Campaign GPS Measurements in Saudi Arabia: Preliminary Results
Current investigations of the motions of the Arabian and its neighboring plates are primarily based on GPS measurements obtained in the surrounding areas of the Arabian plate, with few stations actually located on the Arabian plate itself in the Kingdom of Saudi Arabia. In order to advance the knowledge of the dynamics of the Arabian plate and its intra-plate deformations, the General Directorate of Military Survey (GDMS), through collaboration with the Institute of Engineering Surveying and Space Geodesy (IESSG), densified the GPS network in Saudi Arabia, covering nearly two thirds of the tectonic plate. Since July 2002, a network of 32 GPS stations has been established at locations of the Saudi Arabia geodetic network. At all of these GPS stations a concrete pillar has been used as the monument and the locations have been selected in order to give the broadest distribution of observing sites. During 2005, 27 additional GPS stations in the Hejaz and Asser Mountains, and the Farasan Islands, all in south-western Saudi Arabia, have been established, for which the past and future campaign GPS measurements will provide valuable data for investigations of crustal deformations close to the plate boundaries between the Nubia, Somalian and Arabian plates. In this presentation we will show results in the form of velocity field and plate motion estimates based on data from at least three campaigns occupying the initial 32 GDMS GPS network stations, but also from a number of IGS stations in the region. Our reference frame is aligned to ITRF2005 and uses approximately 40 IGS reference frame stations located on all major tectonic plates, e.g. Nubia and Somalia, surrounding the Arabian plate. Furthermore, we apply absolute satellite and receiver antenna phase center models together with newly available GPS products from a recent global re-processing effort.
G21C-0664
Mechanics and Partitioning of Deformation of the Northwestern Okhostk Plate, Northeast Russia
The tectonic evolution and present day deformation of northeastern Russia remains one of the major challenges in plate tectonics. Arguments over the existence of at least a separate Okhotsk plate between North America and Eurasia appear to be resolved on the basis of the latest GPS studies combined with elastic modeling. The question of the mechanical behaviour of the Okhotsk plate, caught between the slowly, obliquely converging North American and Eurasian plates now becomes important. We present an analysis of geological lineaments, micro-seismicity, total seismic moment release and seismic deformation rate and GPS campaign data and global plate tectonic model data (REVEL) to estimate the likelihood of future seismicity and the relative amount of elastic and viscous deformation of the lithosphere of the northwestern Okhotsk plate. We find that it is likely that the Okhotsk plate is cracked into slivers, but that rates of relative motion of these slivers are close to indistinguishable from the behaviour of a single, rigid plate. The analysis also suggests the upper bound for large earthquakes in the region to be Mw 7-7.5 which we expect to occur only on the plate boundary fault itself. This fits geological evidence for a long term offset rate 5-10 times higher on the major plate boundary fault than other lineaments cutting the Okhotsk plate itself.
G21C-0665
An Introduction to the Tibet cGPS pilot project: TigiCAS
The convergence between India and Eurasia is the¡¡prototype of continental collision in action. Compared¡¡to geological history and fault kinematics studies, the present-day, regional pattern of strain-partitioning¡¡is still inadequately known. Among limited geodetic¡¡efforts in the past decade or two, most have been focused¡¡on refining measurements of the current crustal¡¡shortening rate across the Himalaya. The vast region¡¡immediately to the north is sparsely instrumented, with only one continuous GPS station (Lhasa) within¡¡the plateau proper. Campaign stations are few and¡¡ill-positioned, mostly along major roads, providing¡¡poor constraints on present-day slip-rates on individual¡¡active faults. The extant GPS network configuration is thus still insufficient to discriminate between block vs continuum deformation. In November 2006, the¡¡Chinese Academy of Sciences led a pilot program and¡¡installed 6 continuous GPS stations in southern Tibet, crossing the NS-trending normal fault systems and¡¡complementing the Nepal cGPS profiles. We present¡¡here the new sites, preliminary data processing results, and the spatial relationship with ongoing or planned¡¡continuous GPS sites from a couple of other projects. Together with such projects, TigiCAS will provide¡¡a substantial increase in geodetic data in the¡¡Himalayan-Tibet convergent belt in the next few¡¡years, and lead to a better understanding of¡¡contemporary deformation of the region.
G21C-0666
Distributed Active Deformation in Central Mongolia
We identify and describe a series of east-west left-lateral strike-slip faults (named the Songino, the Hag Nuur, the Uliastay and the South Hangay fault systems) in the Hangay mountains of central Mongolia: an area that has little in the way of recorded seismicity and which is often considered as a rigid block within the India-Eurasia collision zone. Each of the strike-slip faults show indications of late Cenozoic activity in the form of aligned sequences of sag-ponds and pressure-ridges developed in alluvial deposits. Total displacements of ~ 3 km are measured on both the Songino and South Hangay fault systems. The total displacement across the Hag Nuur fault may be as much as 11 km. Cumulative offset across the Uliastay fault systems are unknown but are unlikely to be large. The ≤ 20 km of cumulative slip on the Hangay faults could be accommodated in no more than ~ 5 Ma at the present-day rate of ~ 4 mm/yr measured by GPS. Initiation of strike-slip faulting in the Hangay thus appears to postdate the Oligocene uplift of the Hangay mountains themselves, but may be synchronous with initiation of faulting in the Gobi-Altay fault systems to the south, and might indicate a regional initiation of left-lateral shear across the whole of central Mongolia at ~ 5 Ma ago. The strike-slip faults of central Mongolia constitute a previously unrecognized hazard in this part of Mongolia. Our observations show that, despite the complete absence of instrumentally recorded seismicity in the Hangay, this part of Mongolia is cut through by numerous distributed strike-slip faults that accommodate regional left-lateral shear between Siberia and China. Central Mongolia is thus an important component of the India-Eurasia collision that would be overlooked in models of the active tectonics based on the distribution of seismicity. We suggest that active faults such as those identified in the Hangay of Mongolia might exist in other, apparently aseismic, regions within continental collision zones.
