Rupture Forecast Research Articles

Relaxing Segmentation on the Wasatch Fault Zone: Impact on Seismic Hazard

ABSTRACTThe multisegment Wasatch fault zone is a well-studied normal fault in the western United States that has paleoseismic evidence of recurrent Holocene surface-faulting earthquakes. Along the 270 km long central part of the fault, four primary structural complexities provide possible along-strike limits to these ruptures and form the basis for models of fault segmentation. Here, we assess the impact that the Wasatch fault segmentation model has on seismic hazard by evaluating the time-independent long-term rate of ruptures on the fault that satisfy fault-slip rates and paleoseismic event rates, adapting standard inverse theory used in the Uniform California Earthquake Rupture Forecast, Version 3, and implementing a segmentation constraint in which ruptures across primary structural complexities are penalized. We define three models with varying degrees of rupture penalization: (1) segmented (ruptures confined to individual segments), (2) penalized (multisegment ruptures allowed, but penalized), and (3) unsegmented (all ruptures allowed). Seismic-hazard results show that, on average, hazard is highest for the segmented model, in which seismic moment is accommodated by frequent moderate (moment magnitude Mw 6.2–6.8) earthquakes. The unsegmented model yields the lowest average seismic hazard because part of the seismic moment is accommodated by large (Mw 6.9–7.9) but infrequent ruptures. We compare these results to model differences derived from other inputs such as slip rate and magnitude scaling relations and conclude that segmentation exerts a primary control on seismic hazard. This study demonstrates the need for additional geologic constraints on rupture extent and methods by which these observations can be included in hazard-modeling efforts.

Characteristic Earthquake Magnitude Frequency Distributions on Faults Calculated From Consensus Data in California

AbstractAn estimate of the expected earthquake rate at all possible magnitudes is needed for seismic hazard forecasts. Regional earthquake magnitude frequency distributions obey a negative exponential law (Gutenberg‐Richter), but it is unclear if individual faults do. We add three new methods to calculate long‐term California earthquake rupture rates to the existing Uniform California Earthquake Rupture Forecast version 3 efforts to assess method and parameter dependence on magnitude frequency results for individual faults. All solutions show strongly characteristic magnitude‐frequency distributions on the San Andreas and other faults, with higher rates of large earthquakes than would be expected from a Gutenberg‐Richter distribution. This is a necessary outcome that results from fitting high fault slip rates under the overall statewide earthquake rate budget. We find that input data choices can affect the nucleation magnitude‐frequency distribution shape for the San Andreas Fault; solutions are closer to a Gutenberg‐Richter distribution if the maximum magnitude allowed for earthquakes that occur away from mapped faults (background events) is raised above the consensus threshold ofM = 7.6, if the moment rate for background events is reduced, or if the overall maximum magnitude is reduced fromM = 8.5. We also find that participation magnitude‐frequency distribution shapes can be strongly affected by slip rate discontinuities along faults that may be artifacts related to segment boundaries.

Improving Earthquake Rupture Forecasts Using California as a Guide

Research Article| October 03, 2018 Improving Earthquake Rupture Forecasts Using California as a Guide Edward H. Field; Edward H. Field aU.S. Geological Survey, Denver Federal Center, P.O. Box 25046, MS‐966, Denver, Colorado 80225‐0046 U.S.A., field@usgs.gov Search for other works by this author on: GSW Google Scholar Working Group on California Earthquake Probabilities Working Group on California Earthquake Probabilities Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2018) 89 (6): 2337–2346. https://doi.org/10.1785/0220180151 Article history first online: 03 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edward H. Field, Working Group on California Earthquake Probabilities; Improving Earthquake Rupture Forecasts Using California as a Guide. Seismological Research Letters 2018;; 89 (6): 2337–2346. doi: https://doi.org/10.1785/0220180151 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 SocietySeismological Research Letters Search Advanced Search ABSTRACT This article discusses ways in which earthquake rupture forecast models might be improved. Because changes are most easily described in the context of specific models, the third Uniform California Earthquake Rupture Forecast (UCERF3) and its presumed successor, UCERF4, is used as a basis for discussion. Virtually all of the issues and possible improvements discussed are nevertheless general and should therefore be applicable to other regions as well. Two common themes are a need for better epistemic uncertainty representation and the potential utility of physics‐based simulators. Given the large number of possible improvements, coupled with challenges in defining the potential value of each, which will vary among uses, community feedback is invaluable in terms of setting priorities. We should also strive to define more objective valuation metrics. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Analysis of Mean Seismic Ground Motion and Its Uncertainty Based on the UCERF3 Geologic Slip‐Rate Uncertainty for California

