6:15 PM - 7:30 PM
[SSS32-P03] Seismic activity and attenuation structure aroud the Fukushima and Yamagata Prefectures' border after Tohoku earthquake
Keywords:attenuation structure, Seismic activity, swarm, hypocenter migration, fluid
Around the border between Fukushima and Yamagata prefectures, seismicity was suddenly activated after off the Pacific coast of Tohoku earthquake. We estimated distribution and focal mechanisms of the earthquakes to clarify features of the seismic activity. A three?dimensional seismic structure in the northeastern Japan derived in a previous study showed that fluid might affect the seismic activity in this swarm. A Qp/Qs and Qs value greatly change by existence of fluid. Then, we estimated the Q values to clarify physical properties in this region by taking velocity amplitude spectral ratio between P and S waves.
First, we found that hypocenters were concentrated into four clusters. We also observed hypocenter migration to lateral and vertical direction in some clusters. Most earthquakes have the thrust-type focal mechanisms. Average Qp/Qs and Qs values on the ray paths from hypocenters to stations show high Qp/Qs and low Qs at relatively near stations from source region. While paths from the hypocenters to far stations show low Qp/Qs and high Qs. This feature might indicate that high attenuation region exists in nearby source region. Further, we estimated a detail Q structure of the swarm area by using combination of spectra ratio data which have very similar ray paths. As a result, the blocks in which many earthquakes occurred have high Qp/Qs and low Qs, whereas those for the region between the clusters show vice versa. When we compared with other geophysical data, this high Qp/Qs and low Qs values seem to reflect the influence of fluid. Hypocenter migration would be explained by upward migration of fluids due to difference of density.
First, we found that hypocenters were concentrated into four clusters. We also observed hypocenter migration to lateral and vertical direction in some clusters. Most earthquakes have the thrust-type focal mechanisms. Average Qp/Qs and Qs values on the ray paths from hypocenters to stations show high Qp/Qs and low Qs at relatively near stations from source region. While paths from the hypocenters to far stations show low Qp/Qs and high Qs. This feature might indicate that high attenuation region exists in nearby source region. Further, we estimated a detail Q structure of the swarm area by using combination of spectra ratio data which have very similar ray paths. As a result, the blocks in which many earthquakes occurred have high Qp/Qs and low Qs, whereas those for the region between the clusters show vice versa. When we compared with other geophysical data, this high Qp/Qs and low Qs values seem to reflect the influence of fluid. Hypocenter migration would be explained by upward migration of fluids due to difference of density.