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Summary: Megathrust earthquake sequences, comprising mainshocks and triggered aftershocks along the subduction interface and in the overriding crust, can impact multiple buildings and infrastructure in a city. The time between the mainshocks and aftershocks usually is too short to retrofit the structures; therefore, moderate‐size aftershocks can cause additional damage. To have a better understanding of the impact of aftershocks on city‐wide seismic risk assessment, a new simulation framework of spatiotemporal seismic hazard and risk assessment of future M9.0 sequences in the Cascadia subduction zone is developed. The simulation framework consists of an epidemic‐type aftershock sequence (ETAS) model, ground‐motion model, and state‐dependent seismic fragility model. The spatiotemporal ETAS model is modified to characterise aftershocks of large and anisotropic M9.0 mainshock ruptures. To account for damage accumulation of wood‐frame houses due to aftershocks in Victoria, British Columbia, Canada, state‐dependent fragility curves are implemented. The new simulation framework can be used for quasi‐real‐time aftershock hazard and risk assessments and city‐wide post‐event risk management. [ABSTRACT FROM AUTHOR]

We investigate the physical processes that generate seismicity during hydraulic fracturing. Fluid processes (increases in pore pressure and poroelastic stress) are often considered to be the primary drivers. However, some recent studies have suggested that elastic stress interactions may significantly contribute to further seismicity. In this work we use a microseismic data set acquired during hydraulic fracturing to calculate elastic stress transfer during a period of fault activation and induced seismicity. We find that elastic stress changes may have weakly promoted initial failure, but at later times stress changes generally acted to inhibit further slip. Sources from within tight clusters are found to be the most significant contributor to the cumulative elastic stress changes. Given the estimated in situ stress field, relatively large increases in pore pressure are required to reach the failure envelope for these faults—on the order of 10 MPa. This threshold is far greater than the reliable cumulative elastic stress changes found in this study, with the vast majority of events receiving no more than 0.1 MPa of positive Δ CFS, further indicating that elastic stress changes were not a significant driver, and that interaction with the pressurized fluid was required to initiate failure. Thus, cumulative stress transfer from small events near the injection well does not appear to play a significant role in the reactivation of nearby faults. [ABSTRACT FROM AUTHOR]

We study the stability conditions of a class of branching processes prominent in the analysis and modeling of seismicity. This class includes the epidemic-type aftershock sequence (ETAS) model as a special case, but more generally comprises models in which the magnitude distribution of direct offspring depends on the magnitude of the progenitor, such as the branching aftershock sequence (BASS) model and another recently proposed branching model based on a dynamic scaling hypothesis. These stability conditions are closely related to the concepts of the criticality parameter and the branching ratio. The criticality parameter summarizes the asymptotic behavior of the population after sufficiently many generations, determined by the maximum eigenvalue of the transition equations. The branching ratio is defined by the proportion of triggered events in all the events. Based on the results for the generalized case, we show that the branching ratio of the ETAS model is identical to its criticality parameter because its magnitude density is separable from the full intensity. More generally, however, these two values differ and thus place separate conditions on model stability. As an illustration of the difference and of the importance of the stability conditions, we employ a version of the BASS model, reformulated to ensure the possibility of stationarity. In addition, we analyze the magnitude distributions of successive generations of the BASS model via analytical and numerical methods, and find that the compound density differs substantially from a Gutenberg-Richter distribution, unless the process is essentially subcritical (branching ratio less than 1) or the magnitude dependence between the parent event and the direct offspring is weak. [ABSTRACT FROM AUTHOR]

The Regional Earthquake Likelihood Models (RELM) working group designed a 5-year experiment to forecast the number, spatial distribution, and magnitude distribution of subsequent target earthquakes, defined to be those with magnitude ≥4.95 (M 4.95+) in a well-defined California testing region. Included in the experiment specification were the description of the data source, the methods for data processing, and the proposed evaluation metrics. The RELM experiment began on 1 January 2006 and involved 17 time-invariant forecasts constructed by seismicity modelers; by the end of the experiment on 1 January 2011, 31 target earthquakes had occurred. We analyze the experiment outcome by applying the proposed consistency tests based on likelihood measures and additional comparison tests based on a measure of information gain. We find that the smoothed seismicity forecast by Helmstetter et at., 2007 based on M 2+ earthquakes since 1981, is the best forecast, regardless of whether aftershocks are included in the analysis. The RELM experiment has helped to clarify ideas about testing that can be applied to more wide-ranging earthquake forecasting experiments conducted by the Collaboratory for the Study of Earthquake Predictability (CSEP). [ABSTRACT FROM AUTHOR]

In the aftermath of a devastating earthquake, the ensuing cascade of aftershocks can turn out to be even more destructive than the mainshock. Operational earthquake forecasting seeks to provide reliable real-time information about the time-dependence of earthquake hazard to the public. Two forecasting approaches tend to be used: statistical modelling, which employs empirical laws to predict the clustering characteristics (e.g. ETAS); and physics-based modelling, that combines the stress transfer hypothesis with rate-and-state friction laws (CRS models). Our work is partly motivated by recent retrospective experiments that reveal the comparable performance of physics-based and statistical forecasts.Here, we assess the influence of individual CRS model choices driven by real-time conditions, and clarify the effect of input data quality on the predictive skills of CRS forecasts during the 2016-17 Central Italy earthquake cascade. In the first year of the 2016-17 Central Italy sequence 1160 M ≥ 3.0 events were detected, spreading along a 60-km long normal fault system. Among these, seven M ≥ 5.4 earthquakes occurred from a few hours to five months after the first Mw 6.0 Amatrice event. We develop seven physics-based models with gradually increasing level of refinement as part of a pseudo-prospective experiment. The preliminary forecasts include data available just a few minutes after each M ≥ 5.4 event, featuring synthetic source models with empirically determined fault length and fault constitutive parameters from previous regional studies. Increasingly complex models incorporate: (1) optimized rate-state parameters, (2) spatially heterogeneous receiver fault planes, (3) best available slip models, and (4) secondary triggering effects from M ≥ 3.0 aftershocks. We evaluate and track models' performance in space and time using CSEP log-likelihood statistics for a 1-year time horizon after the Mw 6.0 Amatrice earthquake and compare against a benchmark ETAS model. When including refined input data and increasing model complexity, we observe a significant performance improvement of physics-based forecasts. The most enhanced CRS forecasts reach dramatic probability gains per earthquake of up to 1,000 over the preliminary physics-based realizations. The results confirm that CRS models are about as informative as ETAS only when secondary triggering effects are included together with realistic slip models, spatially heterogeneous receiver fault planes and optimized fault constitutive parameters. [ABSTRACT FROM AUTHOR]