Geologic Slip Rates Research Articles

Surface slip and off‐fault deformation patterns in the 2013 MW 7.7 Balochistan, Pakistan earthquake: Implications for controls on the distribution of near‐surface coseismic slip

AbstractComparison of 398 fault offsets measured by visual analysis of WorldView high‐resolution satellite imagery with deformation maps produced by COSI‐Corr subpixel image correlation of Landsat‐8 and SPOT5 imagery reveals significant complexity and distributed deformation along the 2013 Mw 7.7 Balochistan, Pakistan earthquake. Average slip along the main trace of the fault was 4.2 m, with local maximum offsets up to 11.4 m. Comparison of slip measured from offset geomorphic features, which record localized slip along the main strand of the fault, to the total displacement across the entire width of the surface deformation zone from COSI‐Corr reveals ∼45% off‐fault deformation. While previous studies have shown that the structural maturity of the fault exerts a primary control on the total percentage of off‐fault surface deformation, large along‐strike variations in the percentage of strain localization observed in the 2013 rupture imply the influence of important secondary controls. One such possible secondary control is the type of near‐surface material through which the rupture propagated. We therefore compared the percentage off‐fault deformation to the type of material (bedrock, old alluvium, and young alluvium) at the surface and the distance of the fault to the nearest bedrock outcrop (a proxy for sediment thickness along this hybrid strike slip/reverse slip fault). We find significantly more off‐fault deformation in younger and/or thicker sediments. Accounting for and predicting such off‐fault deformation patterns has important implications for the interpretation of geologic slip rates, especially for their use in probabilistic seismic hazard assessments, the behavior of near‐surface materials during coseismic deformation, and the future development of microzonation protocols for the built environment.

Using Surface Creep Rate to Infer Fraction Locked for Sections of the San Andreas Fault System in Northern California from Alignment Array and GPS Data

Surface creep rate, observed along five branches of the dextral San Andreas fault system in northern California, varies considerably from one section to the next, indicating that so too may the depth at which the faults are locked. We model locking on 29 fault sections using each section’s mean long‐term creep rate and the consensus values of fault width and geologic slip rate. Surface creep rate observations from 111 short‐range alignment and trilateration arrays and 48 near‐fault, Global Positioning System station pairs are used to estimate depth of creep, assuming an elastic half‐space model and adjusting depth of creep iteratively by trial and error to match the creep observations along fault sections. Fault sections are delineated either by geometric discontinuities between them or by distinctly different creeping behaviors. We remove transient rate changes associated with five large ( M ≥5.5) regional earthquakes. Estimates of fraction locked, the ratio of moment accumulation rate to loading rate, on each section of the fault system provide a uniform means to inform source parameters relevant to seismic‐hazard assessment. From its mean creep rates, we infer the main branch (the San Andreas fault) ranges from only 20%±10% locked on its central creeping section to 99%–100% on the north coast. From mean accumulation rates, we infer that four urban faults appear to have accumulated enough seismic moment to produce major earthquakes: the northern Calaveras ( M 6.8), Hayward ( M 6.8), Rodgers Creek ( M 7.1), and Green Valley ( M 7.1). The latter three faults are nearing or past their mean recurrence interval. Online Material: High‐resolution fault system map, and tables of creep observations and geographic coordinates of fault model.

Evolution of the central Garlock fault zone, California: A major sinistral fault embedded in a dextral plate margin