G21C-0667
Listric earthquake faulting and triggering in the Mach/Shiragh/ Quetta earthquakes 1931--5
Between 1931 and 1935, three major earthquakes occurred between the Bolan Pass and Quetta in what is now Pakistan. The first two earthquakes struck within 66 hours of each other and caused 120 deaths. Less than four years later, and 85 km to the west, the Mw7.7 sinestral Quetta earthquake caused 35,000 deaths. All three earthquakes were located in the transpressively deforming western region of the Indian-Eurasian plate boundary. Immediately after the Quetta earthquake, a first-order leveling line through the Bolan Pass revealed 650 mm maximum uplift. This was initially interpreted without structural control as caused by approximately 1 m of slip on an east-dipping reverse fault, and subseqently as slip on a wedge thrust structure [Garcia et al. 2006]. The recent availability of a detailed structural section controlled by seismic reflection and borehole data permits a more satisfactory solution based on slip on one or more listric faults dipping to the west. We find the most likely mechanism for the earthquake to consist of up to 3 m of slip on the flat portion of a listric ramp-flat-ramp structure, with minor slip on faults to the east and west. We examine a possible stress-triggering sequence that links all three earthquakes in the sequence.
G21C-0668
Holocene slip rate for the central Altyn Tagh Fault: Preliminary results from the Tuzidun site based on 14C and 10Be dating of a displaced fluvial terrace riser.
The active, left-slip Altyn Tagh Fault (ATF) defines the northern boundary of the Tibetan Plateau and is among the world's longest intracontinental strike-slip faults. Despite a decade of concentrated work, the Holocene slip rate for the central ATF is still disputed, with millennial slip rates derived from faulted landforms ranging from 9 to 27 mm/yr. To address this factor-of-three difference, we are investigating a new slip-rate site near Tuzidun (37.73N, 86.72E) along the Cherchen He reach of the fault. The new site is situated where a south-flowing, ephemeral stream channel crosses the N70E-striking, active trace of the ATF. This channel is flanked by a set of inset fluvial terraces along its eastern bank. North of the ATF, these terraces include both a younger/lower T1 tread and an older/higher T2 terrace, which are vertically separated by an intervening T2/T1 riser. South of the fault, the stream is inset into an alluvial fan, F1. The F1 fan is separated from the higher, T2 fluvial terrace tread to the east, by the T2/F1 riser. Our neotectonic mapping and survey data indicate that the T2/F1 riser on the south/downstream side of the ATF and the T2/T1 riser on the north/upstream side have been displaced from one another by left slip along the ATF. The present separation between these riser segments is ~56 m, though lateral erosion of the riser may have diminished the true offset. To account for this possibility, we have developed three end-member reconstructions that yield offsets ranging from 56 to105 m. Ongoing geochronologic and geomorphic analyses are designed further limit the range of possible displacements. Preliminary age analyses from the Tuzidun site include 22 new radiocarbon dates from buried organic materials and 7 analyses of 10Be concentration in quartz extracted from amalgamated samples of terrace conglomerates. The 14C analyses are from samples collected from within the T2 tread on both sides of the ATF and from loess deposits that cap the downstream T2/F1 riser face. The 10Be analyses are from samples collected in two depth profiles, north and south of the ATF, dug into the T2 deposit at the crest of the displaced riser. The calibrated 14C dates and 10Be surface-exposure ages are compatible, and indicate that the surfaces at the crest and toe of the riser were abandoned at ~6 ka and ~4.4 ka, respectively. To bracket the millennial slip rate at this site, we consider three end-member reconstructions. The first is an upper-terrace reconstruction, in which the riser started recording displacement as soon as the upper-terrace, T2, was abandoned, providing a minimum constraint on the slip rate of ~9 mm/yr since ~6 ka. An intermediate interpretation is a lower-terrace reconstruction, in which the riser accumulated no displacement until the lower surface, F1, was abandoned, yielding a slip rate of ~13 mm/yr since ~4.4 ka. The final reconstruction is one in which erosion of the upstream T2 surface, prior to T1/F1 deposition, diminished the present-day observed offset. In this case, up to 105 m of displacement has occurred since abandonment of the T2 surface, which permits a slip rate as high as ~18 mm/yr since ~6 ka. The new slip rate of 9-18 mm/yr for the Tuzidun site is consistent with our preliminary results from three additional slip-rate sites along the central ATF, and taken together, provides an upper limit of 18 mm/yr for the Holocene slip rate along this reach of the fault.
G21C-0669
Paleoearthquake History of the Cherchen He Reach of the Central Altyn Tagh Fault Xinjiang, China.
