Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment

The Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near-real time the temporal and spatial variations in the average earthquake size distribution (the b-value) and the performance of the newly introduced foreshock traffic-light system. In normally decaying aftershock sequences, in the past studies, the b-value of the aftershocks was found, on average, to be 10%–30% higher than the background b-value. A drop of 10% or more in “aftershock” b-values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b-values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b-value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b-value increase of 29% after the mainshock. Our results suggest that the real-time monitoring of b-values is feasible in California and may add important information for aftershock hazard assessment.

Variability of ETAS Parameters in Global Subduction Zones and Applications to Mainshock–Aftershock Hazard Assessment

ABSTRACT Megathrust earthquake sequences can impact buildings and infrastructure due to not only the mainshock but also the triggered aftershocks along the subduction interface and in the overriding crust. To give realistic ranges of aftershock simulations in regions with limited data and to provide time-dependent seismic hazard information right after a future giant shock, we assess the variability of the epidemic-type aftershock sequence (ETAS) model parameters in subduction zones that have experienced M≥7.5 earthquakes, comparing estimates from long time windows with those from individual sequences. Our results show that the ETAS parameters are more robust if estimated from a long catalog than from individual sequences, given individual sequences have fewer data including missing early aftershocks. Considering known biases of the parameters (due to model formulation, the isotropic spatial aftershock distribution, and finite size effects of catalogs), we conclude that the variability of the ETAS parameters that we observe from robust estimates is not significant, neither across different subduction-zone regions nor as a function of maximum observed magnitudes. We also find that ETAS parameters do not change when multiple M 8.0–9.0 events are included in a region, mainly because an M 9.0 sequence dominates the number of events in the catalog. Based on the ETAS parameter estimates in the long time period window, we propose a set of ETAS parameters for future M 9.0 sequences for aftershock hazard assessment (K0=0.04±0.02, α=2.3, c=0.03±0.01, p=1.21±0.08, γ=1.61±0.29, d=23.48±18.17, and q=1.68±0.55). Synthetic catalogs created with the suggested ETAS parameters show good agreement with three observed M 9.0 sequences since 1965 (the 2004 M 9.1 Aceh–Andaman earthquake, the 2010 M 8.8 Maule earthquake, and the 2011 M 9.0 Tohoku earthquake).

Aftershock Hazard Assessment Based on Utilization of Observed Main Shock Demand

This paper presents an aftershock hazard assessment method that is based on taking into account the macroseismic indicators of the main shock observed at the site. The proposed method is referred to as conditional aftershock hazard assessment (CAHA). The essence of the CAHA method is to estimate the aftershock hazard at the site conditioned on the main shock intensity exhibited at that location. This is achieved by exploiting the correlation between the ground motion intensities exhibited during the main shock and the aftershock events. Specifically, the correlation of the epsilons registered for the two events, is utilized. Investigation of the epsilon correlation indicates that highest correlation occurs at the range of periods between 0.8 and 1.0 s. Based on the estimated epsilon correlation, the mean and the dispersion of the aftershock ground motion intensity, are estimated. An application of the proposed method to a set of sites affected by the 2011 Van (Turkey) M w7.2 earthquake sequence is illustrated. The performance of the method is assessed in comparison with the conventional approaches. For the considered example application, the hazard estimated using the proposed method shows a better agreement with the actual aftershock recordings, compared to the existing approaches.