The Garlock fault is an integral part of the plate-boundary deformation system inboard of the San Andreas fault (California, USA); however, the Garlock is transversely oriented and has the opposite sense of shear. The slip history of the Garlock is critical for interpreting the deformation of the through-going dextral shear of the Walker Lane belt–Eastern California shear zone. The Lava Mountains–Summit Range (LMSR), located along the central Garlock fault, is a Miocene volcanic center that holds the key to unraveling the fault slip and development of the Garlock. The LMSR is also located at the intersection of the NNW-striking dextral Blackwater fault and contains several sinistral WSW-striking structures that provide a framework for establishing the relationship between the sinistral Garlock fault system and the dextral Eastern California shear zone. New field mapping and geochronology data ( 40 Ar/ 39 Ar and U-Pb) show five distinct suites of volcanic-sedimentary rock units in the LMSR overlain by Pliocene exotic-clast conglomerates. This stratigraphy coupled with fifteen fault slip markers define a three-stage history for the central Garlock fault system of 11–7 Ma, 7–3.8 Ma, and 3.8–0 Ma. Pliocene to recent slip occurs in a ~12-km-wide zone and accounts for ~33 km or 51% of the total 64 km of left-lateral offset on the Garlock fault in the vicinity of the LMSR since 3.8 Ma. This history yields slip rates of 6–9 mm/yr for the younger stage and slower rates for older stages. The LMSR internally accommodates northwest-directed dextral slip associated with the Eastern California shear zone–Walker Lane belt via multiple processes of lateral tectonic escape, folding, normal faulting, and the creation of new faults. The geologic slip rates for the Garlock fault in the LMSR match with and explain along-strike variations in neotectonic rates.

A Method and Example of Seismically Imaging Near-Surface Fault Zones in Geologically Complex Areas Using VP, VS, and their Ratios

Abstract The determination of near‐surface (vadose zone and slightly below) fault locations and geometries is important because assessment of ground rupture, strong shaking, geologic slip rates, and rupture histories occurs at shallow depths. However, seismic imaging of fault zones at shallow depths can be difficult due to near‐surface complexities, such as weathering, groundwater saturation, massive (nonlayered) rocks, and vertically layered strata. Combined P ‐ and S ‐wave seismic‐refraction tomography data can overcome many of the near‐surface, fault‐zone seismic‐imaging problems because of differences in the responses of elastic (bulk and shear) moduli of P and S waves to shallow‐depth, fault‐zone properties. We show that high‐resolution refraction tomography images of P ‐ to S ‐wave velocity ratios ( V P / V S ) can reliably identify near‐surface faults. We demonstrate this method using tomography images of the San Andreas fault (SAF) surface‐rupture zone associated with the 18 April 1906 ∼ M 7.9 San Francisco earthquake on the San Francisco peninsula in California. There, the SAF cuts through Franciscan melange, which consists of an incoherent assemblage of greywacke, chert, greenstone, and serpentinite. A near‐vertical zone (∼75° northeast dip) of high P ‐wave velocities (up to 3000 m/s), low S ‐wave velocities (∼150–600 m/s), high V P / V S ratios (4–8.8), and high Poisson’s ratios (0.44–0.49) characterizes the main surface‐rupture zone to a depth of about 20 m and is consistent with nearby trench observations. We suggest that the combined V P / V S imaging approach can reliably identify most near‐surface fault zones in locations where many other seismic methods cannot be applied.

Slip rates and spatially variable creep on faults of the northern San Andreas system inferred through Bayesian inversion of Global Positioning System data

AbstractFault creep, depending on its rate and spatial extent, is thought to reduce earthquake hazard by releasing tectonic strain aseismically. We use Bayesian inversion and a newly expanded GPS data set to infer the deep slip rates below assigned locking depths on the San Andreas, Maacama, and Bartlett Springs Faults of Northern California and, for the latter two, the spatially variable interseismic creep rate above the locking depth. We estimate deep slip rates of 21.5 ± 0.5, 13.1 ± 0.8, and 7.5 ± 0.7 mm/yr below 16 km, 9 km, and 13 km on the San Andreas, Maacama, and Bartlett Springs Faults, respectively. We infer that on average the Bartlett Springs fault creeps from the Earth's surface to 13 km depth, and below 5 km the creep rate approaches the deep slip rate. This implies that microseismicity may extend below the locking depth; however, we cannot rule out the presence of locked patches in the seismogenic zone that could generate moderate earthquakes. Our estimated Maacama creep rate, while comparable to the inferred deep slip rate at the Earth's surface, decreases with depth, implying a slip deficit exists. The Maacama deep slip rate estimate, 13.1 mm/yr, exceeds long‐term geologic slip rate estimates, perhaps due to distributed off‐fault strain or the presence of multiple active fault strands. While our creep rate estimates are relatively insensitive to choice of model locking depth, insufficient independent information regarding locking depths is a source of epistemic uncertainty that impacts deep slip rate estimates.