Although the Altyn Tagh Fault (ATF), Xinjiang, China, is a first-order structure within the Indo-Asian collision zone, its Holocene paleoseismic and slip history remain contested. Studies of offset fluvial terraces and other landforms indicate slip rates of 9-27mm/yr over millennial timescales. However, little is known about the record of paleoearthquakes along the ATF. Defining a paleoearthquake record for the last few millennia complements slip rate studies spanning 5-10 kyr as well as fault-behavior studies. A classification of slip rate over different time scales is vital to understanding the role such structures play in the large-scale tectonics of the Indo-Asian collision. We mapped ~20km of this reach of the ATF (N 37.43°, E 86.40°). Geomorphic and fault trace mapping (1:3,800) revealed a continuous fault morphology. This reach of the fault is characterized by north-facing scarps ~0.5-2m high superimposed on ~50m high south-facing escarpments. The north- side up component of slip may be associated with ~10 right steps of 10-50m along the fault trace. Along with mapping, measurements of drainages offset between 5 and 30m provide information about slip along this reach for the last few earthquakes. The smallest of these offsets were consistently 5-8m. We infer they result from the most recent earthquake. Two 15m-long and ~2m deep trenches, sites 123 and 105, were excavated across narrow fault traces where fan deposits from south-draining watersheds abut the 0.5m-high fault scarps. These scarps dam the south-draining watersheds, which were offset by the fault and form depressions in which younger sediment was deposited. Trench 123 was logged at a scale of 1:10 and trench 105, 5 km to the west, was logged at a scale of 1:12.5. The stratigraphy in both trenches revealed interfingering alluvial deposits and eolian fine silts and loess. Offset units, unconformities, onlap and drape sequences, and fissures clearly define the fault traces and paleoearthquakes within the trenches. Both sites preserve evidence for 2-3 events. We also collected nine organic samples for radiocarbon dating at site 105 from individual stratigraphic layers within the trench. We expect that ongoing 14C analyses on these samples will bracket the ages of these events. Because of the consistency of 2-3 events at each of the paleoseismic sites, we conclude that their composite record reveals 2-3 late Holocene earthquakes. A clear result of this mapping is that the most recent ATF earthquake along this reach resulted in 5-8m of surface offset (consistent with Mw 7.2). These results have important implications for both slip-rate studies and studies of strike-slip fault behavior. For example, if 5m is representative of slip in each earthquake along this fault reach, earthquake recurrence intervals of 555 years are expected with a 9mm/yr slip rate, whereas 185 years is expected if the slip rate is 27mm/yr. Paleoseismic investigations such as this can distinguish between this factor of three difference in event recurrence. A recurrence interval will help also in classifying which model of fault-behavior is appropriate for the ATF.
G21C-0670
Modeling the Philippine Mobile Belt: Tectonic blocks in a deforming plate boundary zone
The Philippine Mobile Belt, a seismically active, rapidly deforming plate boundary zone situated along the convergent Philippine Sea/Eurasian plate boundary, is examined using geodetic and seismological data. Oblique convergence between the Philippine Sea Plate and the Eurasian plate is accommodated by nearly orthogonal subduction along the Philippine Trench and the Manila Trench, as well as by strike-slip faulting along the Philippine Fault system. We develop a model of active plate boundary deformation in this region, using elastic block models constrained by known fault geometries, published GPS observations and focal mechanism solutions. We then present an estimate of block rotations, fault coupling, and intra-block deformation, based on the best-fit model that minimizes the misfit between observed and predicted geodetic vectors and earthquake slip vectors. Slip rates along the Philippine fault vary from ~22 - 36 mm/yr in the Central Visayas and about 10 to 40 mm/yr in Luzon, trending almost parallel to the fault trace. In northern Luzon, Philippine Fault splays accommodate transpressional strain. The Central Visayas block experiences convergence with the Sundaland block along the Negros Trench and the Mindoro-Palawan collision zone. On the eastern side of Central Visayas, sinistral strike-slip faulting occurs along the NNW-SSE-trending Philippine Fault. Mindanao Island in southern Philippines is dominated by east-verging subduction along the Cotabato Trench, and strain partitioning (strike- slip faulting with west-verging subduction) in eastern Mindanao along the southern Philippine Fault and Philippine Trench, respectively. Oblique active sinistral strike slip faults in Central and Eastern Mindanao that were hypothesized to be responsible for basin formation are obvious boundaries for tectonic blocks. Located south of Mindanao Island we define an adjoining oceanic block defined by the N-S trending complex dual subduction zone of Sangihe and Halmahera, primarily delineated by seismicity, bathymetric profiles and E-W thrust mechanisms. In our preferred model, the Philippine Mobile Belt can be represented by at least 12 independently moving rigid tectonic blocks, separated by active faults and subduction zones.
G21C-0671
Modeling of Crustal Deformation in SW Japan using GPS Data -Simultaneous Estimation of Interplate Coupling and Block Rotation-
We applied a new method devised by McCaffery [2002] to estimate both slip deficit and block rotation simultaneously to SW Japan where the tectonics is complex and the crustal deformation is not uniform. We estimated the distribution of the slip deficit on the subducting Philippine Sea Plate and other assumed block boundaries in SW Japan together with block rotation parameters simultaneously. As a result we can clarify the along-arc variation of the slip deficit on subduction zone, and relative block motion derived from block rotation. We assume some geological faults, grabens and linearly-aligned hypocenters as block boundaries. We also considered the Okinawa micro-plate (ON) proposed by Bird [2003]. The data used in the analysis is the GPS site velocities obtained from GEONET. The results show the larger slip deficit of the subducting slab in the northern Hyuga-nada and off Shikoku, while smaller in the southern Hyuga-nada. The coupling across the Median Tectonic Line (MTL) and Beppu-Shimabara Graben is very shallow (~10km depth), however, Amurian plate (AM) and Mid-Kyushu block (MK) couple strongly across the other part inland Kyushu on the contrary. The Euler poles of AM-ON and AM-MK are located at long. 133°E, and lat. 29°N with an angular velocity of 3.1°/Ma and at long. 126°E, and lat. 40°N with 0.5°/Ma, respectively. The AM-ON angular velocity obtained in this study is about 1/5 of that estimated by Kodama [1995] and shows fair agreement. It is necessary to perform more detailed analyses by adding more data from the Nansei Islands and Shikoku area. The long-term relative motions between adjacent blocks are also estimated. The result shows grabens inland Kyushu are opening, the blocks across the MTL are moving right-laterally, and the MK and the southernmost Kyushu blocks are also moving right-laterally in long term.
