Aftershocks following the 2011 Tohoku-Oki earthquake driven by both stress transfer and afterslip

Abstract

Aftershocks are a fundamental characteristic of seismicity, and their generation mechanism is mainly characterized by two physical models, stress transfer from large earthquakes and afterslip-induced stress loading. However, the contribution of each mechanism to aftershock generation remains unclear. Here we investigate the spatiotemporal variations in aftershock activity following the 2011 Tohoku-Oki earthquake by applying the Hierarchical Space-Time Epidemic-Type Aftershock Sequence (HIST-ETAS) model to the decade of recorded seismicity since the mainshock. Using the estimated HIST-ETAS model, we categorize the aftershocks into background earthquakes (which are caused by aseismic phenomena) and triggered earthquakes (which are caused by earthquake-to-earthquake interactions). Most of the earthquakes that occurred updip of the large coseismic slip zone along the Japan Trench are triggered earthquakes, consistent with the lack of afterslip in this area. Conversely, background earthquakes are the predominant earthquake type in the long-term downdip of the large coseismic slip zone, and they positively correlate with the afterslip evolution. Our results suggest the importance of combining these two end-member aftershock generation models to explain aftershock activity and thus provide new insights into the relationship between afterslip and spatiotemporal aftershock distribution. Our classifications may also contribute to the monitoring of afterslip in a given region that hosts a large earthquake, particularly where geodetic observation networks are too sparse to evaluate afterslip evolution.

Seasonal Modulation of Crustal Seismicity in Northeastern Japan Driven by Snow Load

“…Aftershocks should be removed via a declustering technique prior to investigating the seasonal modulation of seismicity. Here we apply the Hierarchical Space‐Time Epidemic‐Type Aftershock Sequence (HIST‐ETAS) model (Ogata, 2004; Ogata et al., 2003; Ueda et al., 2021) to the earthquake catalog and the probability that each event is a background event is used as the declustered catalog (Ueda & Kato, 2023; Zhuang et al., 2002). A comparison of the cumulative numbers of total and background earthquakes (Figure 5c) shows that the aftershocks are successfully removed from the original earthquake catalog by the HIST‐ETAS model.…”