G14A-01
Evidence for the Existence of the Bering Plate
GPS measurements are used to examine the hypothesis of a clockwise rotating Bering plate. Originally proposed based on seismicity, the Bering plate encompasses the Bering Sea, western Alaska, and the Aleutian Islands. GPS measurements from the Bering plate's interior show south to southwest motions of 3 to 5 mm/yr. We construct elastic dislocation models to determine the spatial distribution and intensity of locked patches on the Aleutian subduction interface, and use these to remove interseismic strain from the GPS observation and determine an arc translation velocity for each region of the Aleutians. Translation velocity estimates range from 4 to 8 mm/yr mm/yr oriented south to southwest. We combine the arc translation rates with measurements from Bering plate's interior sites and estimate the angular velocity for the Bering plate relative to North America to have an angular speed of 6.0°/my about a pole located at 42.5° N, 121.3° E. The clockwise rotation of the Bering plate causes the Bering Sea crust to override the Aleutian trench, and may drive left lateral faulting in interior Alaska. The Bering plate's interaction with southcentral Alaska may be responsible for the decreased slip-rate on the western Denali fault, and for contraction across the central Alaska Range. We analyze slip partitioning along the Aleutian arc based on both GPS measurements and slip azimuths of thrust earthquakes. We find a systematic discrepancy between plate convergence direction and slip azimuths and find that slip partitioning in the back-arc only develops west of Amchitka Pass, whereas slip partitioning in the forearc is present throughout the arc.
G14A-02
Crustal Deformation and Strain Localization in the Saint Elias Orogen, Alaska Observed by GPS
The Saint Elias orogen of southcentral Alaska and the adjacent region of Canada contains one of the highest coastal mountain ranges on earth and over half of the 25 highest peaks in North America. This superlative topography is a result of the collision of the Yakutat block with southern Alaska. Nearly 45 mm/yr of NW-SE directed convergence from this collision is accomodated within the Saint Elias orogen. The locations of the structures taking up the convergence remain uncertain. As part of the Saint Elias Erosion/Tectonics Project (STEEP), a project designed to elucidate the complex tectonics of the region, we re-occupied 30 campaign GPS sites throughout the orogen during the summer of 2007 and used the data to generate velocities at each site. The velocities show a remarkable degree of Fairweather fault-parallel motion. Since the Yakutat block motion is also parallel to the Fairweather fault, the velocities show that the Yakutat block is the dominant tectonic influence within the orogen. The influence of the subducting Pacific plate is not clearly seen until the western edge of the orogen and Prince William Sound. Velocities along the edges of Icy Bay indicate that over 7 mm/yr of convergence occurs over a distance of 11 km, producing a strain rate of -0.6 microstrains/yr. Contraction rates in the Nepal Himalaya are less than a fifth of this value while rates in the eastern Kenai Peninsula in Alaska only reach half of the Icy Bay rate. The strain rate between Yakutat Bay and the eastern edge of Icy Bay is only -0.0419 microstrains/yr. We use elastic dislocation models to describe the tectonic elements in the area and evaluate two possible end member models. The observed velocities may be due to a creeping fault that comes to the surface in Icy Bay. If this is the case, Icy Bay is the current locus of permanent deformation and mountain building in the orogen. On the other hand, the velocities may be able to be explained by a shallow thrust fault beneath the region that has the down dip end of its locked zone centered under Icy Bay.