G21C-0672
Estimation of interplate coupling at the Nankai Trough based on observation of ocean bottom crustal deformation at the Kumano Basin
At the Nankai Trough, the Philippine Sea plate (PH) subducts beneath the southwest Japan with a rate of about 4- 6 cm/yr, where great interplate earthquakes have repeatedly occurred every 100-200 years. A number of researchers have investigated crustal deformation caused by subduction of the PH based on geodetic measurements as represented by GPS observation. However it is difficult to infer the strength of the plate coupling in offshore areas, due to the poverty of offshore geodetic data. For this issue, we have conducted seafloor geodetic observation using GPS/Acoustic techniques around the Nankai Trough since 2004. In this system, we estimate the position of a surveying vessel by Kinematic GPS analysis and measure the distance between the vessel and the benchmark on the seafloor by Acoustic measurements. Next we determine the location of the benchmark and detected crustal movement of the seafloor. There are three seafloor sites in this region, and then the location of benchmark is determined within a precision of 2-3 cm at horizontal components for each observation (Tadokoro et al., 2006). As a result of observations from November 2005 to February 2007, one of the seafloor sites is moving at a rate of 6.96 +/- 4.03 cm/yr in the WNW direction with respect to the Amurian plate (AM). In this study, we investigate interplate coupling at the Nankai Trough using onshore GPS velocities derived from Geophysical Survey Institute of Japan and our estimated offshore GPS site velocity. We assume that observed GPS velocities are represented by the superposition of elastic deformation associated with subduction of the PH, rigid block motion of the overriding plate, and error. The plate interface along the Nankai Trough is represented by multiple rectangular faults. The strength of the plate coupling for each fault segment varies with depth according to a thermal model (Hyndman et al., 1995). Moreover relative plate motion of the PH-AM (Miyazaki and Heki, 2001) is assigned to the plate interface as a priori constraint.
G21C-0673
Interplate coupling of the central Nankai Trough and shortening of the central Japan estimated from GPS data
Interplate megathrust earthquakes have repeatedly occurred with a repeat time of 100-150 years along the Nankai subduction zone. The latest megathrust events there were the 1944 Tonankai (M7.9) and the 1946 Nankai (M8.0) earthquakes. It is very important to investigate tectonic loading processes toward the next large earthquake along the Nankai Trough. For this purpose, we estimate slip deficit distribution of the source region of the 1944 Tonankai earthquake from GPS data inversion analysis and also examine E-W shortening in Central Japan where tectonic contraction between the northeastern and the southwestern Japan arcs. We invert GPS displacement rate data estimated from daily coordinates from April 1996 to March 2000 to estimate interplate coupling in the Tonankai region by using the method of Yoshioka et al. (1993). We reconstruct the plate boundary configuration based on hypocenters determined by the Japan Meteorological Agency and results of seismic explorations in this region. We find that GPS data analysis with a single fixed point is inappropriate in the Tonankai region. With such analysis, the resultant slip deficit distribution becomes unrealistic, showing large slip deficit vectors deviating from the direction of the plate convergence. Such a result can be attributed to the E-W shortening in the inland region. Therefore, this shortening effect must be eliminated to estimate correct distribution of slip deficits. For the elimination of E-W shortening, we divide the GPS stations into the eastern and western groups and introduce one fixed point for each group. We calculate displacement rate of GPS stations with respect to the fixed point of each group, and invert those relative displacement rates to estimate the slip deficit on the plate boundary. We test various cases of partitioning of GPS stations into two groups, and find that a group division near the Yoro-Kuwana-Yokkaichi fault yields a reasonable result. Slip deficit is estimated to be the 4.0-4.3cm/yr and largest below the Sea of Kumano, southeastern of Kii Peninsula, where the plate boundary depth is 10-25km. Vertical displacement rates calculated from our slip deficit distribution are consistent with those obtained from leveling survey during 1983-2000, indicating the result represent a persistent feature for the interseismic period. Estimated slip deficit distribution is consistent with the result of a similar analysis in the Tokai region by Ohta et al. (2004). In addition, the large slip deficit region corresponds to the coseismic slip distribution of the 1944 Tonankai earthquake estimated by Sagiya and Thatcher (1999), but is inconsistent with the seismological analysis results by Ichinose et al. (2003) and Yamanaka (2004). This discrepancy suggests a possibility of significant postseismic slip under the Sea of Kumano, but these exists an open question whether a strongly locked patch can exhibit a velocity-strengthening behavior or not. As for the inland deformation, 5mm/yr E-W shortening is inferred between two fixed points. This inland contraction rate is consistent with the geological displacement rate of Yoro-Kuwana -Yokkaichi fault, indicating that this active fault is playing an important role in the long-term E-W shortening of the Japan mainland.
