NS11E-0827
Comparison of Seismic Methods for Fault Characterization at the Nevada Test Site, Nevada
As part of a multi-method geophysical study, we collected compressional- (P) and shear- (S) wave seismic data along three transects that cross the Yucca Fault or associated splays at the Nevada Test Site, Nevada. In this presentation, we focus on the peculiarities of the seismic data and on the processing schemes that we used to produce tomographic images of the fault. The faults offset caliche-rich alluvial materials in the shallow subsurface and show varying ranges of displacement; the Yucca Fault extends to pre-Cenozoic basement, where it shows 200-400 m of offset, while the smaller faults are thought to be shallow features with significantly smaller offsets (less than one meter). The Yucca Fault has a prominent (approximately 4 m) surface scarp, whereas the other faults show little surface expression. The caliche layer presents challenges for processing seismic data of both wave types. In the S-wave data we see significant converted P-wave energy that makes accurate interpretation of true S-wave arrivals nearly impossible. The P-wave data show generally low noise due to efficient propagation in the partially cemented alluvium; interpretable reflections are not evident, however. Inversion of the P-wave first arrival times yields nonsensical results using a standard commercial seismic tomography algorithm that employs a smoothed grid velocity model. Inversion of these first arrivals using a method that employs a blocky velocity model defined with a smaller number of parameters yields more reasonable velocity models that fit with known geology. In the model corresponding with the transect that crosses the Yucca Fault, we see a distinct low-velocity zone (approximately 50 m wide) at the surface exposure of the fault and extending to the base of the model (60 m). We interpret that these low velocities indicate a zone of mechanically deformed material, although the true width of the deformed zone is likely narrower than the width indicated by the velocity model. Along the other transects, we see lesser velocity anomalies corresponding with smaller magnitudes of mechanical deformation on the faults that have less offset.
NS11E-0828
Imaging the Active Faults in Lanzhou, Northwest China
Although earthquake is difficult to predict, hazards generated by the earthquake can be assessed and certain mitigation measures can be taken accordingly. For example, a better planning and design for buildings can be taken to reduce potential damage caused by earthquake. Earthquakes, particularly those strong or large ones that are of safety concern, are generally associated with active faults. Therefore, determination and characterization of active fault is one of the key elements in seismic hazard assessment. Buildings and infrastructures can be sited away from the active faults. An early warning system of an eminent earthquake may be installed, and post-earthquake relief can also be planned if the active faults are known. Therefore, detecting the active faults in a populated city is of great significance. City of Lanzhou, located on the northeastern edge of Qinghai-Tibet plateau where neo-tectonic deformation is high, is the capital city of Gansu Province, and experienced many earthquakes; the largest one is M7.0 in 1125. This is the reason that Lanzhou was selected by the Chinese Central and Gansu Provincial Governments for conducting a comprehensive project to assess seismic hazard and to formulate mitigation measures in the urban area. One of key components of the project is to image active faults in and around Lanzhou. Shallow high-resolution seismic methods are considered as the most effective and reliable method for detecting the subsurface deformation, particularly in Lanzhou due to the unique seismological and geological conditions. Seismic reflection and refraction profiles across several active faults, especially the Jinchengguan fault and the Liujiapu projected fault, were collected in Lanzhou. In conjunction with other geophysical investigations such as electrical, magnetic, gravity surveys, seismic investigations confirm the Jinchengguan active fault and suggest that Liujiapu projected fault may not be an active one. The seismic investigations will help to assess potential surface offsets of the active faults in Lanzhou.