Influence of fault connectivity on slip rates in southern California: Potential impact on discrepancies between geodetic derived and geologic slip rates

AbstractAlong the San Bernardino strand of the San Andreas fault (SAF) and across the eastern California shear zone (ECSZ), geologic slip rates differ from those inverted from geodetic measurements, which may partly be due to inaccurate fault connectivity within geodetic models. We employ three‐dimensional models that are mechanically compatible with long‐term plate motion to simulate both fault slip rates and interseismic surface deformation. We compare results from fault networks that follow mapped geologic traces and resemble those used in block model inversions, which connect the San Jacinto fault to the SAF near Cajon Pass and connect distinct faults within the ECSZ. The connection of the SAF with the San Jacinto fault decreases strike‐slip rates along the SAF by up to 10% and increases strike‐slip rates along the San Jacinto fault by up to 16%; however, slip rate changes are still within the large geologic ranges along the SAF. The insensitivity of interseismic surface velocities near Cajon Pass to fault connection suggests that inverse models may utilize both an incorrect fault geometry and slip rate and still provide an excellent fit to interseismic geodetic data. Similarly, connection of faults within the ECSZ produces 36% greater cumulative strike‐slip rates but less than 17% increase in interseismic velocity. When using overconnected models to invert GPS for slip rates, the reduced off‐fault deformation within the models can lead to overprediction of slip rates. While the nature of fault intersections at depth remains enigmatic, fault geometries should be chosen with caution in crustal deformation models.

Is there a discrepancy between geological and geodetic slip rates along the San Andreas Fault System?

AbstractPrevious inversions for slip rate along the San Andreas Fault System (SAFS), based on elastic half‐space models, show a discrepancy between the geologic and geodetic slip rates along a few major fault segments. In this study, we use an earthquake cycle model representing an elastic plate over a viscoelastic half‐space to demonstrate that there is no significant discrepancy between long‐term geologic and geodetic slip rates. The California statewide model includes 41 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known historical ruptures and geologic recurrence intervals. The slip rates are constrained by 1981 secular velocity measurements from GPS and L‐band intereferometric synthetic aperture radar. A model with a thick elastic layer (60 km) and half‐space viscosity of 1019Pa s is preferred because it produces the smallest misfit to both the geologic and the geodetic data. We find that the geodetic slip rates from the thick plate model agrees to within the bounds of the geologic slip rates, while the rates from the elastic half‐space model disagree on specific important fault segments such as the Mojave and the North Coast segment of the San Andreas Fault. The viscoelastic earthquake cycle models have generally higher slip rates than the half‐space model because most of the faults along the SAFS are late in the earthquake cycle, so today they are moving slower than the long‐term cycle‐averaged velocity as governed by the viscoelastic relaxation process.

Temporal variations in Holocene slip rate along the central Garlock fault, Pilot Knob Valley, California

Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5–7 mm/yr since at least the Late Pleistocene, yet present-day rates inferred from geodetic velocity fields are indistinguishable from zero. We evaluate the possibility of non-steady slip over millennial timescales using displaced Late Holocene alluvium along the central Garlock fault in Pilot Knob Valley. Truncation of a Late Holocene alluvial fan deposit against a shutter ridge requires a minimum of 30–37 m of displacement since deposition of the fan; maximum allowable displacement is 43–50 m. The extent of soil development atop the fan surface and optically stimulated luminescence ages bracket fan deposition between 3.5 and 4.5 ka. Together, these data require that slip rates during the Late Holocene were ∼7–14 mm/yr, with a preferred rate of ∼11–13 mm/yr. Our results, in conjunction with previous estimates of displacement over the past ∼15 ka, require significant temporal variations in strain release along the Garlock fault and confirm previous suggestions that interactions among fault systems in eastern California give rise to alternating periods of fault activity and quiescence.