G21C-0674
Biannually repeating slow slip events in an uncoupled segment of the Ryukyu Arc, SW Japan
At the Ryukyu Trench, SW Japan, the Philippine Sea Plate (ph) subduct toward NW. The overriding Ryukyu Arc is composed of three nearly rigid blocks with little interseismic elastic deformation. The South Ryukyu Block (sr), the westernmost part of Japan, is known for extremely fast plate convergence (~12 cm/yr) by the the ph subduction and the oceanward movement of the arc due to active opening of the Okinawa Trough. There ph and sr are gdecoupled,h i.e. large interplate thrust events do not occur. Yarai et al. (2004) found repeating slow slip events (SSE) there by analyzing the movement of Hateruma, the southernmost island of the arc, by GPS observations. Here we studied these SSE from the movements of six GPS points in five islands on sr during 1997-2007, and report their time constants, recurrence intervals, consistency between slips and plate convergence, predictability of events. The events occur mostly in spring and fall, but this apparent seasonality is caused by their rather regular biannual recurrences. The time constants ranged between 0.1 and 0.15 years, twice as long as those in Cascadia although their recurrence intervals (~6 months) were only ~1/2 of those in Cascadia (~13 months). SSE signals were clearly seen both in horizontal and vertical coordinates of GPS points in the islands Ishigaki and Iriomote, as well as Hateruma, and we estimated the fault parameters of the SSE as the ph-sr interplate events. Their slip directions were consistent with the plate convergence direction, and the typical amount of slip (~ 6 cm, Mw ~6.6) coincided with the convergence in 1/2 year. The depths of the faults were 20-40 km, which corresponds to the transient depth between seismogenic and freely sliding zones. This situation is similar to Shikoku and Cascadia except that there are no large thrust events at gseismogenic zonesh in Ryukyu. The amounts of slips had significant correlation with the time lengths until the next events, suggesting the time-predictable characteristics of the events. These SSE are unique in two points, i.e. (1) the first repeating SSE found in an uncoupled subduction zone, (2) best recorded SSE sequence with nearly 20 events and 3-D crustal movement data associated with the events.
G21C-0675
Intraplate Strain Accumulation in Central Japan
Crustal deformation in central Japan is caused by the combination of the elastic strain accumulation associated with interseismic locking of subduction thrusts and the deformation due to intraplate tectonic activities. Townend and Zoback (2006, JGR) showed that intraplate stress directions in central Japan (which they obtained from focal mechanism inversions of crustal earthquakes) are essentially the same as the intraplate strain directions, determined by Mazzotti et al. (2001, EPSL) by subtracting a modeled elastic deformation field due to plate subduction from the observed geodetic deformation field. The agreement between the horizontal principal stress directions and the directions of geodetic principal strains indicates a causal relation between the intraplate stress and strain fields. In order to compare the rate of strain release associated with intraplate seismicity with the rate of intraplate strain determined by Mazzotti et al, we carried out a moment tensor summation using the same set of earthquake focal mechanism as Townend and Zoback. We find that the principal horizontal strain directions associated with the earthquakes agree with those determined by subtracting modeled elastic deformation from geodetic observations (as well as the stress orientations obtained from focal mechanism inversions). However, the strain rates determined from moment tensor summation are only about 1/3, on average, of the geodetic strain. We interpret this difference as the accumulation of intraplate strain in central Japan, which could be released by intraplate earthquakes eventually. By mapping the differences between the long-term geodetic strain and intraplate seismic strain, we attempt to express the relative seismic hazards in central Japan in terms of the relative rate of strain accumulation.
G21C-0676
Spatiotemporal Filtering of Eastern Canadian GPS Data
Geodetic data, the spatial and temporal surface expression of complex geophysical processes in the earth, is being acquired today at unprecedented rates and accuracies. Previous research has demonstrated that an eigenpattern decomposition technique known as Karhunen-Loeve expansion (KLE) analysis can be used to identify a unique, finite set of correlated deformation patterns for a given regional network of GPS stations. Similar in nature to the empirical orthogonal functions historically employed in the analysis of atmospheric and oceanographic phenomena, the method derives the eigenvalues and eigenstates from the diagonalization of the correlation matrix. After decomposing large data sets into their orthonormal eigenvectors and associated time series, based upon the spatiotemporal relationships that exist in the data, it can be used to study those modes most responsible for these correlations and their sources (Tiampo et al., 2004) or to remove those uninteresting modes in the system (Dong et al., 2006). One additional benefit is that these modes are ordered naturally by the predominant spatial wavelength of the signal. Here we demonstrate its application to continuous GPS data acquired between 2001 and 2006 in the intraplate region of eastern Canada. We successfully isolate the common mode error and examine some of the potential sources for the additional vertical and horizontal deformation modes, including glacial isostatic rebound (GIA).
G21C-0677
Neotectonic observations along US Highway 50, towards estimating net extension across the Basin and Range, Nevada.
In Central Nevada, limited availability of paleoseismic data has hindered comparisons between modern geodetic and geologic (late Quaternary) fault slip rates. In an effort to build a new paleoseismic database across US HWY 50, we have performed Quaternary geologic mapping along parts of 9 ranges, including the Desatoya, Toiyabe, Monitor, Toquima, Antelope, Fish Creek, Butte, Egan, and Schell Creek Ranges. Based on this mapping, we selected locations for detailed site-specific paleoseismic investigation. Fault trenches were excavated across the Eastern Toiyabe Range and western Simpson Park Mountains faults in 2005-6. During the last year we have added two new paleoseismic sites including a trench across the western Butte Range fault and a natural stream cut exposure of the eastern Schell Creek Range fault. Additionally, we have surveyed fault scarp profiles along all 9 ranges with a backpack differential GPS. We plan to excavate trenches along the Egan and Schell Creek Ranges in September 2007. In all the trench exposures, Late Pleistocene deposits are offset, providing evidence that long-term extensional strain related to the Pacific-North American plate boundary has been accommodated on range-front faults distributed across the HWY 50 transect. Observations from our mapping, trench investigations, and fault scarp diffusion analyses will place estimates on the amount and timing of offsets as well as the net extension rate across the transect.
