ALBERT

Description: Active transpressional fault systems are typically associated with the development of broad zones of deformation and topographic development; however, the complex geometries typically associated with these systems often make it difficult to isolate the important boundary conditions that control transpressional orogenic growth. The Denali fault system is widely recognized as transpressional due to the presence of the Denali fault, a major, active, right-lateral fault, and subparallel zones of thrust faults and fault-related folding along both the north and south flanks of the Alaska Range. Measured Quaternary and Holocene slip rates exist for the Denali fault system and portions of the adjacent thrust system, but the partitioning of fault slip between contractional and translational components of this transpressional system has not been previously studied in detail. Exploiting the relatively simple geometry of the Denali fault, we analyze the style and distribution of active faulting within the Alaska Range to define patterns of strain accommodation and determine how contractional and translational strain is partitioned across the Denali fault system. As the trace of the Denali fault curves by ~70° across central Alaska, the mean strike of the thrust system to the north remains subparallel to the Denali fault, while to the south, the few faults with known or suspected Quaternary offset are oblique to the Denali fault. This relationship suggests that as the Denali fault system accommodates local fault-parallel strike slip, it partitions the residual part of the regional NW-directed plate motion into NW-SE shortening south of the Denali fault and shortening perpendicular to the Denali fault to the north. The degree of slip partitioning is consistent with a balanced slip budget for the two primary faults that contribute displacement to the Denali fault system (the eastern Denali fault and Totschunda fault). The current obliquity of displacement south of the Denali fault is the result of the late Cenozoic development of the Totschunda fault, which provides a more direct connection for the transfer of strain from the Fairweather transform fault to the Denali fault system. The transmitted strain is partitioned into right-lateral slip on the Denali fault and into Denali fault-normal shortening that is accommodated by thrust faulting in the Alaska Range and distributed left-lateral slip faulting within interior Alaska to the north.

Description: Central Oregon (northwestern USA), where northern Basin and Range extension diminishes in magnitude across the High Lava Plains, exhibits widespread extensional faulting and Quaternary volcanism, yet the relations between the processes are complex and chronology is poorly constrained. Here we use cosmogenic 3 He exposure dating of basalt lava flows to quantify the timing of normal faulting and emplacement of a lava field on the margins of pluvial Fort Rock Lake. The northwest-trending Christmas Valley fault system displaces High Lava Plains volcanic rocks, forming an ~3-km-wide graben that transects the eastern Fort Rock Basin. A portion of the western edge of the graben is marked by a normal fault displaying flexural shear folding with a prominent vertical hinge crack, called Crack in the Ground. Lava flows of the Four Craters Lava Field flowed into this crack. Exposure dating of the Four Craters Lava Field, on the eastern flank of the older Green Mountain shield volcano, indicates an emplacement age of 14 ± 1 ka. We dated Green Mountain basalt exposed in the walls of the crack (the fault wall), which also yielded exposure ages of 14 ± 1 ka. The similar ages suggest that substantial crack opening occurred at about the same time the Four Craters lava was emplaced. These data indicate a period of synchronous normal faulting and dike-fed cinder cone activity emanating from a fault-parallel fissure ~2 km northeast of the crack ca. 14 ka, with minimal displacement since.

Description: Paleoseismic investigations at the Hazel Dell site on the Santa Cruz mountains section (SAS) of the San Andreas fault provide the first definitive geologic evidence of two pre-1906 nineteenth-century earthquakes based on the presence of anthropogenic artifacts at the antepenultimate earthquake (E3) horizon. We review historic accounts of candidate events and interpret the penultimate earthquake and E3 to be the April 1890 and June 1838 earthquakes, respectively. These new data suggest more frequent surface-rupturing earthquakes within historical time than previously recognized and highlight variability of interseismic intervals on the SAS of the San Andreas fault. We correlate earthquakes between Hazel Dell and nearby paleoseismic sites based on revised timing, similarity of stratigraphy, style, and size of displacement, and build a composite paleoseismic record. The composite record requires at least two modes of behavior in strain release on the SAS through time. One mode is through great multisegment earthquakes, like that in 1906. Historic records and geologic studies suggest that prior to 1906 the Santa Cruz mountains region was characterized by a second mode of moderate seismicity, with three M≥6 earthquakes between 1838 and 1890, including two that caused surface rupture at Hazel Dell. In the 700 years prior to 1800, individual sites have evidence ranging from 1 to 5 events, suggesting that the longer record remains unresolved. Online Material: Additional Hazel Dell trench logs, figure of age ranges for wood chips, Arano Flat OxCal model, and table of radiocarbon samples.