NS11E-0829
Imaging the Chihshang Fault in the Longitudinal Valley of Eastern Taiwan by using the Shallow Seismic Reflection Method
The Longitudinal Valley of eastern Taiwan is the most active fault zone situated in the Island of Taiwan. The most significant fault system in the Longitudinal Valley is the east dipping Coastal Range faults, including the Chihshang fault and the Yuli faults at the eastern side of the valley. In this paper, we will concentrate on imaging the Chihshang fault, which strikes in the SSW direction for about 47 km. The Chihshang fault separates the Lichi Mélange of the Coastal Range from the fluvial deposits in the Longitudinal Valley at shallow depth. Although there are some outcrops of the fault exposed, retreat of fault scarp by erosion still made it difficult to trace the detailed location of the Chihshang fault. In addition, the characteristics of the Mélange of poor image made it more difficult to image the Chihshang fault. To obtain the clear boundary of the Chihshang fault, we have deployed several shallow seismic lines to cross the Lichi Mélange in the Coastal Range and the fluvial deposits in the Longitudinal Valley. Seismic source was stacked around 50 times at each shot point to obtain enough energy for penetrating into the loose deposits. The results of the 24-fold CMP stacked seismic images show clear layering structures of the fluvial deposits in the longitudinal valley for more than 400 meters thick, and the sharp termination of the reflections mark the clear boundary of the Chihshang fault. Based on the characteristics of formations and features of the structural across the seismic lines, we are able to interpret the Chihshang fault and the extension of the fault. The patterns of the subsurface structures are found generally consistent with that on surface, but the features of the subsurface at some locations are offset horizontally to the east, compared to the topography.
NS11E-0830
The Characteristics of Shallow Seismic Reflection Images Across a Fault Between Slate and Conglomerate
The Taitung Longitudinal Valley in eastern Taiwan is the suture zone of the Eurasian and Philippine Sea plates. The valley is bound by the Central Mountain Range and the Coastal Range to the west and east, respectively. Because of rapid uplifting of the Central Mountain Range, there are series of terraces and alluvial fans existed along the eastern foothill of the Central Mountain Range. The Luyeh terrace is one of the terraces, which consists of a flexure scarp caused by the reactivation of the Luyeh fault. The Luyeh fault is a major active faults situated in the western margin of the of the suture zone. The fault is a reverse-slip fault and shows a clear lineament striking from north to south from geomorphic investigation. The Luyeh fault basically separates the slate formation of the Central Mountain Range from the conglomerate of the Luyeh terrace at shallow depth. Several investigations have been conducted to show the structural characteristics of the Luyeh fault; however, none direct contact of the fault was seen. Although seismic reflection method was thought inappropriate for applying over slate formations, the contact of slates and fluvial deposits sometimes becomes a remarkable boundary for mapping. We have conducted several shallow seismic reflection lines to delineate the Luyeh fault. Although the seismic lines were deployed on a difficult terrain across slate formations and conglomerate, the characteristics of shallow seismic reflection images show the eastward dipping Luyeh fault, its possible locations and the branches.
NS11E-0831
Natural Paleoseismometers: Cosmogenic Nuclide Dating of Precariously Balanced Rocks (PBRs) - Integral Constraints on Maximum Ground Accelerations
Precariously balanced rocks (PBRs) act as natural seismometers constraining maximum ground acceleration over the surface exposure history of the PBR. These key paleoseismic indicators have the potential to validate ground motions on the timescale necessary to test earthquake rupture forecasts and Seismic Hazard Assessment estimates, and are an active topic of research to validate CyberShake results and constrain National Seismic Hazard Maps. This research focuses on examining the post-exhumation history of PBRs using in-situ terrestrial cosmogenic nuclides (TCNs). TCNs provide a record of near-surface exposure history. The measured concentrations are a function of the residence time in the upper ~20 m of the subsurface (inherited concentration), the timing and rate of exhumation, and post-exhumation surface spalling and chemical erosion. Our goal in the project is to provide reasonable constraints on the post-exhumation history, specifically the age of the PBRs and evolution of precariousness: we should be able to constrain whether the rocks were of similar precariousness 2.5 ka, 5 ka, and 10 ka ago. These specific targets will provide important constraints on time since exceedance for the CyberShake models. We developed our sampling strategy to address subsurface inheritance, exhumation rate and timing, and post- exhumation spalling and chemical erosion. PBRs were selected to meet a variety of considerations. These rocks constrain ground motions from large earthquakes on the San Jacinto and Elsinore faults, in Southern California. Inherited concentrations lead to an age estimate that is too old; we are investigating inherited concentrations though sampling a rock quarry near Perris CA, with shielded samples at greater than 15 m depth. We also have partially shielded samples from the interior of rocks toppled to measure their stability, and through vandalism. To determine exhumation age and rate, our sampling strategy is to collect 5-6 samples per PBR: 1 on top, 3 on the sides, 1 on the pedestal (on pedestal samples), and 1 on the ground surface to determine modern landscape lowering rate. Our hope is that PBRs experienced rapid exhumation, and that 1 sample near the top will provide a good estimate of the exhumation age for future sampling campaigns. Lateral erosion is most important around the pivot point where the balanced rock rests on the pedestal, and the lower portion of the balanced rock, which can increase precariousness. 2-3 samples from were collected from this region, and selected to determine the effects of post-exhumation jointing and grüssification. Concentrations in this region that are significantly lower than the concentrations on the top and on the lower pedestal would suggest lateral erosion. We also have two samples for in situ 14C analysis; if the sample was exposed for 10 kyr with no erosion, the concentration should be close to secular equilibrium. Lower concentrations would indicate younger exposure ages, erosion, or a combination both.
