[S-SS11] Strong Ground Motion and Earthquake Disaster

[SSS11-15] Comparison of seismic wave fields from the earthquakes off Fukushima prefecture, northeastern Japan

*Yasumaro Kakehi¹ (1.Department of Planetology, Graduate School of Science, Kobe University)

Keywords:island arc, interplate earthquake, intraslab earthquake, attenuation

An M_W 7.1 earthquake occurred off Fukushima prefecture in northeastern Japan on Feb. 13, 2021. This (called event SU1 hereafter) is a down-dip compression type intraslab earthquake occurring on the upper plane of the double seismic zone, and its focal depth is 55km. The PGA (Peak Ground Acceleration) distribution based on NIED K-NET and KiK-net observation networks shows singular distribution, apart from concentric distribution, where high-frequency seismic wave amplitudes decay rapidly in the backarc side of the island arc, while large amplitudes of high-frequency seismic waves are observed in the forearc side. This phenomenon is a reflection of the attenuation structure with low-Q medium in the backarc side specific to island arc, and it comes from attenuation of high-frequency waves during propagation through the low-Q medium in the backarc side (Kakehi, 2015). In order to investigate the difference of seismic wave fields due to focal depths, strong ground motion data from following three M5 class earthquakes that have epicenters close to that of SU1 and various focal depths are analyzed: an interplate earthquake on Oct. 2, 2001 (M_W 5.5, 40.8km depth, event P hereafter), an intraslab (upper plane) earthquake on Aug. 4, 2013 (M_W 5.9, 58.0km depth, event SU2 hereafter), and an intraslab earthquake (lower plane) on Mar. 13, 2010 (M_W 5.5, 77.7km depth, event SL hereafter). The epicentral location and focal depth of SU2 are almost same as those of SU1. From the comparison of the PGA spatial distributions from these three earthquakes, a common feature that a clear contrast is seen between the forearc and backarc sides, where high-frequency seismic wave amplitudes decay rapidly in the backarc side, is observed among them, similar to the SU1 case. Although the focal depths of the three events have difference of about 20km among them, the incident angles of the seismic rays from the sources to the stations are commonly high angles, since the 40km depth of the shallowest P is sufficiently deep and the epicenter is close to the land area. Therefore, significant difference is not seen in the attenuation of high-frequency waves in the backarc side among the three events. On the other hand, a remarkable difference is seen between the shallowest event P and the deeper events of SU2 and SL, with regard to the absolute amplitude of overall PGA distribution. The amplitude level of the large acceleration area close to the epicenter of P is clearly lower than those of the deeper SU2 and SL. Additionally, while the spatial extents of the observed accelerations reach Hokkaido prefecture (= the accelerometers are triggered) in case of the deeper SU2 and SL, that of the shallowest P reaches only the southern part of Aomori prefecture at farthest. This indicates the overall acceleration level brought by the shallow P is lower than those by deeper SU2 and SL. This is an reflection of the finding by Kasatani and Kakehi (2014) based on the spectral inversion of strong ground motions of interplate and intraslab earthquakes in northeastern Japan: "high-frequency levers of source effects depend not on the tectonic environments but simply on the focal depths and the events with deeper focal depths have higher high-frequency levels."