Research Article| April 25, 2018 Analysis of Mean Seismic Ground Motion and Its Uncertainty Based on the UCERF3 Geologic Slip‐Rate Uncertainty for California Yuehua Zeng Yuehua Zeng aUSGS Geologic Hazards Science Center, MS 966, Box 25046, Denver, Colorado 80225 U.S.A., zeng@usgs.gov Search for other works by this author on: GSW Google Scholar Author and Article Information Yuehua Zeng aUSGS Geologic Hazards Science Center, MS 966, Box 25046, Denver, Colorado 80225 U.S.A., zeng@usgs.gov Publisher: Seismological Society of America First Online: 25 Apr 2018 Online Issn: 1938-2057 Print Issn: 0895-0695 © Seismological Society of America Seismological Research Letters (2018) 89 (4): 1410–1419. https://doi.org/10.1785/0220170114 Article history First Online: 25 Apr 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Yuehua Zeng; Analysis of Mean Seismic Ground Motion and Its Uncertainty Based on the UCERF3 Geologic Slip‐Rate Uncertainty for California. Seismological Research Letters 2018;; 89 (4): 1410–1419. doi: https://doi.org/10.1785/0220170114 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 SocietySeismological Research Letters Search Advanced Search ABSTRACT The Uniform California Earthquake Rupture Forecast v.3 (UCERF3) model (Field et al., 2014) considers epistemic uncertainty in fault‐slip rate via the inclusion of multiple rate models based on geologic and/or geodetic data. However, these slip rates are commonly clustered about their mean value and do not reflect the broader distribution of possible rates and associated probabilities. Here, we consider both a double‐truncated 2σ Gaussian and a boxcar distribution of slip rates and use a Monte Carlo simulation to sample the entire range of the distribution for California fault‐slip rates. We compute the seismic hazard following the methodology and logic‐tree branch weights applied to the 2014 national seismic hazard model (NSHM) for the western U.S. region (Petersen et al., 2014, 2015). By applying a new approach developed in this study to the probabilistic seismic hazard analysis (PSHA) using precomputed rates of exceedance from each fault as a Green’s function, we reduce the computer time by about 105‐fold and apply it to the mean PSHA estimates with 1000 Monte Carlo samples of fault‐slip rates to compare with results calculated using only the mean or preferred slip rates. The difference in the mean probabilistic peak ground motion corresponding to a 2% in 50‐yr probability of exceedance is less than 1% on average over all of California for both the Gaussian and boxcar probability distributions for slip‐rate uncertainty but reaches about 18% in areas near faults compared with that calculated using the mean or preferred slip rates. The average uncertainties in 1σ peak ground‐motion level are 5.5% and 7.3% of the mean with the relative maximum uncertainties of 53% and 63% for the Gaussian and boxcar probability density function (PDF), respectively. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Candidate Products for Operational Earthquake Forecasting Illustrated Using the HayWired Planning Scenario, Including One Very Quick (and Not‐So‐Dirty) Hazard‐Map Option

