Ray J. Weldon

Late Quaternary slip rates across the central Tien Shan, Kyrgyzstan, central Asia

[1] Slip rates across active faults and folds show that late Quaternary faulting is distributed across the central Tien Shan, not concentrated at its margins. Nearly every intermontane basin contains Neogene and Quaternary syntectonic strata deformed by Holocene north-south shortening on thrust or reverse faults. In a region that spans two thirds of the north-south width of the central Tien Shan, slip rates on eight faults in five basins range from � 0.1 to � 3 mm/yr. Fault slip rates are derived from faulted and folded river terraces and from trenches. Radiocarbon, optically stimulated luminescence, and thermoluminescence ages limit ages of terraces and aid in their regional correlation. Monte Carlo simulations that sample from normally distributed and discrete probability distributions for each variable in the slip rate calculations generate most likely slip rate values and 95% confidence limits. Faults in basins appear to merge at relatively shallow depths with crustal-scale ramps that underlie mountain ranges composed of pre-Cenozoic rocks. The sum and overall pattern of late Quaternary rates of shortening are similar to current rates of north-south shortening measured using Global Positioning System geodesy. This similarity suggests that deformation is concentrated along major fault zones near range-basin margins. Such faults, separated by rigid blocks, accommodate most of the shortening in the upper crust. INDEX TERMS: 8107 Tectonophysics: Continental neotectonics; 9320 Information Related to Geographic Region: Asia; 8010 Structural Geology: Fractures and faults;

Tectonic role of active faulting in central Oregon

Geologic and geodetic studies in California indicate that about 1 cm yr−1 of right-lateral shear occurs across what has been referred to as the Eastern California Shear Zone. Northwest trending zones of dextral, sinistral, and normal faults splay eastward from the San Andreas system, continuing through the Mojave Desert, east of the Sierra Nevada, and northward along the Central Nevada and Walker Lane fault zones. Aerial photography, field investigations, and fault studies in southern and central Oregon, compiled with a comprehensive analysis of previous studies nearby, indicate that latest Pleistocene and Holocene fault activity is concentrated along four zones that stretch northward into the Cascade volcanic arc and across the northwestern edge of the Basin and Range Province. The Oregon zones appear to continue the activity in eastern California and northwestern Nevada northward and provide a connection to seismically active zones in southern and central Washington. Several techniques are applied to fault data from the Oregon zones in an attempt to estimate the overall direction and rate of motion across them. The orientations and styles of faults younger than middle Tertiary are used with models of oblique rifting to estimate that the motion of western Oregon is ∼N60° ± 20°W, relative to North America. Summation of geologic moment tensors from faults with latest Pleistocene and Holocene slip yields a direction ∼N90° ± 30°W at a rate of ∼0.5 mm yr−1. This result is a minimum since many fault scarps have not been preserved or recognized, and additional deformation is recorded as folding and tilting. Crustal strain associated with slip during 76 of the largest crustal earthquakes in the past 120 years located along this broad zone from northern California and Nevada, across Oregon, to Washington and Vancouver Island, indicates motions at rates of 3 ± 1 mm yr−1 in a direction N55° ± 10°W. Although the motion across central Oregon is much slower, its similarity in style with regions to the north and south suggests that the regional averages are meaningful. Oregon fault zones, taken together, may accommodate as much as 6 mm yr−1 oriented ∼N60° to 70°W. A tectonic model of fault activity reveals that this proposed shear zone through Nevada, Oregon, and Washington can account for 10% to 20% of the total Pacific-North American transform motion and much of the lateral component of relative motion between the Juan de Fuca and North American plates.

Late Cenozoic tectonics of the northwestern San Bernardino Mountains, southern California