NS11E-0832
Understanding Seismic Hazard in the Caucasus. Past, Present and Future.
It took more then a century for mankind first to understand a nature of most devastating catastrophes on the earth – earthquakes and then to reach more or less realistic seismic hazard assessment for particular regions. Seismic observations in the Caucasus had started in the end of XIX century. In 1899 the Tiflis (now Tbilisi) seismic station has been founded in the former Caucasian district of Russian Empire. After the well-known Shemacha earthquakes, The Permanent Central Commission of Seismicity (PCCS) in Russia founded seven seismic stations in the Caucasus region in 1903–1908. The Bulletins of the PCCS also were published. In 40- ties the formation of regional network was finished. From the beginning of 1960 network was equipped by high sensitive seismographs of different types. In eighties more than a hundred seismic stations were operated in Caucasus region, including Georgia, Armenia, Azerbaijan and Russia. Seismic hazard assessment in the region was developing along with the worldwide achievements in the field. First hazard maps dated back to forties and seventies were greatly dependent on observed seismic activity. Understanding of "seismic source zones" and local attenuation models were developed on later versions, though the devastating Earthquakes of Spitak in 1988 and Racha in 1991 have tremendously revised the official Seismic hazard Assessment maps. The most recent hazard maps for the region, created during the number of international projects, are based on probabilistic methodology using logic tree approach, though the main problem remains identification and quantitative characterization of active structures. Due to the extremely complex tectonics of the Caucasus – representing continental collision, it remains to be one of the purely studied areas in the world with rather high seismic activity. This is the main topic addressed by most recent activities in the region, involving GPS and SAR technologies. http://www.seismo.ge
NS11E-0833
Building A Seismotectonic Model For The Tbilisi Region Using Sparse Data
Tbilisi (Capital of the republic of Georgia) has largely been spared of large destructive earthquakes. The maximum affect from historical earthquakes in the Tbilisi area has not exceeded macroseismic Intensity 7 (MSC scale). There is very sparse information with which to identify the location of these earthquakes. The active seismogenic structures and the earthquakes near the city have been poorly investigated. On 25 April 2002, Tbilisi was hit by an Mw = 4.6 earthquake. Observed macroseimic intensity was 6-7 (MSC) in Tbilisi. Seven people were killed, 20 people were injured, ten buildings collapsed, and 10,000 buildings were damaged. In this study we utilize recent geologic investigations, regional tectonic interpretations, historical earthquakes, recent seismicity, moment tensor inversion, and finite fault models of the 2002 Tbilisi earthquake to develop a seismo-tectonic model of the Tbilisi area in order to better identify the hazard from future earthquakes. Unfortunately, there were no permanent local recording stations in place at the time of the 2002 sequence. However, a temporary station was installed following the main event and eleven aftershocks were recorded. With only one recording station it is not possible to obtain local focal mechanism solutions or accurate earthquake locations that might indicate fault geometries. Data for a seismo-tectonic study and identification of the earthquake hazard is also very sparse. Therefore, one aspect of this study is to utilize modern techniques to maximize the use of sparse data.