Research Article| April 18, 2018 Candidate Products for Operational Earthquake Forecasting Illustrated Using the HayWired Planning Scenario, Including One Very Quick (and Not‐So‐Dirty) Hazard‐Map Option Edward H. Field; Edward H. Field aU.S. Geological Survey, Denver Federal Center, P.O. Box 25046, MS 966, Denver, Colorado 80225‐0046 U.S.A. Search for other works by this author on: GSW Google Scholar Kevin R. Milner Kevin R. Milner bUniversity of Southern California, Southern California Earthquake Center, 3651 Trousdale Parkway #169, Los Angeles, California 90089‐0742 U.S.A. Search for other works by this author on: GSW Google Scholar Author and Article Information Edward H. Field aU.S. Geological Survey, Denver Federal Center, P.O. Box 25046, MS 966, Denver, Colorado 80225‐0046 U.S.A. Kevin R. Milner bUniversity of Southern California, Southern California Earthquake Center, 3651 Trousdale Parkway #169, Los Angeles, California 90089‐0742 U.S.A. Publisher: Seismological Society of America First Online: 18 Apr 2018 Online Issn: 1938-2057 Print Issn: 0895-0695 © Seismological Society of America Seismological Research Letters (2018) 89 (4): 1420–1434. https://doi.org/10.1785/0220170241 Article history First Online: 18 Apr 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Edward H. Field, Kevin R. Milner; Candidate Products for Operational Earthquake Forecasting Illustrated Using the HayWired Planning Scenario, Including One Very Quick (and Not‐So‐Dirty) Hazard‐Map Option. Seismological Research Letters 2018;; 89 (4): 1420–1434. doi: https://doi.org/10.1785/0220170241 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 SocietySeismological Research Letters Search Advanced Search ABSTRACT In an effort to help address debates on the usefulness of operational earthquake forecasting (OEF), we illustrate a number of OEF products that could be automatically generated in near‐real time. To exemplify, we use an M 7.1 mainshock on the Hayward fault, which is very similar to the U.S. Geological Survey (USGS) HayWired earthquake planning scenario. Given that there is always some background level of hazard or risk, we emphasize that probability gains (the ratio of short‐term to long‐term‐average estimates) might be of particular interest to users. We also illustrate how such gains are highly sensitive to forecast duration and latency, with the latter representing how long it takes to generate the forecast and/or to take action. The influence of fault‐based information, which has traditionally been ignored in OEF, is also evaluated using the newly developed the third Uniform California Earthquake Rupture Forecast epidemic‐type aftershock sequence (UCERF3‐ETAS) model. We find that the inclusion of faults only makes a difference for hazard and risk metrics that are dominated by large‐event likelihoods. We also show how the ShakeMap of a mainshock represents a decent estimate of the ground motions that have a 6% chance of being exceeded due to aftershocks in the week that follows. The ultimate value of these types of OEF products can only be determined in the context of specific uses, and because these vary widely, institutions responsible for providing OEF products will depend heavily on user feedback, especially when making resource‐allocation decisions. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Probabilistic Estimates of Ground Motion in the Los Angeles Basin from Scenario Earthquakes on the San Andreas Fault

Kinematic source inversions of past earthquakes in the magnitude range of 6–8 are used to simulate 60 scenario earthquakes on the San Andreas fault. The unilateral rupture scenario earthquakes are hypothetically located at 6 locations spread out uniformly along the southern section of the fault, each associated with two hypocenters and rupture directions. Probabilities of occurrence over the next 30 years are assigned to each of these earthquakes by mapping the probabilities of 10,445 plausible earthquakes postulated for this section of the fault by the Uniform California Earthquake Rupture Forecast. Three-component broadband ground motion histories are computed at 636 sites in the greater Los Angeles metropolitan area by superposing short-period (0.2–2.0 s) empirical Green’s function synthetics on top of long-period (>2.0 s) spectral element synthetics. The earthquake probabilities and the computed ground motions are combined to develop probabilistic estimates of ground shaking in the region from San Andreas fault earthquakes over the next 30 years. The results could be useful in city planning, emergency management, and building code enhancement.

A Prototype Operational Earthquake Loss Model for California Based on UCERF3-ETAS – A First Look at Valuation

We present a prototype operational loss model based on UCERF3-ETAS, which is the third Uniform California Earthquake Rupture Forecast with an Epidemic Type Aftershock Sequence (ETAS) component. As such, UCERF3-ETAS represents the first earthquake forecast to relax fault segmentation assumptions and to include multi-fault ruptures, elastic-rebound, and spatiotemporal clustering, all of which seem important for generating realistic and useful aftershock statistics. UCERF3-ETAS is nevertheless an approximation of the system, however, so usefulness will vary and potential value needs to be ascertained in the context of each application. We examine this question with respect to statewide loss estimates, exemplifying how risk can be elevated by orders of magnitude due to triggered events following various scenario earthquakes. Two important considerations are the probability gains, relative to loss likelihoods in the absence of main shocks, and the rapid decay of gains with time. Significant uncertainties and model limitations remain, so we hope this paper will inspire similar analyses with respect to other risk metrics to help ascertain whether operationalization of UCERF3-ETAS would be worth the considerable resources required.