G21C-0678
Asymmetric motion along the San Franciso Bay Area faults. Implication for the magnitude of future seismic events
The San Francisco Bay area is one of the tectonically most deformed areas in the world. This deformation is the result of relative motion of the Pacific and North-America plates. A large part of the strain (75 %) is accommodated along structures lying in a 50 km wide land strip. At least two major seismic events (Mw>6.5) are expected along the San Andreas (SAF) and Hayward faults (HAY) within the next decades. Triggering effects between the two seismic events may not be excluded. The BARD network is a permanent GPS network comprising 40 GPS sites, installed since 1994 in Northern California. Originally started as a collaborative effort of different Bay Area institutions, since the establishment of the Plate Boundary Observatory it has focused on real-time data acquisition from stations operated by UC Berkeley, with plans for expansion in collaboration with USGS/Menlo Park. The BARD network is streaming data to the Berkeley Seismological Laboratory in real-time (sampling rates of 1s and 15s, depending on the site). All sites are transmitting data using Frame Relay technology which makes them safer in case of earthquake occurrence. Data are archived at the Northern California Earthquake Data Center (NCEDC, http://www.ncedc.org) and are freely available. The BARD network is currently able to provide high accuracy (error<1mm/yr) velocities in Northern California. We use the BARD data to show that the motion across the San Andreas fault may be asymmetric. Therefore, the common assumption that the deformation is symmetric across the fault could lead to a biased location of the region of maximum strain in the San Francisco Bay Area. The new location of the maximum static strain based on asymmetry influences estimates of the response of the Hayward Fault to deformation associated with the San Andreas fault. We also present preliminary velocities for PBO sites located in the San Francisco Bay Area and discuss them in the light of a BARD reference frame.
G21C-0679
Surface Geometry and Geomorphology of the Rodgers Creek Fault, San Francisco Bay Area
The Rodgers Creek fault, part of the right-lateral San Andreas fault system in the San Francisco Bay area, is geometrically segmented by bends on multiple scales. North of Sonoma Mountain, along the northern half of the fault, sections of the fault trace trend approximately parallel to the direction of relative plate motion (~N34°W) and display a right-stepping pattern across releasing double bends. Within the releasing bends, the fault trends >5° oblique to plate motion and shows geomorphic evidence of extension. The largest right bend, ~1 km at Santa Rosa, corresponds to the lowest elevations along the fault. To the south, the fault makes a broad restraining double bend around the southwest flank of Sonoma Mountain and trends up to ~13° compressively oblique to plate motion. Long-term uplift (Sonoma Mountain) east of the bend suggests a reduction in slip on the fault to the south. The restraining bend corresponds to the north end of a pronounced aseismic region along the fault that may represent a spatial change in the mode of strain accommodation. Aerial photo analysis (1:6 k) of well-preserved geomorphology at the south end of the Rodgers Creek fault, where the fault makes another left bend with respect to plate motion, reveals a section that is undergoing progressive inversion from localized transtension (at a right bend) to transpression. This inversion is manifest as a northwest- lengthening zone of uplift within the fault zone. The youngest push-ups appear to be overprinting a relict pull-apart and sag pond. This and possibly older sag deposits along the margin of the uplift may mark former positions of a releasing geometry in the fault trace, presently located directly north of the uplift front. Geometric and overprinting relations suggest that the main trace of the fault rotates and translates through the passing bends. This mode of fault-bend migration contrasts with a previously proposed model in which new transverse structures develop progressively in the direction of deformation (Wakabayashi and others, 2004).
G21C-0680
Distributed Plate Boundary Deformation Across the San Andreas Fault System, Central California
Plate boundaries are now recognized as broad zones of complex deformation as opposed to narrow zones with discrete offsets. When assessing how plate boundary deformation is accommodated, both spatially and temporally, it is therefore crucial to understand the relative contribution of the discrete and distributed components of deformation. The creeping segment of the San Andreas fault is an ideal location to study the distribution of plate boundary deformation for several reasons. First, the geometry of the fault system in central California is relatively simple. Plate motion is dominated by slip along the relatively linear strike-slip San Andreas fault, but also includes lesser slip along the adjacent and parallel Hosgri-San Gregorio and Rinconada faults, as well as within the borderlands between the three fault strands. Second, the aseismic character of the San Andreas fault in this region allows for the application of modern geodetic techniques to assess creep rates along the fault and across the region. Third, geologic structures within the borderlands are relatively well-preserved allowing comparison between modern and ancient rates and styles of deformation. Continuous GPS stations, alignment arrays surveys, and other geodetic methods demonstrate that approximately 5 mm/yr of distributed slip is accumulated (on top of the fault slip rate) across a 70-100 km wide region centered on the San Andreas fault. New campaign GPS data also suggest 2-5 mm/yr of deformation in the borderlands. These rates depend on the magnitude of the coseismic and postseismic corrections that must be made to our GPS time series to compensate for the 2003 San Simeon and 2004 Parkfield earthquakes, which rupture faults outside, but near the edges of our GPS network. The off-fault deformation pattern can be compared to the style of permanent deformation recorded in the geologic record. Fold and thrust belts in the borderlands are better developed in the Tertiary sedimentary rocks west of the fault than in their Mesozoic counterparts on the east side of the fault. This combination of geodetic and geologic methods is a powerful way to better understand distributed plate boundary deformation that neither method could provide independently.