G14A-03
Accommodation of Plate-Boundary Strain in the Northern Cordillera of NW Canada and E Alaska from Earthquake Statistics and GPS
Current collision of the Yakutat block with N. America is accommodated over a large region of the Canada-Alaska Northern Cordillera. We investigate this plate-boundary strain distribution using continuous and campaign Global Positioning System (GPS) data and deformation rates estimated from earthquake catalog statistics. Seismic deformation estimates have significant uncertainties, but generally agree with other estimates (GPS, geological, plate models) and provide useful first-order constraints on local long-term tectonics and seismic hazard. Our GPS data infer that the relative Yakutat/N. America motion is accommodated to the east by near-field right- lateral motion (~ 40 mm/yr), mainly on the Fairweather fault, and minor shortening (~ 6 mm/yr). To the north, collision is taken up by fold-and-thrust belt shortening (~ 31 mm/yr), with westward extrusion and possible counter-clockwise rotation of the Yakutat block and Alaskan forearc facilitated by ~ 23 mm/yr distributed dextral shear. We estimate that 50-75% of the relative plate motion is taken up by earthquakes on the Yakutat/N. America boundaries. The remainder must be accommodated by lateral crustal extrusion, other far-field strain, and possibly aseismic creep. Significant seismic strain occurs throughout E. Alaska, Yukon and W. Northwest Territories, inferring that plate- boundary deformation is far-reaching. GPS data enable the extent of this deformation to be mapped, thereby defining the NW edge of "stable" N. America. GPS sites in SW Yukon show motion of 3-10 mm/yr to the NE, confirming strain transfer at least 400 km from the Yakutat collision. We calculate dextral slip at 5-10 mm/yr on the Denali fault system in Alaska, similar to late Pleistocene/Holocene rates from geomorphic offsets. Right-lateral motion bypasses the Denali fault in easternmost Alaska and westernmost Yukon, passing along the Totschunda and Duke River fault systems. Significant seismic deformation (in cases higher than suggested by GPS) occurs throughout the N. Cordillera, where pre-existing faults have a strong control on the pattern and mechanisms of current deformation.
G14A-04
Into the Future: Continuing Evolution of the Pacific-Juan de Fuca-North America Plate System
The death of a series of spreading ridge segments adjacent to California and Mexico starting about 28.5 Ma led to the piecewise destruction of a subduction regime and the opening of slab windows beneath the continent. The formation, growth, and healing of these slab windows has been a transient process marked at the surface by overprinted pulses of volcanism in the former forearc triggered by asthenospheric upwelling behind subducting slab edges. In the subsurface, where windows opened beneath thin forearc lithosphere, the shallow asthenosphere promoted a plastic mode of mechanical behavior in the lithosphere characterized by fault-block deformation. This plastic mode of deformation resulted mainly from (1) asthenosphere in direct contact with the lower crust instead of stronger lithospheric mantle; and (2) lower crust shifting from brittle to ductile behavior as a result of conductive heating. Our finite-rotation model built from quantitative reconstruction of Farallon-North America slab windows and their magmatic signatures allows us to investigate past and future fault-block kinematics of the North America continental margin. While the shallow asthenosphere thermally weakened coastal California and promoted plastic deformation in the lower crust, the actual fragmentation of the upper crust into fault blocks occurred in concert with retrograde motion of a partially subducted Monterey plate fragment following its capture by the Pacific plate ca. 19 Ma. This abrupt change from convergent to transtensional deformation initiated the pulling apart and clockwise pivoting of the adjacent western Transverse Ranges and California Borderlands region. A similar reversal in plate motion occurred adjacent to coastal Mexico following capture of a Magdalena plate fragment by the Pacific plate ca. 12.5 Ma. In this case, the transtensional strain that initiated Baja California pulling away from the Mexican continental margin was localized along already thermally weakened Comondu volcanic arc crust. In California, running our kinematic model into the future suggests that the current locus of strike-slip motion may shift to the east side of the Sierra Nevada Mountains, starting about 2 My from now as a result of convergence between the Sierra Nevada and Peninsular Range batholiths. Alternatively, the locus of fault slip may remain on the San Andreas fault, requiring new fragments to be broken from the southern Sierra Nevada and Mojave blocks, to make room for the Peninsular Range to slip past on their west side. Or perhaps, eastern parts of the Peninsular Range may transfer back to the North America plate, with the Whittier-Elsinore fault accommodating most of the relative plate motion.
