[SSS04-13] Strong ground motions from deep earthquakes in Hokkaido and attenuation structure of island arc
Keywords:strong ground motion, attenuation structure, island arc, Hokkadio, Hidaka collision zone
High-density strong ground motion data by NIED K-NET and KiK-net are analyzed to investigate the relation between the ground motions by deep earthquakes in Hokkaido and the attenuation structure of island arc. In this study, the strong ground motions from the following three events are analyzed: (1) 2020 Southeast Off Nemuro Peninsula earthquake (Mw 5.4, 96.0km depth, intraslab earthquake), (2) 2007 Mid-eastern Iburi earthquake (Mw 5.5, 126.18km, intraslab earthquake), and (3) 2015 East Off Aomori Prefecture earthquake (Mw 6.1, 66.07km, interplate earthquake). In northeastern Japan, a clear feature that the accelareation amplitudes in the backarc side are smaller than those in the forearc side is commonly seen among the three events. This is because of the low-Q medium of the volcanic front and the backarc side (e.g. Kakehi (2015)). On the other hand, spatial distribution of strong ground motions in Hokkaido is more complex. In case of event 1, large acceleration amplitudes are seen only within the eastern side of the Hidaka moutains, and the those in the western area of the Hidaka moutains are small though it is in the forearc side. This feature, small acceleration amplitudes in the western side of the Hidaka mountains, is also seen in event 2 and 3.The Hidaka mountains is an area of arc-arc collision (called "Hidaka collision zone"), and the underground structure (including attenuation structure) in the area is very complex. Previous studies (Kita et al. (2014) , Nakamura and Shiina (2019)) reported that Hokkaido has a low-Q area in the western side of Hidaka collision zone though the area is in the forearc side. The area with small acceleration amplitudes seen for the three events is intepreted to be caused by the attenuation of high-frequency seismic waves propagating through this low-Q area in the forearc side. Thus, in Hokkaido, more complex distribution of strong ground motion is seen than in northeastern Japan because of the existence of low-Q area in the forearc side, in addition to the clear contrast of attenuation structrue between the forearc and backarc sides.