G21C-0681
Implications of new paleoseismological data from the Carrizo section of the San Andreas Fault
How does strain accumulated at a plate boundary get released over several thousand years? Is earthquake recurrence time-dependent or time-independent? The Carrizo section of the San Andreas fault (SAF) is potentially an ideal laboratory for addressing these questions because this section of the fault is structurally simple, it is oriented parallel to the overall Pacific plate motion and long-term slip rate of the fault (34+-3 mm/yr) agrees with geodetically determined decadal strain accumulation rate of ~36mm/yr. The Carrizo section of the SAF has been proposed to rupture relatively infrequently, with centuries long recurrence, and only during the largest earthquakes, thereby controlling the occurrence of great quakes on the southern SAF. We will present paleoseismic evidences from five closely spaced and connected fault-perpendicular trenches at the Bidart Fan site along with ~60 C14 dates which indicate an average recurrence interval of ~100 years. The event chronology obtained from the Bidart site trenches contradicts the slip per event data from 3-D excavations at the Wallace Creek site, ~5 km to the northwest, which show that four of the last six earthquakes were 1857-like large slip events. While we do not have a long-enough record of past earthquakes at the Bidart Fan site to test various fault behavior models, we propose that while some of the surface rupturing events at the Bidart Fan site are part of 1857-like, multi-segment ruptures, others are smaller offsets which could be the tail ends of Cholame ruptures extending south into the northern section of Carrizo segment. New sites for paleoseismic studies along the Cholame and southern Carrizo sections of the SAF are needed to test this hypothesis, which has significant scientific and seismic hazard implications.
G21C-0682
Finite Element Meshing of the SCEC Community Fault Model: Methods and Algorithms
Complex models of fault interactions and crustal deformation have been developed for Southern California. The Southern California Earthquake Center (SCEC) has sponsored the creation of the Community Fault Model (CFM), http://epicenter.usc.edu/cmeportal/cmodels.html, a 3D representation of active faults in southern California that are deemed capable of generating moderate to large earthquakes. In order to integrate the CFM into finite element models of stress and strain, SCEC has also supported development of mesh generation algorithms and tools to create computational meshes that capture CFM geometry. A series of simple benchmark meshes have been created and made available to the community (see URL), as well as more complex meshes that incorporate the CFM geometry for the faults associated with Landers and Hector Mine earthquakes. General methods that take advantage of algorithms for conforming Delaunay tetrahedral meshing of a planar straight line complex (the fault triangulation) are described and their application to meshing over one hundred faults from the CFM are presented. These methods have general application to meshing of other non-manifold geometries. The results of visco-elastic crustal deformation calculations that utilize these finite element meshes illustrate applications. Geodynamics modeling, from data integration, conceptual model development, model construction, algorithm development, and computations and analysis, has become complex enough to necessitate the compartmentalization and specialization of some tasks. http://meshing.lanl.gov/proj/#Crustal_Dynamics_open
G21C-0683
A Three-Dimensional Numerical Investigation of San Andreas Fault Configuration Through the San Gorgonio Pass
The partitioning of deformation among strands of the San Andreas Fault through the San Gorgonio pass depends on fault geometry. We investigate three alternative three-dimensional configurations of the San Andreas Fault to explore the influence of fault geometry on uplift patterns and slip rates. One model has the commonly used vertical geometry for the system. Another uses the Southern California Earthquake Center Community Fault Model to include two north-dipping, discontinuous, alternative stands. We developed a third model that smoothly connects the north-dipping faults to adjacent segments. Regional transform loading is applied as slip at the distal edges of a deep detachment as well as along the distal portions of the primary fault segments. The model with vertical faults fails to produce uplift in the San Bernardino Mountains. Both of the north-dipping models produce significant uplift that may correspond to observations of recent uplift in the San Bernardino Mountains (Spotila et al, 1998). The vertical model has faster strike-slip rates while the north-dipping, discontinuous system has the slowest strike-slip rates. While the vertical model has the greatest net slip and more efficiently transmits deformation through the pass, the north-dipping fault configurations better match uplift patterns. We altered the continuous and north-dipping model to explore the effects of slip partitioning among the San Andreas and San Jacinto faults and secondary faults to the model. Debate continues about the slip partitioning between the Coachella Valley segment of the SAF and the San Jacinto fault. Our study suggests that slip portioning between the two faults has less of an effect on the slip rates and off fault deformation than fault geometry. The addition of secondary faults to the model significantly increases uplift in the northern San Bernardino Mountains. The senses of slip along secondary faults in the model are consistent with geologic observations. Continuing research investigates the evolution of the fault system of the San Gorgonio Pass. Geologic studies have assigned activation and deactivation times to each fault strand in the region. A sequence of numerical models investigates these transitions and the processes controlling fault system evolution.
G21C-0684
Complex Faulting in the Pacific-North America Transform Offshore Southern California And Implications on Plate Boundary Tectonics and Tsunamigenesis
Complexity in the tectonic model for Pacific-North America transform motion in the offshore southern California region is demonstrated by earthquakes near San Clemente Island and Fortymile Bank. Observed focal mechanisms show movements opposite to those predicted by the plate tectonic theory for right-slip on NW- trending transform faults and observed in other parts of the California Continental Borderland. Also, there is evidence suggesting that moderate earthquakes in the Inner Borderland have greater magnitudes based on long-period seismic waves than the nominal Richter local magnitudes reported in earthquake catalogs. With better data showing the geologic structure of the area now available, we can try to derive a more complete understanding of this complex tectonic behavior and resulting consequences for local tsunamigenesis. The "backwards" earthquakes suggest the occurrence of plate boundary deformation and/or microplate tectonics with the western side of a block containing Fortymile Bank moving instantaneously faster to the northwest than the adjacent block to the west. Such motions may be consistent with clockwise rotation of blocks in the continental borderland due to the regional dextral shear couple as proposed by Crouch (1978) and Luyendyk and others (1980) based upon paleomagnetic and other geologic data. Alternatively, as initially observed for the 1986 Offshore Oceanside earthquake (MS=5.8) by Hauksson and Jones (1988), the anomalous focal mechanism may be due to an inaccurate model of crustal seismic velocity structure for the offshore region. Use of a refined velocity model may show these anomalous earthquakes to have oblique-reverse mechanisms like the 1986 mainshock. Furthermore, more recent seismicity, located with greater accuracy due to expansion of the Southern California Seismograph Network (SCSN), has apparent NE alignments suggestive of significant active secondary fault structure located off the major NW-trending right-slip faults. Such faulting was also inferred to be significant during the clockwise vertical-axis block rotations of the western Transverse Ranges. Interaction between faults within conjugate systems may enhance vertical movements including subsidence at basins where blocks diverge and uplift where blocks converge, thereby producing local tsunamis during large earthquakes. It was not until recently that local earthquake sources were identified offshore Southern California as potentially damaging tsunami sources. Erroneous magnitude estimate of offshore earthquakes can have serious implications for tsunamigenesis and tsunami warning. A half magnitude error can make the difference between a non-tsunamigenic and a tsunamigenic event especially when a marginal event is considered. Better magnitude estimates using long-period seismographs may be necessary for more accurate identification of potentially tsunamigenic local earthquakes.