The late Cenozoic structural and stratigraphic history of the northwestern San Bernardino Mountains supports two distinct episodes of uplift, in late Miocene to earliest Pliocene and Quaternary time, that we hypothesize are related to movements on low-angle structures beneath the range. In this paper, we document the nature, distribution, and timing of late Cenozoic deformation and deposition in the northwestern San Bernardino Mountains, and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams. In the first episode of deformation, late Miocene to earliest Pliocene motion on the south-southwest-directed Squaw Peak thrust system disrupted drainage in pre-existing Miocene nonmarine basins and uplifted the western third of the present range to form the ancestral San Bernardino Mountains. Crystalline rocks of the San Bernardino Mountains were thrust southward across the present site of the San Andreas fault between 9.5 and 4.1 Ma, at a time when the San Gabriel fault was the active strand of the San Andreas transform system. We speculate that the Liebre Mountain crystalline block at the northern margin of the Ridge Basin may be the missing upper plate of the Squaw Peak thrust, now offset along the San Andreas fault. The second episode of deformation began with uplift of the northern plateau of the modern San Bernardino Mountains on north-directed, range-front thrusts in early Pleistocene time, between 2.0 and 1.5 Ma. Synchronous uplift of the northern plateau, recorded in early Pleistocene fanglomerates on the northwestern margin of the range, is interpreted to be the result of movement of a relatively coherent crustal block northward up a south-dipping detachment ramp beneath the central range. In middle Pleistocene time, activity on the northern range front began to wane, and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the San Andreas fault, which subsequently migrated rapidly northwestward along the San Andreas fault from the western San Bernardino Mountains into the northeastern San Gabriel Mountains. We attribute this pattern of deformation to the passage of a bulge or strike-slip ramp attached to the southwest side of the San Andreas fault at depth.

Stress near geometrically complex strike‐slip faults: Application to the San Andreas Fault at Cajon Pass, southern California

Slip on an undulatory strike-slip fault induces predictable residual stresses in the adjacent crust. Elastic analytic and finite element models are developed to quantify these stresses for arbitrary fault geometries. Across fault-parallel planes, domains of reverse, normal, right-lateral and left-lateral residual stresses are induced near the bends in the fault. These residual stresses increase in magnitude from one slip event to the next, typically reaching failure level after several events. A two-dimensional analytic elastic plate model for slip on a sinusoidal fault provides general results on the scales and patterns of slip-induced stresses, and Fourier synthesis allows for the solution with arbitrary but small fault distortions. Maximum fault-normal residual stress is proportional to the square of the wavenumber of the sinusoidal trace and decays exponentially away from the fault, reduced to 2/e times the maximum value at a distance equal to 1/wavenumber. This fault-parallel residual shear stress oscillates between dextral and sinistral along the fault, with maximum magnitude adjacent to the maximum excursions in the fault trace. Fault-parallel residual shear stress is maximum at a distance from the fault of 1/wavenumber, where its magnitude is 1/e times the maximum normal stress on the fault. Two- and three-dimensional finite element analyses extend the analytic model; they account for depth-decaying fault perturbation amplitude and slip deficit and are valid for large fault undulations. Application of the model to the San Andreas fault in the Cajon Pass region produces the complex distribution of fault-parallel normal, reverse, right-lateral and left-lateral structures recognized near the main trace. Left-lateral stresses on planes subparallel to the San Andreas fault at the Cajon Pass well are predicted by this model.

Paleoseismic Event Dating and the Conditional Probability of Large Earthquakes on the Southern San Andreas Fault, California

We introduce a quantitative approach to paleoearthquake dating and apply it to paleoseismic data from the Wrightwood and Pallett Creek sites on the southern San Andreas fault. We illustrate how stratigraphic ordering, sedimentolog- ical, and historical data can be used quantitatively in the process of estimating earth- quake ages. Calibrated radiocarbon age distributions are used directly from layer dating through recurrence intervals and recurrence probability estimation. The method does not eliminate subjective judgements in event dating, but it does provide a means of systematically and objectively approaching the dating process. Date dis- tributions for the most recent 14 events at Wrightwood are based on sample and contextual evidence in Fumal et al. (2002) and site context and slip history in Weldon et al. (2002). Pallett Creek event and dating descriptions are from published sources. For the five most recent events at Wrightwood, our results are consistent with pre- viously published estimates, with generally comparable or narrower uncertainties. For Pallett Creek, our earthquake date estimates generally overlap with previous results but typically have broader uncertainties. Some event date estimates are very sensitive to details of data interpretation. The historical earthquake in 1857 ruptured the ground at both sites but is not constrained by radiocarbon data. Radiocarbon ages, peat accumulation rates, and historical constraints at Pallett Creek for event X yield a date estimate in the earliest 1800s and preclude a date in the late 1600s. This event is almost certainly the historical 1812 earthquake, as previously concluded by Sieh et al. (1989). This earthquake also produced ground deformation at Wrightwood. All events at Pallett Creek, except for event T, about A.D. 1360, and possibly event I, about A.D. 960, have corresponding events at Wrightwood with some overlap in age ranges. Event T falls during a period of low sedimentation at Wrightwood when conditions were not favorable for recording earthquake evidence. Previously pro- posed correlations of Pallett Creek X with Wrightwood W3 in the 1690s and Pallett Creek event V with W5 around 1480 (Fumal et al., 1993) appear unlikely after our dating reevaluation. Apparent internal inconsistencies among event, layer, and dating relationships around events R and V identify them as candidates for further inves- tigation at the site. Conditional probabilities of earthquake recurrence were estimated using Poisson, lognormal, and empirical models. The presence of 12 or 13 events at Wrightwood during the same interval that 10 events are reported at Pallett Creek is reflected in mean recurrence intervals of 105 and 135 years, respectively. Average Poisson model 30-year conditional probabilities are about 20% at Pallett Creek and 25% at Wrightwood. The lognormal model conditional probabilities are somewhat higher, about 25% for Pallett Creek and 34% for Wrightwood. Lognormal variance rln estimates of 0.76 and 0.70, respectively, imply only weak time predictability. Conditional probabilities of 29% and 46%, respectively, were estimated for an em- pirical distribution derived from the data alone. Conditional probability uncertainties are dominated by the brevity of the event series; dating uncertainty contributes only secondarily. Wrightwood and Pallett Creek event chronologies both suggest varia- tions in recurrence interval with time, hinting that some form of recurrence rate modulation may be at work, but formal testing shows that neither series is more ordered than might be produced by a Poisson process.