G14A-05
The Development of the San Andreas Plate Boundary through Northern California: Insights from GPS, Crustal Structure, and Lithospheric Modeling
The San Andreas plate boundary lengthens in the wake of the Mendocino triple junction (MTJ), and over the last ca. 7-10 Ma it has developed into a localized plate boundary shear zone between the North America and Pacific plates. The pathway from a diffuse deformation swath to a few major fault related plate boundary structures reflects the interplay of thermal and deformational processes acting on the inherited structures of the Cascadia forearc. Furlong and Govers (1998) proposed the Mendocino Crustal Conveyor (MCC) model (supported by numerical modeling) that argued for temporal and spatial variations in lithospheric deformation in association with MTJ passage, which have led to the formation of the main plate boundary structures. The general concept of faults developing and eventually coalescing into a primary plate boundary structure after MTJ passage serves as the framework for most tectonic and geodetic analyses of the fault system. What has been less well understood or quantified is specifically how the fault systems form, what drives fault localization, and how does the concomitant crustal evolution play a role in the plate boundary development. The substantial augmentation of the geodetic data for northern California through a combination of campaign and most recently (through the PBO component of EarthScope) continuous GPS observations in concert with seismological analyses of crustal structure now allows us to test, calibrate, and refine the MCC model. Specifically, the (1) crustal thickening at and north of the MTJ, predicted by MCC processes, is clearly seen in the crustal velocity and GPS derived strain fields, (2) the approx, E-W extent of MCC deformation is delineated by the GPS data to occur primarily through the core of the northern Coast Ranges – consistent with the topographic and fluvial evolution of the region, (3) compatible with seismic observations, the GPS data imply that the upper crust is only a minor participant in the MCC crustal thinning that occurs approximately 200 km south of the MTJ (i.e. ca. 4-5 million years after MTJ passage), and (4) development of the precursor faults to the San Andreas plate boundary structures appear to be driven by the combination of MCC crustal deformation and the development of localized shear within the MTJ-formed slab window. Further structural complexities arise in developing these precursor faults in the upper crust with its pre- existing convergent margin structures, which are either overprinted or reactivated as a result of their orientation and segmentation. The addition of GPS observations to the existing catalog of geophysical and tectonic characteristics of the northern San Andreas system allow us to place the transition from a mature convergent margin to an active translational plate boundary into a physically constrained framework. The MCC model of plate boundary evolution is consistent with these kinematic and structural constraints and thus provides a useful framework model for unraveling the processes that drive the development of the San Andreas plate boundary after MTJ passage and the cessation/removal of subduction from the western margin of North America.
G14A-06
Slip Rates and Rheology of the Southern San Andreas-San Jacinto Fault System From Earthquake Cycle Models Constrained by GPS and InSAR Observations
Estimates of fault slip rates across deforming plate margins from geodetic data are nonunique and depend upon assumptions in model parameters that may be poorly constrained and that trade-off with other parameters. Surface velocity profiles across active fault systems are a product of far-field loading, long-term slip rates and locking depths of the individual faults, the rheology of the crust and upper mantle, time since the last major earthquake on each fault, and the system's seismic history. The southern San Andreas fault system (San Andreas, San Jacinto, and Elsinore faults, principally) has been the subject of a number of recent studies that seek to understand the effects of lateral variations in elastic and/or vertical variations in viscoelastic mechanical structure on estimates of fault slip rates as deduced from surface velocity measurements. There are known trade-offs in fault slip, mechanical structure, earthquake repeat interval, and the current relative time into the present earthquake cycle that make estimation of these parameters somewhat difficult, with differences between significantly different models resulting in rather small differences in the surface velocity profile. We apply two dimensional (2D) viscoelastic finite element models (FEM) of faults undergoing periodic earthquakes to explore the effects of rheology, inter-event time, and time into the earthquake cycle on estimates of long-term slip rates across the southern San Andreas fault system in southern California. Specifically we will explore a range of models incorporating lateral rheology (both elastic and time dependent) variations across this fault system that best fit the interferometric synthetic aperture radar (InSAR) mean line-of- sight (LOS) velocity field, and geodetic velocities from the SCEC CMM3 solution projected into the SAR LOS. The InSAR LOS velocities were calculated from a least-squares inversion of 61 interferograms from 33 SAR data images spanning the interval between the Landers (1992) and Hector Mine (1999) earthquakes. 2D viscoelastic FEM Green's functions are calculated using the GeoFEST v. 4.5 software. Green's functions are calculated for a fully "spun- up" edge driven earthquake cycle with an assumed fault locking depth and mechanical structure across the fault system. We use a Bayesian inversion method to solve for both fault slip on the San Jacinto and San Andreas faults and the most appropriate rheology across the fault system.