Description: 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-independent 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 methodology 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 importance of logic-tree branches, vary throughout the region and depend on the evaluation 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.

Description: Probabilistic forecasting of earthquake-producing fault ruptures informs all major decisions aimed at reducing seismic risk and improving earthquake resilience. Earthquake forecasting models rely on two scales of hazard evolution: long-term (decades to centuries) probabilities of fault rupture, constrained by stress renewal statistics, and short-term (hours to years) probabilities of distributed seismicity, constrained by earthquake-clustering statistics. Comprehensive datasets on both hazard scales have been integrated into the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3). UCERF3 is the first model to provide self-consistent rupture probabilities over forecasting intervals from less than an hour to more than a century, and it is the first capable of evaluating the short-term hazards that result from multievent sequences of complex faulting. This article gives an overview of UCERF3, illustrates the short-term probabilities with aftershock scenarios, and draws some valuable scientific conclusions from the modeling results. In particular, seismic, geologic, and geodetic data, when combined in the UCERF3 framework, reject two types of fault-based models: long-term forecasts constrained to have local Gutenberg–Richter scaling, and short-term forecasts that lack stress relaxation by elastic rebound.

Description: The 2014 Working Group on California Earthquake Probabilities (WGCEP14) present the time-independent component of the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), which provides authoritative estimates of the magnitude, location, and time-averaged frequency of potentially damaging earthquakes in California. The primary achievements have been to relax fault segmentation and include multifault ruptures, both limitations of UCERF2. The rates of all earthquakes are solved for simultaneously and from a broader range of data, using a system-level inversion that is both conceptually simple and extensible. The inverse problem is large and underdetermined, so a range of models is sampled using an efficient simulated annealing algorithm. The approach is more derivative than prescriptive (e.g., magnitude–frequency distributions are no longer assumed), so new analysis tools were developed for exploring solutions. Epistemic uncertainties were also accounted for using 1440 alternative logic-tree branches, necessitating access to supercomputers. The most influential uncertainties include alternative deformation models (fault slip rates), a new smoothed seismicity algorithm, alternative values for the total rate of M w ≥5 events, and different scaling relationships, virtually all of which are new. As a notable first, three deformation models are based on kinematically consistent inversions of geodetic and geologic data, also providing slip-rate constraints on faults previously excluded due to lack of geologic data. The grand inversion constitutes a system-level framework for testing hypotheses and balancing the influence of different experts. For example, we demonstrate serious challenges with the Gutenberg–Richter hypothesis for individual faults. UCERF3 is still an approximation of the system, however, and the range of models is limited (e.g., constrained to stay close to UCERF2). Nevertheless, UCERF3 removes the apparent UCERF2 overprediction of M  6.5–7 earthquake rates and also includes types of multifault ruptures seen in nature. Although UCERF3 fits the data better than UCERF2 overall, there may be areas that warrant further site-specific investigation. Supporting products may be of general interest, and we list key assumptions and avenues for future model improvements.

Description: An alluvial succession on the northeast side of the San Bernardino strand of the San Andreas fault includes distinctive aggradational and degradational features that can be matched with correlative features on the southwest side of the fault. Key among these are (1) a terrace riser on the northeast side of the fault that correlates with an offset channel wall on the southwest side of the fault and forms a basis for slip estimates for the period ca. 35 ka to the present, and (2) a small alluvial fan on the southwest side of the fault that has been matched with its most likely source gullies on the northeast side of the fault and forms a basis for slip estimates for the last 10.5 k.y. Slip-rate estimates for these two separate intervals are nearly identical. The rate for the older feature is most likely between 8.3 and 14.5 mm/yr, with a 95% confidence interval of 7.0–15.7 mm/yr. The rate for the younger feature is most likely between 6.8 and 16.3 mm/yr, with a 95% confidence interval of 6.3–18.5 mm/yr. These rates are only half the previously published slip rate for the San Andreas fault 35 km to the northwest in Cajon Pass, a rate that traditionally is extrapolated southeastward along the San Bernardino section of the fault. Results from Plunge Creek suggest that about half of the 25 mm/yr rate at Cajon Pass transfers southeastward to the San Jacinto fault, as proposed by other workers on the basis of regional geologic relations. These results indicate that the discrepancy between latest Quaternary slip rates and present-day rates of strain accumulation across the San Bernardino section of the San Andreas fault from geodesy can be largely explained by slip transfer between faults, leading to spatial variation in rate along the San Andreas fault. Nonetheless, the latest Pleistocene and Holocene slip rate at Plunge Creek is still somewhat faster than rates inferred for the San Bernardino section of the San Andreas fault based on elastic block modeling of geodetic data and may be more appropriate than those rates for hazard estimation.

