[S-SS10] Strong Ground Motion and Earthquake Disaster

[SSS10-05] Characteristics of the strong ground motions from the 2022 Hyuganada intraslab earthquake

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

Keywords:strong ground motions, island arc, focal depth, attenuation

Characteristics of the strong ground motions from the 2022 Hyuganada intraslab earthquake (Jan. 22, M_W 6.4, 44.6 km depth) are investigated using strong ground motion data recorded by the NIED K-NET and KiK-net high-density observation network. First, spatial distribution of peak ground accelerations (called PGA hereafter) from the 2006 Oita-ken Seibu intraslab earthquake (M_W 6.4, 145.17 km) ((A) of Figure) is explained for comparison. The figure clearly shows that PGA distribution of this event is singular one, i.e., obviously eastward spatially-biased distribution. This spatially-biased distribution is interpreted to have been caused by the attenuation structure of the island arc. More specifically, the spatially-biased distribution of shaking is interpreted to have been brought by the following three causes, (a) high-frequency seismic waves attenuate in the low-Q backarc side, (b) extent of large PGA area is enlarged along the high-Q forearc side, and (c) extent of large PGA area is enlarged toward the shallower extension of the high-Q slab. Additionally, in the case of the 2006 event, the path lengths propagating in the low-Q or high-Q media are remarkably long due to the extremely deep focal depth of 145.17 km, and it brings the more emphasized spatial contrast between the areas with attenuation and those without attenuation. On the other hand, clear spatial bias is not seen in the PGA distribution of the 2022 Hyuganada intraslab earthquake. Strictly speaking, however, spatial bias does exist also in the case of this event. In (B) of figure, distribution of large acceleration shows an elliptical shape, rather than concentric distribution, as clearly shown by the 10 cm/s² contour line (red solid line in the figure). This elliptical-shape distribution is composed of following three elements, (1) amplitude is small in the western Kyushu, northwestern side of the epicenter, (2) extent of large PGA area is enlarged slightly toward southwestern side, and (3) extent of large PGA area is enlarged slightly toward northeastern side. Each of these three elements are estimated to come from the above three causes (a), (b), and (c), respectively. Thus, spatially-biased distribution of shaking reflecting the attenuation structure is also seen in the case of the 2022 event, but the strength of the bias is by far inferior to that of the 2006 event case. This difference comes from the difference of the focal depths of the two events. In the 2022 event case with the relatively shallow focal depth of 44.6 km, clear amplitude difference cause by the attenuation structure is not produced since the path lengths propagating in the low-Q or high-Q media are short. Additionally, the effect of geometrical spreading factor becomes significant due to the shallow focal depth, and large PGA area near the epicenter becomes outstanding, and it masks the spatially-biased distribution of shaking. Since this effect is added in the 2022 event case, spatial bias becomes more unclear.