Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG65] 20th anniversary of the Kobe earthquake: Progress in the studies of active faults and strong motion

Tue. May 26, 2015 9:00 AM - 10:45 AM A04 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Hisashi Nakahara(Solid Earth Physics Laboratory, Department of Geophysics, Graduate School of Science, Tohoku University), Haruo Horikawa(Active Fault and Earthquake Research Center, National Institute for Advanced Science and Technology), Tadashi Maruyama(Ministry of Education, Culture, Sports, Science and Technology), Chair:Haruo Horikawa(Institute of Earthquake and Volcano Geology, National Institute for Advanced Science and Technology), Tadashi Maruyama(Institute of Earthquake and Volcano Geology, National Institute of Advanced Industrial Science and Technology)

10:25 AM - 10:40 AM

[SCG65-06] Characteristics of radiated short-period seismic energy from moderate-to-large inland earthquakes

*Hisashi NAKAHARA1 (1.Graduate School of Science, Tohoku University)

Keywords:inland earthquakes, short-period seismic energy, envelope inversion

It is in 1990s that envelope inversion methods have been developed to study earthquake source processes in shorter period ranges than about 1s (e.g. Gusev et al., 1991; Zeng et al., 1993; Kakehi and Irikura, 1996). Motivated by the occurrence of the 1995 Hyogo-ken Nanbu earthquake, the authors developed an envelope inversion method (Nakahara et al., 1998) to estimate the spatial distribution of short-period seismic energy radiation on earthquake source faults. So far, we have applied the inversion method to more than 10 moderate-to-large earthquakes. Compiling the results, we consider statistical characteristics of short-period seismic energy radiation from the earthquakes especially focusing on inland earthquakes.

We have so far analyzed 6 inland earthquakes: the 1995 Hyogo-ken Nanbu (Kobe) earthquake (Mw6.9), the 1998 Northern Iwate Prefecture earthquake (5.8), the 1999 Chi-Chi, Taiwan, earthquake (7.6), the 2000 Tottori-ken Seibu earthquake (6.6), the 2004 Niigata-ken Chuetsu earthquake (6.6), and the 2008 Iwate-Miyagi Inland earthquake (6.6). We compile the results in terms of 3 items:

1. Scaling law of short-period seismic energy
The logarithm of seismic energy in short period bands (1-2, 2-4, 4-8, and 8-16Hz) is found to be proportional to the moment magnitude. In other words, the short-period seismic energy is proportional to the fault area. The absolute values for 6 inland earthquakes are explained well by the omega-squared model with the short-period spectral level A obtained by Dan et al. (2001). On the contrary, the absolute values for the other plate-boundary type and intra-slab type earthquakes are about 10 times larger than those for the inland earthquakes.

2. Spatial relationship between asperities and short-period sources
We compare relations between locations of asperities estimated by waveform inversions in longer periods and locations of short-period energy radiation obtained by the envelope inversion in shorter periods. The relation is found to be complimentary for 3 earthquakes of the 1998 Iwate, the 1999 Chichi, and the 2000 Tottori. But the relation is complex for the other 3 earthquakes.

3. Statistics of short-period seismic energy radiation
Seismic energy radiation is estimated for each subfault and each period band by the envelope inversion method. Plotting energies from all the subfaults in a descending order, we find that the logarithms of the energies are linearly decrease against the order. This is explained by the two-parameter Weibull distribution. The slope is characterized by the shape-parameter which ranges from 0 and 2 depending on earthquakes and period bands. The shape parameter of 1 corresponds to the exponential distribution and that of smaller than 1 means more heavy-tailed distributions.

Given these results, we need to physically understand the statistical characteristics of short-period seismic energy radiations and to take them into account in predicting strong-ground motions.