Present-day vertical deformation of the Cascadia Margin, Pacific Northwest, United States

We estimate present-day uplift rates along the Cascadia Subduction Zone in California, Oregon, and Washington in the Pacific Northwest, United States, by utilizing repeated leveling surveys and tide gauge records. These two independent data sets give similar profiles for latitudinal variation of contemporary uplift rates along the coast. Uplift rates are extended inland through east-west leveling lines that connect the north-south line along the coast to the north-south line along the inland valleys just west of the Cascades. The results are summarized as a contour map of present-day uplift rates for the western Pacific Northwest. We find that rates of present-day uplift vary latitudinally along the coast and inland valleys, as well as longitudinally along transects connecting the coast to the inland valleys. Long-term tidal records of Neah Bay, Astoria, and Crescent City indicate uplift of land relative to sea level of 1.6±0.2, 0.0±0.2, and 0.9±0.2 mm/yr, respectively (±1 standard error). Unlike previous estimates of relative sea level change at Astoria, we adjust for discharge effects of the Columbia River, including human management influences. After approximating an absolute framework by using 1.8±0.1 mm/yr to compensate for global sea level rise, results indicate that much of the western Pacific Northwest is rising at rates between 0 and 5 mm/yr. The most rapid uplift rates are near the coast, particularly near the Olympic Peninsula, the mouth of the Columbia River, Cape Blanco, and Cape Mendocino. Two axes of uplift are identified: one trends northeast from the southwest Oregon coast, and the other trends south-southeasterly from the Olympic Peninsula to the Columbia River. The Puget Sound vicinity and a small east-west region from the north central Oregon coast to the inland Willamette Valley are subsiding at rates up to 1 mm/yr. We interpret the overall pattern of rapid present-day uplift to be generated by interseismic strain accumulation in the subduction zone. This interseismic elastic strain accumulation implies significant seismic hazard.

Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California

This study focuses on uncertainties in estimates of the geologic slip rate along the Mission Creek strand of the southern San Andreas fault where it offsets an alluvial fan (T2) at Biskra Palms Oasis in southern California. We provide new estimates of the amount of fault offset of the T2 fan based on trench excavations and new cosmogenic 10Be age determinations from the tops of 12 boulders on the fan surface. We present three alternative fan offset models: a minimum, a maximum, and a preferred offset of 660 m, 980 m, and 770 m, respectively. We assign an age of between 45 and 54 ka to the T2 fan from the 10Be data, which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface, and with ages from U-series geochronology on pedogenic carbonate from T2, described in a companion paper by Fletcher et al. (this volume). These new constraints suggest a range of slip rates between ∼12 and 22 mm/yr with a preferred estimate of ∼14–17 mm/yr for the Mission Creek strand of the southern San Andreas fault. Previous studies suggested that the geologic and geodetic slip-rate estimates at Biskra Palms differed. We find, however, that considerable uncertainty affects both the geologic and geodetic slip-rate estimates, such that if a real discrepancy between these rates exists for the southern San Andreas fault at Biskra Palms, it cannot be demonstrated with available data.