Fault network modeling of crustal deformation in California constrained using GPS and geologic observations

Abstract We have developed a kinematic fault network model of crustal deformation in an elastic half-space. Surface deformation is calculated using this model assuming each fault segment slipping beneath a locking depth. Each fault segment connects to its adjacent elements with slip vector continuity imposed at fault nodes or intersections; the degree of the constraints determines whether deformation is block-like or not. We apply this model to invert GPS observations for slip rates on major faults in California with geological rate constraints. Based on the F-test result, we find that lesser block-like models fit the data significantly better than the strictly block-like model. Our final inversion shows a slip rate varying from 20 to 23 mm/yr along the northern San Andreas from the Santa Cruz to the North Coast segment. Slip rates vary from 9 to 13 mm/yr along the Hayward to the Maacama fault segment, and from 15 to 3 mm/yr along the central Calaveras to the West Napa fault segment. For the central California Creeping Zone, the result suggests a depth dependent creep rate with an average of 22 mm/yr over the top 5 km and 32 mm/yr underneath. From the Mojave to San Bernardino Mountain segments, we also find a significant decrease in slip rate along the San Andreas in comparison with the geologic rates, in contrast to a significant increase in slip rate on faults along the eastern California shear zone. Along the southern San Andreas, slip rates vary from 21 to 25 mm/yr from the Coachella Valley to Imperial Valley segments. Slip rates range from 0 to 3 mm/yr across the western Transverse Ranges faults, which is consistent with the regional crustal thickening. Overall slip rates derived from geodetic observations correlate strongly with the geologic slip rates statistically, suggesting high compatibility between geodetic and geologic observations.

Viscous roots of active seismogenic faults revealed by geologic slip rate variations

Viscous flow in the deep crust and uppermost mantle can contribute to the accumulation of strain along seismogenic faults in the shallower crust1. It is difficult to evaluate this contribution to fault loading because it is unclear whether the viscous deformation occurs in localized shear zones or is more broadly distributed2. Furthermore, the rate of strain accumulation by viscous flow has a power law dependence on the stress applied, yet there are few direct estimates of what the power law exponent is, over the long term, for active faults. Here we measure topography and the offset along fault surfaces created during successive episodes of slip on seismically active extensional faults in the Italian Apennines during the Holocene epoch. We show that these data can be used to derive a relationship between the stress driving deformation and the fault strain rate, averaged over about 15 thousand years (kyr). We find that this relationship follows a well-defined power law with an exponent in the range of 3.0–3.3 (1σ). This exponent is consistent with nonlinear viscous deformation in the deep crust and, crucially, strain localization promoted by seismogenic faulting at shallower depths. Although we cannot rule out some distributed deformation, we suggest that fault strain and thus earthquake recurrence in the Apennines is largely controlled by viscous flow in deep, localized shear zones, over many earthquake cycles.

Nailing down the slip rate of the Altyn Tagh fault

AbstractPrevious estimates of the geodetic and geologic slip rates of the 1500 km long Altyn Tagh fault bordering the northern edge of the Tibetan plateau vary by a factor of five. Proposed reasons for these discrepancies include poor GPS geometry, interpretative errors in terrace morphology, and changes in fault slip rate over time. Here we present results from a new dense GPS array orthogonal to the fault at ~86.2°E that indicates a velocity of 9.0−3.2/+4.4 mm/yr, in close agreement with geomorphologic estimates at the same location. Our estimated geodetic slip rate is consistent with recent geological slip rates based on terrace offsets. The resulting mean combined geological and geodetic slip rate (9.0 ± 4.0 mm/yr) is remarkably uniform for the central ~800 km of the Altyn Tagh fault, significantly lower than early kinematic estimates and consistent with deformation elsewhere in Tibet and central Asia.