G14A-07
Effects of Material Heterogeneity on Interseismic and Geologic Deformation Southern California Sedimentary Basins
Conventional methods involving the inversion of geodetic data for fault slip can become cumbersome in structurally complex regions such as the Los Angeles and Ventura basins. To incorporate any number fault intersections at depth along dipping faults, we develop a technique that allows for faults of finite height. We show that the analytical solution for a semi-infinite vertical strike-slip fault is identical to that of a vertical fault of finite height that soles into two horizontal detachments with opposite senses of slip. Based on this analytical solution, we formulate a two-step numerical simulation of the earthquake cycle that allows complex fault surfaces to interact and accumulate slip. In the first step, we solve for the distribution of total geologic fault slip (i.e. entire earthquake cycle) at all crustal levels along multiple, non-planar, three-dimensional faults. Then, to simulate interseismic deformation, slip from the geologic model below the seismogenic locking depth is mapped onto fault surfaces; above the locking depth slip is zero. We apply this technique to the greater Los Angeles and Ventura regions of southern California and find that geologic model results driven by geodetic shortening rates match well geologic slip rate data. Interseismic model results match well the heterogeneous GPS velocity pattern in both regions; however, analysis of model residuals indicates that localized convergence in the San Gabriel and Ventura sedimentary basins is under-predicted by these homogeneous models. To explore the effects of large sedimentary basins in southern California, we create a second set of models that simulate sedimentary basin compliance by populating basin regions with arrays of randomly-oriented micro-cracks. We find that incorporation of the effects of large sedimentary basins into interseismic models of southern California reduces the model-GPS residuals at sites within sedimentary basins. Our results indicate that in southern California, long-term geologic deformation rates are compatible with short-term GPS rates and that models that approach geologic realism best match geodetic data. http://www.people.umass.edu/stmarsha/la_basin.html
G14A-08
Using Finite Element Meshes Derived from the SCEC Community Fault Model to Evaluate the Effects of Detailed Fault Geometry and Material Inhomogeneities
To make realistic assessments of fault slip inferred from surface geodetic observations, we need to know the sensitivity of our predicted surface deformation field to additional factors that may not be accurately represented in the model. Two primary factors that may influence our predicted results are the detailed geometry of the faults in the model and the variations in material properties in the region under investigation. As a step in addressing the potential importance of these factors, we compare finite element computations of varying model complexity against those obtained using the analytical model of Meade and Hager (JGR, 2005). We use meshes derived from the Community Fault Model (CFM) to represent the detailed geometry for a small portion of the San Andreas fault system. This is a departure from previous work, where we used a derived Community Block Model (CBM) to provide airtight volumes for meshing. This new approach allows us to more easily include more faults and greater geometrical detail depending on the problem under consideration, and is more in keeping with the fractal nature of fault networks. Using this method, we have produced meshes including as many as 90 of the faults from the CFM, with the capability to include all CFM faults. We perform three different comparisons using four different models. We first compare analytical results using CFM-R (a coarser rectangularized version of the CFM) against a finite element representation of CFM-R assuming homogeneous elastic properties. This allows us to evaluate the accuracy of the finite element solution to insure the validity of our results. We then compare finite element solutions using CFM and CFM-R to evaluate the effects of including detailed fault geometry, again assuming homogeneous material properties. Finally, we compare finite element solutions with homogeneous and vertically-layered elastic property variations, both using the same CFM-derived mesh, thus providing a first-order estimate of the influence of material property variations on the predicted surface deformation field.