G21C-0685
Strain pattern in Southern California from geodetic measurements
Geodetic measurements above all GPS data provide outstanding possibilities in identifying and parametrizing active faults. However, the data has to be interpreted by a strain accumulation model to obtain fault parameters like slip rate, locking depth, or earthquake recurrence times. Different rheological assumptions about the earth deformation behaviour resulted in a variety of models, like elastic half space or elastic layer over viscoelastic half space. Depending on which model chosen for the geodetic data interpretation, the obtained fault parameters may vary strongly. We tested for the variation of fault parameters due to the use of models with different rheological assumptions (Savage and Burford, 1973; McCaffrey, 2006; Meade 2005; Savage and Prescott, 1978). We chose our test site, Southern California, San Andreas Fault Zone, due to the presence of dense networks of both GPS sites and well studied active faults and interacting strain accumulation patterns. We merged GPS velocity data sets from SCEC, SOPAC, PBO and the University of Miami to obtain dense spatial data coverage. We improve the accuracy of the model estimates by including geologic information like the spatial distribution of faults and seismic information like the thickness of the seismogenic crust to obtain the slip rate distribution among San Andreas Fault, San Jacinto Fault and Elsinore Fault and its change along this system.
G21C-0686
New insights on Southern Coyote Creek Fault and Superstition Hills Fault
Recent field work has confirmed an extension of the southern Coyote Creek (CCF) branch of the San Jacinto fault in the western Salton trough. The fault marks the western edge of an area of subsidence caused by groundwater extraction, and field measurements suggest that recent strike-slip motion has occurred on this fault as well. We attempt to determine whether this fault connects at depth with the Superstition Hills fault (SHF) to the southeast by modeling observed surface deformation between the two faults measured by InSAR. Stacked ERS (descending) InSAR data from 1992 to 2000 is initially modeled using a finite fault in an elastic half-space. Observed deformation along the SHF and Elmore Ranch fault is modeled assuming shallow (< 5 km) creep. We test various models to explain surface deformation between the two faults.
G21C-0687
Implications of the Baja California Microplate Motion on the Deformation Within the Western North America Plate Boundary Zone
Geodetic analysis have shown that the Baja California microplate is moving at ~10% slower rate but in the same direction as the Pacific plate, with respect to North America. Maintenance of the shear zone between Pacific plate and Baja California may be due to the collision of Baja California with the North American plate in the region of the Transverse Ranges in southern California. This restraining bend was created about 16-12 Ma when the North America - Pacific plate boundary jumped inland Baja California, connecting the strike-slip faults of the San Andreas Fault Zone in the north to the transtensional faults along the Gulf of California in the south. The implications of the restraining bend and the Baja California microplate motion onto North America is thought to be responsible for broad deformation along this plate boundary. We study the interaction between the Baja California microplate and the Western North America, in particular the role of inherited geologic structures on strain localization along the Eastern California Shear Zone, south of the Garlock fault. The inherited structures we study include normal faults in the Basin and Range area that become reactivated as strike slip faults, and the Sierra Nevada microplate. We use the numerical modelling technique of finite elements applying the code G- Tecton. All models are 2D spherical caps driven by displacement/velocity boundary conditions calculated from our GPS analysis of rigid plate rotations in the study area.
G21C-0688
Fore-arc Counterclockwise Rotation In Southern Mexico: Implications For The NA-CA-CO Triple Junction
Numerous studies, mainly based on structural and paleomagnetic data, consider the Southern Mexico as a crustal block (Southern Mexico Block, SMB) uncoupled from the North American plate with a southeast motion with respect to North America accommodated by extension through the central Trans-Mexican Volcanic Belt (TMVB). On the other hand, the accommodation of this motion on the southeastward boundary, especially at the Cocos/Caribbean/North American triple junction is still debated. The boundary between the SMB and the North American plate is constituted by three connected zones of deformation: (1) left-lateral transtension across the central TMVB, (2) left-lateral strike-slip faulting along the eastern TMVB and Veracruz area and (3) reverse and left- lateral strike-slip faulting in the Chiapas area. We show that these three active deformation zones accommodate a couterclockwise rotation of the SMB with respect to the North American plate. We specially discuss the Quaternary motion of the SMB with respect to the surrounding plates near the Cocos/Caribbean/North American triple junction. The model we propose predicts a Quaternary couterclockwise rotation of 0.45 deg/Ma with a pole located at 24.2N and 91.8W. Finally we discuss the geodynamic implications of this counterclockwise rotation. The Southern Mexico Block motion is generally assumed to be the result of slip partitioning at the trench. However the obliquity of the subduction is too small to explain slip partitioning. The motion could be facilitated by the high thermal gradient and gravitational collapse that affects central Mexico or by partial coupling with the eastward motion of the Caribbean plate.