Slip rates and off‐fault deformation in Southern California inferred from GPS data and models

In Southern California, fault slip rate estimates along the San Andreas fault (SAF) and Garlock fault from geodetically constrained kinematic models are systematically at the low end or lower than geologic slip rate estimates. The sum of geodetic model slip rates across the eastern California shear zone is higher than the geologic sum. However, the ranges of reported model and geologic slip rate estimates in the literature are sufficiently large that it remains unclear whether these apparent discrepancies are real or attributable to epistemic uncertainties in the two types of estimates. We further examine uncertainties in geodetically derived slip rate estimates on major faults in Southern California by conducting a suite of inversions with four kinematic models. Long‐term rigid elastic block models constrained by the geologic slip rates cannot fit the present‐day GPS‐derived velocity field. Deforming (permanent off‐fault strain) elastic block models and viscoelastic earthquake cycle block models constrained by geologic slip rates can fit the present‐day GPS‐derived velocity field with 28–33% of the total geodetic moment rate occurring as distributed deformation off of the major faults. Models incorporating viscoelastic mantle flow predict systematically higher slip rates than purely elastic models on many of the major Southern California faults with ranges of (elastic/viscoelastic) 29–34/30–37 mm/yr for the Carrizo SAF segment, 20–24/20–32 mm/yr for the Mojave SAF segment, 14–17/18–22 mm/yr for the Coachella SAF segment, 13–19/14–22 mm/yr for the San Jacinto fault, and 5–11/5–11 mm/yr for the western Garlock fault.

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

Research Article| October 01, 2013 Inference of Multiple Earthquake‐Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation Brendan J. Meade; Brendan J. Meade Department of Earth & Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138meade@fas.harvard.edu Search for other works by this author on: GSW Google Scholar Yann Klinger; Yann Klinger Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F‐75005 Paris, Franceklinger@ipgp.fr Search for other works by this author on: GSW Google Scholar Eric A. Hetland Eric A. Hetland Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109ehetland@umich.edu Search for other works by this author on: GSW Google Scholar Author and Article Information Brendan J. Meade Department of Earth & Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138meade@fas.harvard.edu Yann Klinger Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, F‐75005 Paris, Franceklinger@ipgp.fr Eric A. Hetland Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109ehetland@umich.edu Publisher: Seismological Society of America First Online: 14 Jul 2017 Online ISSN: 1943-3573 Print ISSN: 0037-1106 Bulletin of the Seismological Society of America (2013) 103 (5): 2824–2835. https://doi.org/10.1785/0120130006 Article history First Online: 14 Jul 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brendan J. Meade, Yann Klinger, Eric A. Hetland; Inference of Multiple Earthquake‐Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation. Bulletin of the Seismological Society of America 2013;; 103 (5): 2824–2835. doi: https://doi.org/10.1785/0120130006 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract Characterizing surface deformation throughout a full earthquake cycle is a challenge due to the lack of high‐resolution geodetic observations of duration comparable to that of characteristic earthquake recurrence intervals (250–10,000 years). Here we approach this problem by comparing long‐term geologic slip rates with geodetically derived fault slip rates by sampling only a short fraction (0.001%–0.1%) of a complete earthquake cycle along 15 continental strike‐slip faults. Geodetic observations provide snapshots of surface deformation from different times through the earthquake cycle. The timing of the last earthquake on many of these faults is poorly known, and may vary greatly from fault to fault. Assuming that the underlying mechanics of the seismic cycle are similar for all faults, geodetic observations from different faults may be interpreted as samples over a significantly larger fraction of the earthquake cycle than could be obtained from the geodetic record along any one fault alone. As an ensemble, we find that geologically and geodetically inferred slip rates agree well with a linear relation of 0.94±0.09. To simultaneously explain both the ensemble agreement between geologic and geodetic slip‐rate estimates with observations of rapid postseismic deformation, we consider the predictions from simple two‐layer earthquake‐cycle models with both Maxwell and Burgers viscoelastic rheologies. We find that a two‐layer Burgers model, with two relaxation timescales, is consistent with observations of deformation throughout the earthquake cycle, whereas the widely used two‐layer Maxwell model with a single relaxation timescale, is not, suggesting that the earthquake cycle is effectively characterized by a largely stress‐recoverable rapid postseismic stage and a much more slowly varying interseismic stage. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Insights into distributed plate rates across the Walker Lane from GPS geodesy

