Are Aftershock Sequences Pertinent to Long‐Term Seismic Hazard Assessments? Insights From the Temporal ETAS Model

Abstract

Aftershocks are commonly removed from observed earthquake catalogs in probabilistic seismic hazard analyses, using declustering techniques. We use stationary and temporal Epistemic‐Type Aftershock Sequence (ETAS) models to generate aftershocks from background seismicity and preceding aftershocks. We assume that the mainshocks equal the background seismicity, to divide the synthetic earthquake catalogs into mainshock (declustered) and complete (nondeclustered) versions. Only mainshocks follow a Poissonian distribution. We then evaluate how accurately we can forecast the recurrence of the largest events based on the simulated catalogs. A single b value is derived from the simulated catalogs and used in the magnitude‐frequency forecast. When the b value of the mainshocks is considerably smaller than the b value of the aftershocks, the information derived from the mainshock catalog leads to accurate predictions of the occurrence of the largest events. Conversely, when the mainshock and aftershocks have comparable b values, only the complete catalog produces representative estimates for the occurrence statistics of the largest events. We also show that using Poisson statistics leads to representative assessment of long‐term recurrences, even if aftershocks have a non‐Poissonian distribution. Finally, we analyze a recent case of induced seismicity, Oklahoma, USA, where the complete catalog displays a kink in the magnitude‐frequency distribution. Declustering removes this kink, leading to better b value estimations for the largest magnitude events. We conclude that temporal declustering for seismic hazard assessment is only recommended in catalogs with a large number of earthquakes and in catalogs where the b values of the mainshocks are significantly different from the b values of the complete catalog.

Evolution of Short‐Term Seismic Hazard in Alberta, Canada, From Induced and Natural Earthquakes: 2011–2020

“…Underestimation of potentially damaging events may also occur if aftershocks have similar $$b$$‐values to mainshocks, but temporal declustering is applied to remove the aftershocks. Conversely, temporal declustering is recommended if aftershocks have larger $$b$$‐values than the mainshocks in order to correct for kinked magnitude‐frequency distributions (Reyes Canales & van der Baan, 2020). In this study, declustering was not applied due to the sparsity of the earthquake catalogs.…”

“…One noticeable caveat in the proposed seismic hazard analysis is the occurrence of earthquakes with kinked magnitude‐frequency distributions, particularly if they underpredicted moderate and large events when assuming constant b ‐value (Reyes Canales & van der Baan, 2020). This is the case for the Red Deer area in 2019, where the mainshock event ( M L = 4.24), deviates from the magnitude‐frequency distribution for the area, resulting in biased hazard forecasts.…”

“…Seismicity declustering is a standard methodology applied in PSHA to discriminate between mainshocks, aftershocks, and foreshocks. However, seismicity declustering is not advised for small catalogs, as it might lead to hazard underestimations (Reyes Canales & van der Baan, 2020).…”

The Rise, Peak and Decline of the Seismic Hazard Related to Hydraulic Fracturing Activities in the Duvernay Play, Fox Creek Area, Alberta

“…The use of Equation 9 for seismic hazard forecasts is permitted even if the earthquake distributions are different from a Poissonian process, for instance, due to the existence of aftershock sequences, as shown by Reyes Canales and van der Baan (2020). Additionally, some studies, including Hajati et al.…”