Methodology for earthquake rupture rate estimates of fault networks: example for the western Corinth rift, Greece

Abstract. Modeling the seismic potential of active faults is a fundamental step of probabilistic seismic hazard assessment (PSHA). An accurate estimation of the rate of earthquakes on the faults is necessary in order to obtain the probability of exceedance of a given ground motion. Most PSHA studies consider faults as independent structures and neglect the possibility of multiple faults or fault segments rupturing simultaneously (fault-to-fault, FtF, ruptures). The Uniform California Earthquake Rupture Forecast version 3 (UCERF-3) model takes into account this possibility by considering a system-level approach rather than an individual-fault-level approach using the geological, seismological and geodetical information to invert the earthquake rates. In many places of the world seismological and geodetical information along fault networks is often not well constrained. There is therefore a need to propose a methodology relying on geological information alone to compute earthquake rates of the faults in the network. In the proposed methodology, a simple distance criteria is used to define FtF ruptures and consider single faults or FtF ruptures as an aleatory uncertainty, similarly to UCERF-3. Rates of earthquakes on faults are then computed following two constraints: the magnitude frequency distribution (MFD) of earthquakes in the fault system as a whole must follow an a priori chosen shape and the rate of earthquakes on each fault is determined by the specific slip rate of each segment depending on the possible FtF ruptures. The modeled earthquake rates are then compared to the available independent data (geodetical, seismological and paleoseismological data) in order to weight different hypothesis explored in a logic tree.The methodology is tested on the western Corinth rift (WCR), Greece, where recent advancements have been made in the understanding of the geological slip rates of the complex network of normal faults which are accommodating the ∼ 15 mm yr−1 north–south extension. Modeling results show that geological, seismological and paleoseismological rates of earthquakes cannot be reconciled with only single-fault-rupture scenarios and require hypothesizing a large spectrum of possible FtF rupture sets. In order to fit the imposed regional Gutenberg–Richter (GR) MFD target, some of the slip along certain faults needs to be accommodated either with interseismic creep or as post-seismic processes. Furthermore, computed individual faults' MFDs differ depending on the position of each fault in the system and the possible FtF ruptures associated with the fault. Finally, a comparison of modeled earthquake rupture rates with those deduced from the regional and local earthquake catalog statistics and local paleoseismological data indicates a better fit with the FtF rupture set constructed with a distance criteria based on 5 km rather than 3 km, suggesting a high connectivity of faults in the WCR fault system.

A new earthquake forecast for California

Earthquakes Earthquakes cannot be predicted, but rupture models can estimate the regional likelihood of an earthquake within a certain time window. Field et al. incorporate new data and fault-based information for the Third Uniform California Earthquake Rupture Forecast (UCERF3). The new model better accounts for potential multiple fault ruptures and provides self-consistent forecasting windows from hours to more than a century. UCERF3 is an important development for operational forecasts that are vital for assessing the evolving seismic hazard in California. Seismol. Res. Lett. 10.1785/0220170045 (2017).

Trimming a Hazard Logic Tree with a New Model-Order-Reduction Technique

The size of the logic tree within the Uniform California Earthquake Rupture Forecast Version 3, Time-Dependent (UCERF3-TD) model can challenge risk analyses of large portfolios. An insurer or catastrophe risk modeler concerned with losses to a California portfolio might have to evaluate a portfolio 57,600 times to estimate risk in light of the hazard possibility space. Which branches of the logic tree matter most, and which can one ignore? We employed two model-order-reduction techniques to simplify the model. We sought a subset of parameters that must vary, and the specific fixed values for the remaining parameters, to produce approximately the same loss distribution as the original model. The techniques are (1) a tornado-diagram approach we employed previously for UCERF2, and (2) an apparently novel probabilistic sensitivity approach that seems better suited to functions of nominal random variables. The new approach produces a reduced-order model with only 60 of the original 57,600 leaves. One can use the results to reduce computational effort in loss analyses by orders of magnitude.