NS11E-0834
New evidence for active deformation off the Annaba region (easternmost Algerian margin): Estimates of Quaternary shortening and slip rates from folding analyses
From the first marine investigations off the region of Annaba (easternmost part of Algeria) made after the MARADJA2 survey in November 2005, a set of large active faults and folds was discovered near the foot of the margin. This active thrust system resumes a previous passive margin and creates growth strata deposition on the backlimbs of large Quaternary folds, resulting in the development of perched basins at the foot of the margin since less than 1 Ma. The system forms a set of overlapping fault segments verging towards the Algerian basin. From the seismic line analysis (high-resolution and Chirp data), and using sedimentation rates obtained from coring and extrapolated, a shortening rate of about 0.9 mm/yr is estimated. If we consider a single fault dipping at about 45° (hypothesized from the Boumerdès 2003 rupture zone), a fault slip rate of about 1.3 mm/yr can be postulated. Considering recent kinematic studies, this fault-related fold system could accommodate a significant part of the present-day submarine shortening rate between the European and African plates, predicted by Stich et al. (2006) to be 1.5 mm/yr from GPS modelling. This faulting is apparently active at a faster rate than the similar faulting observed at the foot of the margin NW of Algiers. This system of faults off Annaba represents a major threat for the region. Indeed, the location of this system changes drastically the seismic hazard assessment of the region, since no large historical earthquake is reported there since 3 centuries at least.
NS11E-0835
New Seismic CHIRP evidence for Transpression and Transtension Beneath the Salton Sea, California
The Salton Trough is a critical structure that separates spreading center dominated deformation in the Gulf of California and dextral strike-slip deformation along the San Andreas Fault (SAF) System. Geological and geophysical data suggest there is a transition within the Salton Trough near the town of Bombay Beach that separates transpression to the north from transtension in the south. To date, however, this transition remains poorly understood, in large part, due to a lack of geophysical subsurface data in the Salton Sea. We present preliminary analysis of > 350 line-km of high-resolution seismic CHIRP data acquired in 2006 and 2007 that imaged the different deformational styles beneath the Salton Sea, including several previously unidentified tectonic structures. The Extra Fault Zone (EFZ), which has been mapped onshore, is imaged in CHIRP profiles as an ~5 km wide deformation zone and can be traced offshore > 15 km along strike. Along the northern extent of the EFZ, ramp-flat deformation is observed with southward vergence. A marked angular unconformity between the Brawley and Cahuilla formations records fault-bend folding often predicted for ramp-flat thrust systems. Uplift and truncation of Pleistocene sediments along the northern edge of the EFZ is observed across the entire sea and appears to systematically increase towards the west. Onlapping sediments and growth folds in the Holocene Lake Cahuilla section to the north and south of the fault zone record ongoing uplift. Compression is also manifested in the bathymetry, with a mid-lake bathymetric high trending parallel the EFZ and separating southern and northern lake basins. The EFZ is interpreted to accommodate sinistral transpression related to clockwise- rotating crustal blocks. In contrast, at Bombay Beach the SAF takes an ~15 km releasing step towards the Imperial Fault, producing transtensional deformation through the Brawley Seismic Zone (BSZ). CHIRP profiles across the western margin of the BSZ image several down-to-the southeast, en echelon normal faults. Vertical offset of Lake Cahuilla sediments extends to within .5 m of the lake floor, suggesting recent deformation (possibly post-1905), and increases down-section to a maximum of ~6-8 m, thus providing evidence for several earthquakes during the past 1,300 - 1,500 years. The trend of these normal faults is approximately northeast, an orientation that is not aligned with the "ladder and rung" patterns in the relocated seismicity. Facies changes documented in ~15 m sediment cores collected by United Research Services (URS) allow us to identify at least four lake sequences. Semblance between coarse-grained materials in core logs and high-amplitude reflections in CHIRP data allow us to correlate strata throughout the basin and assign tentative ages. With an improved chronology for Lake Cahuilla sequences, the CHIRP data will allow us to constrain slip rates, paloeseismic event timing across the EFZ and faults in the BSZ, and to investigate relationships between events along the EFZ, SAF and San Jacinto Fault (SJF). The overall structure observed in CHIRP profiles might be a result of transrotation caused by strain interactions between the SAF and SJF and overprinted by regional transpression and transtension. Doming and uplift of LC sediments above the thrust ramp, as well as en echelon normal faults along the western edge of the BSZ suggest transpression and transtension are currently active and play an important role in the late Pleistocene- Holocene tectonic evolution of the Salton Trough.