AbstractContemporary geodetic slip rates are observed to be approximately two times greater than late Pleistocene geologic slip rates across the southern Walker Lane. Using a dense GPS network, we compare the present‐day crustal velocities to observed geologic slip rates in the region. We find that the Walker Lane is characterized by a smooth transition from westward extension in the Basin and Range to northwestward motion of the Sierra Nevada block. The GPS velocity field indicates that (1) plate parallel (N37°W) velocities define a velocity differential of 10.6 ± 0.5 mm/yr between the western Basin and Range and the Sierra Nevada block, (2) there is ~2 mm/yr of contemporary extension perpendicular to the normal faults of the Silver Peak‐Lone Mountain extensional complex, and (3) most of the observed discrepancy in long‐ and short‐term slip rates occurs across Owens Valley. We believe the discrepancy is due to distributed strain and underestimated geologic slip rates.

How do “ghost transients” from past earthquakes affect GPS slip rate estimates on southern California faults?

In this study, we investigate the extent to which viscoelastic velocity perturbations (or “ghost transients”) from individual fault segments can affect elastic block model‐based inferences of fault slip rates from GPS velocity fields. We focus on the southern California GPS velocity field, exploring the effects of known, large earthquakes for two end‐member rheological structures. Our approach is to compute, at each GPS site, the velocity perturbation relative to a cycle average for earthquake cycles on particular fault segments. We then correct the SCEC CMM4.0 velocity field for this perturbation and invert the corrected field for fault slip rates. We find that if asthenosphere viscosities are low (3 × 1018 Pa s), the current GPS velocity field is significantly perturbed by viscoelastic earthquake cycle effects associated with the San Andreas Fault segment that last ruptured in 1857 (Mw = 7.9). Correcting the GPS velocity field for this perturbation (or “ghost transient”) adds about 5 mm/a to the SAF slip rate along the Mojave and San Bernardino segments. The GPS velocity perturbations due to large earthquakes on the Garlock Fault (most recently, events in the early 1600s) and the White Wolf Fault (most recently, the Mw = 7.3 1952 Kern County earthquake) are smaller and do not influence block‐model inverted fault slip rates. This suggests that either the large discrepancy between geodetic and geologic slip rates for the Garlock Fault is not due to a ghost transient or that un‐modeled transients from recent Mojave earthquakes may influence the GPS velocity field.

Latest Quaternary paleoseismology and slip rates of the Longriba fault zone, eastern Tibet: Implications for fault behavior and strain partitioning

AbstractAlthough much work has been done on active tectonics of eastern Tibet, little is known about the Longriba fault zone and its role in strain partitioning. Whether its two sub‐parallel strands (Longriqu and Maoergai faults) can rupture simultaneously in a large earthquake remains unknown. We conducted trenching combined with the interpretation of satellite imagery, field investigations, topographic surveys, and radiocarbon and Optically Stimulated Luminescence (OSL) dating to reconstruct paleoseismic history, and we used displaced terrace risers to estimate geological slip rates. Our results demonstrate that the Longriba fault zone is predominantly right‐lateral with a small southeast‐verging thrust component. Four surface‐rupturing events occurred on the Longriqu fault at 5080 ± 90, 11,100 ± 380, 13,000 ± 260, and 17,830 ± 530 cal yr B.P. Together with our previous trenches on the Maoergai fault, we found that the last event probably ruptured both the Longriqu and Maoergai faults. Prior to the last event, the two strands of the Longriba fault zone experienced alternating earthquakes. The fault zone has a high potential for an earthquake larger than Mw 7. The slip rate of the Longriba fault zone decreases from ~7.5 mm/yr in latest Pleistocene to ~2.1 mm/yr in the Holocene, probably related to a slowing down of the eastern motion of the Tibetan Plateau. The comparison with slip rates at the Longmen Shan fault zone suggests that the Longriba fault zone has an equally important role in strain partitioning in eastern Tibet. This study is helpful to seismic hazard assessment and an understanding of deformation mechanism in eastern Tibet.