A Synoptic View of the Third Uniform California Earthquake Rupture Forecast (UCERF3)

ABSTRACT 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.

Examining associations between citizens' beliefs and attitudes about uncertainty and their earthquake risk judgments, preparedness intentions, and mitigation policy support in Japan and the United States

Although hazards are inherently uncertain, research on citizens’ judgments of risk, hazard preparedness, and support for mitigation policies has rarely accounted for citizens’ beliefs about the uncertainty of fields estimating hazard risk or in science as providing accurate, unbiased knowledge, nor citizens’ need to achieve quick, certain answers. Parallel online surveys of residents of earthquake-prone areas of Japan and the United States revealed that belief in scientific positivism increased policy support in both countries (as did need for closure among Americans), and belief in seismological uncertainty reduced judged earthquake risk in Japan, with small effect sizes. Preparedness was unaffected by these predictors. Associations of other factors (quake experience; trust in experts; demographics) with dependent variables were consistent with other studies, and Japanese-American differences were small on dependent variables and in most predictors. Motivation (i.e., high involvement with the topic, relevance of the fictional earthquake rupture forecast in a quasi-experiment embedded in the survey, and judged ability to use its information) strongly affected judged risk, preparedness and policy support. Low-motivation Japanese and high-motivation Americans exhibited associations most similar to overall findings for their nations. Implications of these findings for hazards research and risk communication are discussed.

A Spatiotemporal Clustering Model for the Third Uniform California Earthquake Rupture Forecast (UCERF3‐ETAS): Toward an Operational Earthquake Forecast

We, the ongoing Working Group on California Earthquake Probabilities, present a spatiotemporal clustering model for the Third Uniform California Earthquake Rupture Forecast (UCERF3), with the goal being to represent aftershocks, induced seismicity, and otherwise triggered events as a potential basis for operational earthquake forecasting (OEF). Specifically, we add an epidemic‐type aftershock sequence (ETAS) component to the previously published time‐independent and long‐term time‐dependent forecasts. This combined model, referred to as UCERF3‐ETAS, collectively represents a relaxation of segmentation assumptions, the inclusion of multifault ruptures, an elastic‐rebound model for fault‐based ruptures, and a state‐of‐the‐art spatiotemporal clustering component. It also represents an attempt to merge fault‐based forecasts with statistical seismology models, such that information on fault proximity, activity rate, and time since last event are considered in OEF. We describe several unanticipated challenges that were encountered, including a need for elastic rebound and characteristic magnitude–frequency distributions (MFDs) on faults, both of which are required to get realistic triggering behavior. UCERF3‐ETAS produces synthetic catalogs of M ≥2.5 events, conditioned on any prior M ≥2.5 events that are input to the model. We evaluate results with respect to both long‐term (1000 year) simulations as well as for 10‐year time periods following a variety of hypothetical scenario mainshocks. Although the results are very plausible, they are not always consistent with the simple notion that triggering probabilities should be greater if a mainshock is located near a fault. Important factors include whether the MFD near faults includes a significant characteristic earthquake component, as well as whether large triggered events can nucleate from within the rupture zone of the mainshock. Because UCERF3‐ETAS has many sources of uncertainty, as will any subsequent version or competing model, potential usefulness needs to be considered in the context of actual applications. [Electronic Supplement:][1] Figures showing discretization, verification of the DistanceDecayCubeSampler , average simulated participation rate, and average cumulative magnitude–frequency distributions (MFDs). [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160173/-/DC1

Disaggregating UCERF3 for Site-Specific Applications

We present methods to simplify the use of the Uniform California Earthquake Rupture Forecast 3 (UCERF3) in site-specific seismic hazard analyses. UCERF3 defines the “state-of-practice” for hazard assessments in California and underlies the current USGS National Seismic Hazard Map (NSHM). UCERF3 contains many more ruptures than previous models and fault-to-fault connectivity in the model makes ruptures less readily incorporated into conventional hazard analyses. We demonstrate a decomposition of the UCERF3 model using site-specific subsection magnitude-frequency distributions (S3MFDs) that reduces computational demands while still including every rupture. The decomposed hazard can be used to evaluate site-specific fault source epistemic uncertainties. We illustrate using spectral acceleration at 1 Hz at three trial sites, one near a single dominant fault, one near Long Beach, where many low slip-rate faults contribute, and one in San Jose, where fault hazard is concentrated in a few high slip-rate faults.