NS11E-0836
A High-Resolution Seismic Survey Across the State Line fault, NV
During the summer of 2007, an investigation of the faulting in Stewart Valley was under taken, located within the central Basin and Range province ~90 km west of Las Vegas, Nevada. The goal of this study was to resolve the seismic hazard potential of the State Line fault, a right-lateral strike-slip fault that runs the length of Stewart Valley. Four seismic reflection lines were acquired, two perpendicular and two parallel to the State Line fault. What is presented is an analysis of the western and eastern seismic lines parallel to the State Line fault. The western line was acquired utilizing a 144-channel geode system with each of the 4.5 Hz vertical geophones set out at 5 m intervals to form a 715 m long profile. The eastern line employed 120 of these geophones in a 595 m long profile. A mini-vibroseis served as the seismic source every ten meters, between geophones. The vibroseis was programmed to produce an 8 s linear sweep from 20-160 Hz. Three sweeps were recorded at each shot location without acquisition filters at a sampling rate of 0.5 ms. The three shot gathers were then stacked at each location to reduce noise. The data collected had minimal noise, although; during the processing of the eastern line a notch filtered was used to remove the 60 Hz noise created by adjacent power line. These lines, acquired parallel to the State Line fault, contain matching features that serve to determine how much lateral displacement the fault has undergone. The amount of the displacement can indicate how active the fault is, and thus, what magnitude of earthquake can be expected in the future. This will in turn contribute to determining the seismic hazard potential for southern Nevada. A preliminary interpretation of the seismic reflection sections indicates an average displacement of about 20 - 38 m with greater displacement in the deeper sections of the image. The shallow depth displacement calculations are consistent with previous work in the area. The State Line fault is believed to be a result of strain partitioning from the San Andreas Fault. By studying this more localized active strike-slip fault system the results of this study can contribute to a model that provides a better understanding of the tectonics in the central Basin and Range.
NS11E-0837
Preliminary Pseudo 3-D Imagery of the State Line Fault, Stewart Valley, Nevada Using Seismic Reflection Data
The Pahrump Fault system is located in the central Basin and Range region and consists of three main fault zones: the Nopah range front fault zone, the State Line fault zone and the Spring Mountains range fault zone. The State Line fault zone is made up north-west trending dextral strike-slip faults that run parallel to the Nevada- California border. Previous geologic and geophysical studies conducted in and around Stewart Valley, located ~90 km from Las Vegas, Nevada, have constrained the location of the State Line fault zone to within a few kilometers. The goals of this project were to use seismic methods to definitively locate the northwestern most trace of the State Line fault and produce pseudo 3-D seismic cross-sections that can then be used to characterize the subsurface geometry and determine the slip of the State Line fault. During July 2007, four seismic lines were acquired in Stewart Valley: two normal and two parallel to the mapped traces of the State Line fault. Presented here are preliminary results from the two seismic lines acquired normal to the fault. These lines were acquired utilizing a 144-channel geode system with each of the 4.5 Hz vertical geophones set out at 5 m intervals to produce a 595 m long profile to the north and a 715 m long profile to the south. The vibroseis was programmed to produce an 8 s linear sweep from 20-160 Hz. These data returned excellent signal to noise and reveal subsurface lithology that will subsequently be used to resolve the subsurface geometry of the State Line fault. This knowledge will then enhance our understanding of the evolution of the State Line fault. Knowing how the State Line fault has evolved gives insight into the stick-slip fault evolution for the region and may improve understanding of how stress has been partitioned from larger strike-slip systems such as the San Andreas fault.
