2021 Volume 21 Issue 2 Pages 2_1-2_20
Probabilistic seismic hazard analysis (PSHA) was implemented and the exceedance frequency of peak ground acceleration (PGA) on the ground surface for aftershocks over one year following the 2011 Tohoku earthquake (Mw9.0) was estimated. The magnitude-frequency relationship for aftershocks was estimated using two models, in view of the validity assessment and prediction. The first used the aftershock catalog of the Tohoku earthquake (Model 1) and the second used the catalog of the aftershocks accompanying the mainshocks occurring before the Tohoku earthquake with a smaller magnitude up to 8 compared to the Tohoku earthquake (Model 2). Exceedance frequency was estimated at four stations of the Strong-motion Seismograph Networks, operated by the National Research Institute for Earth Science and Disaster Resilience of Japan, along the coastline of the Pacific Ocean in the Tohoku and Kanto districts, Japan. The estimated frequency exceedances for the PGA based on the two models are mostly equivalent for all stations except for one station which is far from an aftershock zone. Moreover, exceedance frequencies at the four stations were directly compared to those estimated based on the acceleration seismograms recorded at these stations. The results show that predicted exceedance frequencies for both models are nearly consistent with those observed for a PGA less than 200 cm/s2 at all stations. However, the former are not consistent with the latter for a PGA significantly greater than 200 cm/s2 at some stations. Finally, exceedance frequencies considering both the mainshock and aftershocks were estimated based on the sequence-based probabilistic seismic hazard model proposed by Iervolino et al. (2014). Here, Model 2 was used for the magnitude-frequency relationship of the aftershocks. The results show that the exceedance frequency from the aftershocks is greater than 20% of the total exceedance frequency (which was the combined exceedance frequency from the mainshock and aftershocks) within a PGA range from approximately 300 to 1500 cm/s2 (at which buildings may be damaged) for the three stations. Our results suggest that it is important to consider the effects of aftershocks when a PSHA is conducted for a site with the potential of a mega earthquake occurrence, such as the Tohoku earthquake, in terms of earthquake disaster prevention following an earthquake disaster.