Description: We present evidence of 11-14 earthquakes that occurred between 3000 and 1500 B.C. on the San Andreas fault at the Wrightwood paleoseismic site. Earthquake evidence is presented in a novel form in which we rank (high, moderate, poor, or low) the quality of all evidence of ground deformation, which are called "event indicators." Event indicator quality reflects our confidence that the morphologic and sedimentologic evidence can be attributable to a ground-deforming earthquake and that the earthquake horizon is accurately identified by the morphology of the feature. In four vertical meters of section exposed in ten trenches, we document 316 event indicators attributable to 32 separate stratigraphic horizons. Each stratigraphic horizon is evaluated based on the sum of rank (Rs), maximum rank (Rm), average rank (Ra), number of observations (Obs), and sum of higher-quality event indicators (Rs (sub 〉1) ). Of the 32 stratigraphic horizons, 14 contain 83% of the event indicators and are qualified based on the number and quality of event indicators; the remaining 18 do not have satisfactory evidence for further consideration. Eleven of the 14 stratigraphic horizons have sufficient number and quality of event indicators to be qualified as "probable" to "very likely" earthquakes; the remaining three stratigraphic horizons are associated with somewhat ambiguous features and are qualified as "possible" earthquakes. Although no single measurement defines an obvious threshold for designation as an earthquake horizon, Rs, Rm, and Rs (sub 〉1) correlate best with the interpreted earthquake quality. Earthquake age distributions are determined from radiocarbon ages of peat samples using a Bayesian approach to layer dating. The average recurrence interval for the 10 consecutive and highest-quality earthquakes is 111 (93-131) years and individual intervals are + or -50% of the average. With comparison with the previously published 14-15 earthquake record between A.D. 500 and present, we find no evidence to suggest significant variations in the average recurrence rate at Wrightwood during the past 5000 years.

Description: The 2007 Working Group on California Earthquake Probabilities (WGCEP, 2007) presents the Uniform California Earthquake Rupture Forecast, Version 2 (UCERF 2). This model comprises a time-independent (Poisson-process) earthquake rate model, developed jointly with the National Seismic Hazard Mapping Program and a time-dependent earthquake-probability model, based on recent earthquake rates and stress-renewal statistics conditioned on the date of last event. The models were developed from updated statewide earthquake catalogs and fault deformation databases using a uniform methodology across all regions and implemented in the modular, extensible Open Seismic Hazard Analysis framework. The rate model satisfies integrating measures of deformation across the plate-boundary zone and is consistent with historical seismicity data. An overprediction of earthquake rates found at intermediate magnitudes (6.5〈 or =M〈 or =7.0) in previous models has been reduced to within the 95% confidence bounds of the historical earthquake catalog. A logic tree with 480 branches represents the epistemic uncertainties of the full time-dependent model. The mean UCERF 2 time-dependent probability of one or more M〉 or =6.7 earthquakes in the California region during the next 30 yr is 99.7%; this probability decreases to 46% for M〉 or =7.5 and to 4.5% for M〉 or =8.0. These probabilities do not include the Cascadia subduction zone, largely north of California, for which the estimated 30 yr, M〉 or =8.0 time-dependent probability is 10%. The M〉 or =6.7 probabilities on major strike-slip faults are consistent with the WGCEP (2003) study in the San Francisco Bay Area and the WGCEP (1995) study in southern California, except for significantly lower estimates along the San Jacinto and Elsinore faults, owing to provisions for larger multisegment ruptures. Important model limitations are discussed.