NS11E-0838
Seismic Imaging of the Vallo di Diano Fault System (Southern Italy) by Integrating High- Resolution Reflection Profiles and Shallow Tomography
The NW-striking, SW-dipping range-bounding extensional faults (Vallo di Diano Fault System, VDFS) drive the evolution of the Vallo di Diano basin (southern Italy). Despite of structural and geomorphic investigations, the Late Pleistocene activity of this complex fault system is still a matter of debate. We have interpreted seismic commercial profiles (down to 2 sec TWT) suggesting a 1000 m maximum infill thickness, with depositional geometries controlled by the VDFS" activity probably since late Pliocene times. Geomorphological analysis in the central sector of the basin revealed unreported small scarps aligned with the VDFS and affecting two late Pleistocene fans. With the scope to define the shallower portion of the VDFS and to assess its possible recent activity, we get two parallel high-resolution seismic profiles crossing the aforementioned scarps. The first is 1200 m long and crosses a 30 m high scarp affecting a middle Pleistocene fan. The second is 400 m long and crosses two small scarps affecting a late Pleistocene fan. Seismic profiles were collected with a non-conventional dense/extended-offset geometry using 168 geophones and a Minivib source. The two high-quality stack sections obtained show coherent reflections, together with two high-resolution P-velocity images from first-arrival travel-time tomography. Stack sections depict the subsurface structure up to 1 km depth, while the tomographic models image the shallower portion (40-50 m). The high-resolution profile presents four distinct seismic units separated by unconformities, where the shallowest unit shows sub-horizontal high-frequency low-amplitude reflectors overlapping an articulated erosion surface. Strong and coherent reflectors below this unconformity can be interpreted as two separate alluvial fans with southward progradational patterns. The third and deeper unconformity is represented by a low-frequency and continuous reflector downlapped by the alluvial fans to the south. The very-high resolution profile also shows evidence of truncations in the upper reflective portion, here interpreted as a high-angle SW-dipping normal fault. It is not clear if the fault displaces shallower portions of the alluvial fans, however the fault position well matches a surface scarp. These features correlate with deeper reflection truncations on the stack sections and partially with the observed scarps suggesting recent faulting activity in the area. Now, we are going to acquire a new data set along the south- eastern part of the main tectonic alignment to define its shallower branch.
NS11E-0839
Combined Seismic Reflection and Refraction Profile Across the Hayward Fault in Fremont, Alameda County, California
In late July of 2007, we acquired a ~300-m-long combined seismic reflection and refraction profile across the Hayward fault in Fremont, California. The profile was located in order to image an inferred structural stepover of the main trace of the fault. We used fifty-eight 40 Hz P-wave geophones in a linear array with a spacing of 5 m. Seismic sources consisted of repeated sledgehammer strikes at each geophone location. The P-wave geophones were then replaced by 4.5 Hz S-wave geophones, orientated perpendicular to the profile. Sledgehammer strikes were then repeated at each location using a custom-built S-wave generator consisting of a large aluminum block, which was struck at both ends; cleats on the bottom of the block helped couple the block to the ground. All shots were recorded by all of the geophones, allowing for both refraction and reflection processing. We created a tomographic velocity image from the P-wave data in order to image the structure of the Hayward fault at this location, and to stack the seismic reflection data. We also created a tomographic velocity image from the S-wave data in order to calculate Poisson's ratio and a Vp/Vs ratio along the profile. All of these data contribute to defining fault location, geometry, and structure.
NS11E-0840
High-Resolution Seismic Reflection and Refraction Imaging of the Hayward Fault in Fremont, Alameda County, California
In July 2007, the U.S. Geological Survey acquired a 60-m-long seismic reflection and refraction profile across the main trace of the Hayward fault in Fremont Central Park, Fremont, California. The profile was designed to determine the geometry, seismic velocities, and possible structural complexities of the fault. The study was along a part of the surface rupture of the 1868 M 7.0 Hayward earthquake. We used single-element, 40-Hz vertical geophones placed at 1-m intervals along the profile with 0.5-m lateral offset from the shot points, also with 1-m intervals. Seismic sources were generated by multiple sledgehammer blows at each shot point. Data were recorded unfiltered in the field on a Geometrics Strataview RX-60 seismograph at a sampling rate of 0.5 ms for 2 s. Geophone locations were measured in 3D using differential GPS. We developed a velocity model using the Hole (1992) code to invert P-wave first arrivals of the refraction data. Seismic P-wave velocities range from about 200 m/s near the surface to approximately 800 m/s at a depth of 13 to 16 m. The velocity model was then applied to the reflection data to develop an unmigrated common depth point (CDP) stack. The reflection data indicate the presence of at least three fault strands in an approximately 20-m-wide zone. We believe the three strands define an upwardly flaring ‘flower structure', with the central strand being the main strand of the Hayward fault. The three strands project to merge at a depth of about 150 m; the overall dip of the fault zone in the upper 100 m is to the northeast, at about 88 degrees.