M ≥ 7 earthquake rupture forecast and time‐dependent probability for the sea of Marmara region, Turkey

AbstractWe forecast time‐independent and time‐dependent earthquake ruptures in the Marmara region of Turkey for the next 30 years using a new fault segmentation model. We also augment time‐dependent Brownian passage time (BPT) probability with static Coulomb stress changes (ΔCFF) from interacting faults. We calculateMw > 6.5 probability from 26 individual fault sources in the Marmara region. We also consider a multisegment rupture model that allows higher‐magnitude ruptures over some segments of the northern branch of the North Anatolian Fault Zone beneath the Marmara Sea. A total of 10 differentMw = 7.0 toMw = 8.0 multisegment ruptures are combined with the other regional faults at rates that balance the overall moment accumulation. We use Gaussian random distributions to treat parameter uncertainties (e.g., aperiodicity, maximum expected magnitude, slip rate, and consequently mean recurrence time) of the statistical distributions associated with each fault source. We then estimate uncertainties of the 30 year probability values for the next characteristic event obtained from three different models (Poisson, BPT, and BPT + ΔCFF) using a Monte Carlo procedure. The Gerede fault segment located at the eastern end of the Marmara region shows the highest 30 year probability, with a Poisson value of 29% and a time‐dependent interaction probability of 48%. We find an aggregated 30 year Poisson probability ofM > 7.3 earthquakes at Istanbul of 35%, which increases to 47% if time dependence and stress transfer are considered. We calculate a twofold probability gain (ratio time dependent to time independent) on the southern strands of the North Anatolian Fault Zone.

A Fault‐Based Model for Crustal Deformation, Fault Slip Rates, and Off‐Fault Strain Rate in California

We invert Global Positioning System (GPS) velocity data to estimate fault slip rates in California using a fault‐based crustal deformation model with geologic constraints. The model assumes buried elastic dislocations across the region using Uniform California Earthquake Rupture Forecast Version 3 (UCERF3) fault geometries. New GPS velocity and geologic slip‐rate data were compiled by the UCERF3 deformation working group. The result of least‐squares inversion shows that the San Andreas fault slips at 19–22 mm/yr along Santa Cruz to the North Coast, 25–28 mm/yr along the central California creeping segment to the Carrizo Plain, 20–22 mm/yr along the Mojave, and 20–24 mm/yr along the Coachella to the Imperial Valley. Modeled slip rates are 7–16 mm/yr lower than the preferred geologic rates from the central California creeping section to the San Bernardino North section. For the Bartlett Springs section, fault slip rates of 7–9 mm/yr fall within the geologic bounds but are twice the preferred geologic rates. For the central and eastern Garlock, inverted slip rates of 7.5 and 4.9 mm/yr, respectively, match closely with the geologic rates. For the western Garlock, however, our result suggests a low slip rate of 1.7 mm/yr. Along the eastern California shear zone and southern Walker Lane, our model shows a cumulative slip rate of 6.2–6.9 mm/yr across its east–west transects, which is ∼1 mm/yr increase of the geologic estimates. For the off‐coast faults of central California, from Hosgri to San Gregorio, fault slips are modeled at 1–5 mm/yr, similar to the lower geologic bounds. For the off‐fault deformation, the total moment rate amounts to 0.88×1019 N·m/yr, with fast straining regions found around the Mendocino triple junction, Transverse Ranges and Garlock fault zones, Landers and Brawley seismic zones, and farther south. The overall California moment rate is 2.76×1019 N·m/yr, which is a 16% increase compared with the UCERF2 model. Online Material: Table of geological slip rates.

Forecasting cascading fault rupture

Geophysics Earthquake rupture forecasts provide vital estimates of the likelihood of future earthquakes in a region. However, Nissen et al. show that rupture forecasts can be muddied by not considering cascading multiple-fault ruptures. A combined geodetic and seismological reanalysis of a 1997 earthquake in Pakistan revealed dynamic triggering of a second fault 50 km away shortly after the first rupture. Current forecasts assume that triggering is limited to faults within 5 km. This observation suggests that longer-range, multiple-fault ruptures should be incorporated into forecasts, and it highlights the dangers resulting from this type of earthquake doublet. Nat. Geosci. 10.1038/ngeo2653 (2016).

