Attenuation characteristics of strong ground motion from deep-focus earthquakes around the Ogasawara Islands, Japan

Abstract:

The 2015 deep-focus earthquake off the Ogasawara Islands, Japan (Mw 7.9, 680 km depth) caused strong ground motion that was felt over whole Japan with a maximum seismic intensity of 5-upper on the JMA scale. The observed seismic intensity in east Japan was larger than that in west Japan even if they were observed at the equal hypocentral distance. To understand the attenuation characteristics of strong ground motion from deep-focus earthquakes around the Ogasawara Islands, we investigate the period-dependent characteristics of their strong ground motion using response acceleration spectra (Sa) and conduct a regression analysis of the ground motion prediction equation for Sa.

First, using waveforms recorded by F-net of NIED during the 2015 event, we obtained 5 % damped horizontal Sa in the period from 0.1 s to 10 s at F-net stations. In the long period band (≥ 4 s), Sa in east Japan is comparable to that in west Japan at the equal hypocentral distance. On the other hand, in the short period band (≤ 1 s), Sa in east Japan is much larger than that in west Japan at the equal hypocentral distance. We also obtained Sa at F-net stations for four other deep-focus earthquakes around the Ogasawara Islands. They also have a trend similar to that of the 2015 event.

Then, we evaluated the ground motion prediction equation for Sa whose parameters are Mw and hypocentral distance X: log Sa = a*Mw + (R1*b1+R2*b2)*X - log X + c. In this equation, we assumed the geometric attenuation of body waves. b1 and b2 are coefficients in west and east Japan for the attenuation except for the geometric attenuation, respectively. R1 is 1 and 0 at stations in west and east Japan, respectively, and R2 is vice versa. The regression coefficients (a, b1, b2, and c) were estimated by the two-step linear regression analysis (Kataoka et al., 2008). In the long period band (≥ 4 s), there is not much difference between the values of b1 and b2. On the other hand, in the short period band (≤ 1 s), the absolute value of b2 is small compared to that of b1. This result indicates that the internal and/or scattering attenuation along the wave path from the deep Ogasawara-Islands region to east Japan is weaker than that along the wave path to west Japan. This is expected to be caused by whether there is a high-Q subducting oceanic plate along the wave path or not.