Pliocene sinistral slip across the Adobe Hills, eastern California–western Nevada: Kinematics of fault slip transfer across the Mina deflection

The Adobe Hills region (California and Nevada, USA) is a faulted volcanic field located within the western Mina deflection, a right-stepping zone of faults that connects the northern Eastern California shear zone (ECSZ) to the south with the Walker Lane belt (WLB) to the north. New detailed geologic mapping, structural studies, and 40 Ar/ 39 Ar geochronology in the Adobe Hills allow us to calculate fault slip rates and test predictions for the kinematics of fault slip transfer into the Mina deflection. The Adobe Hills are dominated by Pliocene tuffaceous sandstone, basaltic lavas that yield 40 Ar/ 39 Ar ages between 3.13 ± 0.02 and 3.43 ± 0.01 Ma, and basaltic cinder cones. These Pliocene units unconformably overlie Middle Miocene latite ignimbrite that yields an 40 Ar/ 39 Ar age of 11.17 ± 0.04 Ma, and Quaternary tuffaceous sands, alluvium, and lacustrine deposits cap the sequence. Northwest-striking normal faults, west-northwest–striking dextral faults, and northeast-striking sinistral faults cut all units; the northeast-striking sinistral faults are the youngest and most well developed fault set. We calculate ∼0.1 mm/yr of approximately east-west horizontal extension and northwest dextral shear since the Pliocene. The prominent northeast-striking sinistral faults offset basalt ridgelines, normal fault–hanging-wall intersections, a channelized basalt flow, a basalt flow edge, and a basalt flow contact a net minimum of 921 ± 184 to 1318 ± 264 m across the Adobe Hills. These measured sinistral offsets yield a minimum Pliocene sinistral fault slip rate of 0.2–0.5 mm/yr; our preferred minimum slip rate is 0.4–0.5 mm/yr. The geometry and orientation of the prominent sinistral faults are consistent with simple shear/couple clockwise block rotation within a broad dextral shear zone. Vertical axis block rotation data are needed to test this interpretation. We propose that a set of faults subparallel to Sierra Nevada–North America motion and associated releasing steps, located west of the White Mountains fault zone and east of the Long Valley Caldera, transfer a portion of dextral Owens Valley fault slip northwestward onto the sinistral faults in the Adobe Hills. Dextral slip distributed across faults between the White Mountains fault zone and the Sierra Nevada and east of the Fish Lake Valley fault zone may account for the apparent discrepancy between summed long-term geologic slip rates and present-day geodetic rates across the northern ECSZ. Fault slip in the Adobe Hills is part of a regional pattern of initiation and renewal of dextral, sinistral, and normal fault slip during the Pliocene that extends from lat ∼40°N to ∼36°N within the ECSZ-WLB and along the western margin of the Basin and Range Province. This regional deformation episode may be related to changes in gravitational potential energy.