2014 Update to the National Seismic Hazard Model in California

The 2014 update to the U. S. Geological Survey National Seismic Hazard Model in California introduces a new earthquake rate model and new ground motion models (GMMs) that give rise to numerous changes to seismic hazard throughout the state. The updated earthquake rate model is the third version of the Uniform California Earthquake Rupture Forecast (UCERF3), wherein the rates of all ruptures are determined via a self-consistent inverse methodology. This approach accommodates multifault ruptures and reduces the overprediction of moderate earthquake rates exhibited by the previous model (UCERF2). UCERF3 introduces new faults, changes to slip or moment rates on existing faults, and adaptively smoothed gridded seismicity source models, all of which contribute to significant changes in hazard. New GMMs increase ground motion near large strike-slip faults and reduce hazard over dip-slip faults. The addition of very large strike-slip ruptures and decreased reverse fault rupture rates in UCERF3 further enhances these effects.

Long Valley Caldera and the UCERF Depiction of Sierra Nevada Range-Front Faults

Long Valley caldera lies within a left-stepping offset in the north-northwest-striking Sierra Nevada range-front normal faults with the Hilton Creek fault to the south and Hartley Springs fault to the north. Both Uniform California Earthquake Rupture Forecast (UCERF) 2 and its update, UCERF3, depict slip on these major range-front normal faults as extending well into the caldera, with significant normal slip on overlapping, subparallel segments separated by ∼10 km. This depiction is countered by (1) geologic evidence that normal faulting within the caldera consists of a series of graben structures associated with postcaldera magmatism (intrusion and tumescence) and not systematic down-to-the-east displacements consistent with distributed range-front faulting and (2) the lack of kinematic evidence for an evolving, postcaldera relay ramp structure between overlapping strands of the two range-front normal faults. The modifications to the UCERF depiction described here reduce the predicted shaking intensity within the caldera, and they are in accord with the tectonic influence that underlapped offset range-front faults have on seismicity patterns within the caldera associated with ongoing volcanic unrest.

GEAR1: A Global Earthquake Activity Rate Model Constructed from Geodetic Strain Rates and Smoothed Seismicity

Global earthquake activity rate model 1 (GEAR1) estimates the rate of shallow earthquakes with magnitudes 6–9 everywhere on Earth. It was designed to be reproducible and testable. Our preferred hybrid forecast is a log–linear blend of two parent forecasts based on the Global Centroid Moment Tensor (CMT) catalog (smoothing 4602 m ≥5.767 shallow earthquakes, 1977–2004) and the Global Strain Rate Map version 2.1 (smoothing 22,415 Global Positioning System velocities), optimized to best forecast the 2005–2012 Global CMT catalog. Strain rate is a proxy for fault stress accumulation, and earthquakes indicate stress release, so a multiplicative blend is desirable, capturing the strengths of both approaches. This preferred hybrid forecast outperforms its seismicity and strain‐rate parents; the chance that this improvement stems from random seismicity fluctuations is less than 1%. The preferred hybrid is also tested against the independent parts of the International Seismological Centre‐Global Earthquake Model catalog ( m ≥6.8 during 1918–1976) with similar success. GEAR1 is an update of this preferred hybrid. Comparing GEAR1 to the Uniform California Earthquake Rupture Forecast Version 3 (UCERF3), net earthquake rates agree within 4% at m ≥5.8 and at m ≥7.0. The spatial distribution of UCERF3 epicentroids most resembles GEAR1 after UCERF3 is smoothed with a 30 km kernel. Because UCERF3 has been constructed to derive useful information from fault geometry, slip rates, paleoseismic data, and enhanced seismic catalogs (not used in our model), this is encouraging. To build parametric catastrophe bonds from GEAR1, one could calculate the magnitude for which there is a 1% (or any) annual probability of occurrence in local regions. Online Material: Discussion of forecast‐scoring metrics, tables of scoring results, and source code and data files needed to reproduce forecast.