A 100-year average recurrence interval for the San Andreas fault at Wrightwood, California

Evidence for five large earthquakes during the past five centuries along the San Andreas fault zone 70 kilometers northeast of Los Angeles, California, indicates that the average recurrence interval and the temporal variability are significantly smaller than previously thought. Rapid sedimentation during the past 5000 years in a 150-meter-wide structural depression has produced a greater than 21-meter-thick sequence of debris flow and stream deposits interbedded with more than 50 datable peat layers. Fault scarps, colluvial wedges, fissure infills, upward termination of ruptures, and tilted and folded deposits above listric faults provide evidence for large earthquakes that occurred in A.D. 1857, 1812, and about 1700, 1610, and 1470.

Long-Term Time-Dependent Probabilities for the Third Uniform California Earthquake Rupture Forecast (UCERF3)

The 2014 Working Group on California Earthquake Probabilities (WGCEP 2014) presents time-dependent earthquake probabilities for the third Uniform California Earthquake Rupture Forecast (UCERF3). Building on the UCERF3 time-in- dependent model published previously, renewal models are utilized to represent elastic- rebound-implied probabilities. A new methodology has been developed that solves applicability issues in the previous approach for unsegmented models. The new meth- odology also supports magnitude-dependent aperiodicity and accounts for the historic open interval on faults that lack a date-of-last-event constraint. Epistemic uncertainties are represented with a logic tree, producing 5760 different forecasts. Results for a variety of evaluation metrics are presented, including logic-tree sensitivity analyses and comparisons to the previous model (UCERF2). For 30 yr M ! 6:7 probabilities, the most significant changes from UCERF2 are a threefold increase on the Calaveras fault and a threefold decrease on the San Jacinto fault. Such changes are due mostly to differences in the time-independent models (e.g., fault-slip rates), with relaxation of segmentation and inclusion of multifault ruptures being particularly influential. In fact, some UCERF2 faults were simply too long to produce M 6.7 size events given the segmentation assumptions in that study. Probability model differences are also influential, with the implied gains (relative to a Poisson model) being generally higher in UCERF3. Accounting for the historic open interval is one reason. Another is an effective 27% increase in the total elastic-rebound-model weight. The exact factors influencing differences between UCERF2 and UCERF3, as well as the relative im- portance of logic-tree branches, vary throughout the region and depend on the evalu- ation metric of interest. For example, M ! 6:7 probabilities may not be a good proxy for other hazard or loss measures. This sensitivity, coupled with the approximate nature of the model and known limitations, means the applicability of UCERF3 should be evaluated on a case-by-case basis.

Deformation across the western United States: A local estimate of Pacific-North America transform deformation

We obtain a locally based estimate of Pacific-North America relative motion and an uncertainty in this estimate by integrating deformation rate along three different paths leading west across southwestern North America from east of the Rio Grande Rift to near the continental escarpment. Data are primarily Quaternary geologic slip rate estimates, and resulting deformation determinations therefore are “instantaneous” in a geologic sense but “long term” with respect to earthquake cycles. We deduce a rate of motion of the Pacific plate relative to North America that is 48±2 mm/yr, a rate indistinguishable from that predicted by the global kinematic models RM2 and NUVEL-1; however, we obtain an orientation that is 5–9° counterclockwise of these models. A more westerly motion of the Pacific plate, with respect to North America, is calculated from all three paths. The relatively westerly motion of the Pacific plate is accommodated by deformation in the North American continent that includes slip on relatively counterclockwise-oriented strike-slip faults (including the San Andreas fault), which is especially relevant in and south of the Transverse Ranges, and a margin-normal component of net extension across the continent, which is especially relevant north of the Transverse Ranges. Deformation of the SW United States occurs in regionally coherent domains within which the style of deformation is approximately uniform. In the vicinity of the Transverse Ranges, two important shear systems splay from the San Andreas fault: the eastern California shear zone trending NNW from the eastern Transverse Ranges and the trans-Peninsular faults trending SSE from the western and central Transverse Ranges. Within the Transverse Ranges the right-lateral San Andreas fault steps left, seemingly requiring large amounts of convergence there. However, most of this convergence is avoided through a “funneling flow” of the crust toward the western Transverse Ranges and into the relatively narrow central California Coast Ranges and the northern motion of the Mojave. The former process involves an alternation of rotation direction from counterclockwise (in and south of the central Transverse Ranges) to clockwise (in the western Transverse Ranges).