Sensitivity of the Southern San Andreas Fault System to Tectonic Boundary Conditions and Fault Configurations

The southern Big Bend of the San Andreas fault (SAF) accommodates transpression along numerous non‐vertical, non‐planar, and intersecting active surfaces. Using three‐dimensional boundary element method (BEM) models, we test the sensitivity of fault slip rates to a range of tectonic boundary conditions constrained by Global Positioning System (GPS) studies of the region (45–50 mm/yr and 320°–325°). Using this approach, our models provide a range of distributed fault slip rates associated with both spatial variations in geometry and present‐day uncertainties in plate motion. Model slip rates match most of the available geologic slip rates, and discrepancies may owe to inaccurate fault geometries. More northerly plate velocity (325°) produces greater transpression along the SAF system associated with greater uplift of the San Bernardino Mountains, greater reverse‐slip rates along range bounding reverse thrust faults, lower strike‐slip rates along the San Andreas and San Jacinto faults, and greater strike‐slip rates along the Eastern California Shear Zone (ECSZ) and Garlock fault. These results suggest that the degree of regional transpression controls the partitioning of deformation between uplift, and slip along both the SAF system and the ECSZ. The insensitivity of modeled slip rates to applied velocity along the southern San Andreas and San Jacinto faults suggests that fault geometry greatly influences slip rates within the San Bernardino Mountains region. A northerly shift in plate velocity orientation could account for the abandonment of the Mill Creek strand of the SAF>95 ka and development of the present‐day active geometry, which accommodates greater uplift.

Distributed extensional deformation in a zone of right‐lateral shear: Implications for geodetic versus geologic rates of deformation in the eastern California shear zone‐Walker Lane

The eastern California shear zone (ECSZ)‐Walker Lane belt represents an important, evolving component of the Pacific‐North America plate boundary. Geodetic data suggest the northern ECSZ is accumulating dextral shear at a rate of ∼9.3 mm/a, more than double the total measured late Pleistocene rate at ∼37.5°N. At this latitude, the Silver Peak‐Lone Mountain (SPLM) extensional complex plays an important role in accommodating and transferring slip among the strike‐slip and normal faults of the ECSZ and Walker Lane. To better understand the recent geodynamic evolution of this region, we determined late Pleistocene extension rates for the Clayton Valley fault zone, one of a series of down‐to‐the‐northwest normal faults comprising the SPLM, using geologic mapping, differential GPS fault scarp surveys, and cosmogenic nuclide geochronology. Extension rates along the Clayton Valley fault zone are time‐invariant at 0.1 ± 0.1 to 0.3 ± 0.1 mm/a (depending on fault dip) since ∼137 ka. When combined with other published fault slip rates at this latitude, the cumulative late Pleistocene geologic slip rate is ∼3.3 to 5.2 mm/a. This rate is lower than both the geodetic rate of dextral shear and other long‐term slip rate budgets in the northern ECSZ. Our results suggest that deformation in Clayton Valley is spread across a diffuse set of normal faults and that not all of the deformation is recorded in the surficial geology. We suggest that the low cumulative geologic slip rate in the northern ECSZ may be a result of this distributed extension, which can cause long‐term rates of deformation to be significantly underestimated.

Quaternary seismo‐tectonic activity of the Polochic Fault, Guatemala

The Polochic‐Motagua fault system is part of the sinistral transform boundary between the North American and Caribbean plates in Guatemala and the associated seismic activity poses a threat to ∼70% of the country's population. The aim of this study is to constrain the Late Quaternary activity of the Polochic fault by determining the active structure geometry and quantifying recent displacement rates as well as paleo‐seismic events. Slip rates have been estimated from offsets of Quaternary volcanic markers and alluvial fan using in situ cosmogenic 36Cl exposure dating. Holocene left‐lateral slip rate and Mid‐Pleistocene vertical slip rate have been estimated to 4.8 ± 2.3 mm/y and 0.3 ± 0.06 mm/y, respectively, on the central part of the Polochic fault. The horizontal slip rate is within the range of longer‐term geological slip rates and short‐term GPS‐based estimates. In addition, the non‐negligible vertical motion participates in the uplift of the block north of the fault and seems to be a manifestation of the regional, far‐field stress regime. We excavated the first trench for paleo‐seismological study on the Polochic fault in which we distinguish four large paleo‐seismic events since 17 ky during which the Polochic fault ruptured the ground surface.