11:00 AM - 1:00 PM
[SSS08-P01] S-wave later phase at the northern Ibaraki area and the estimation of the crustal reflector
Keywords:Crustal earthquake, North Ibaraki area, S-wave later phase, Reflector
Seismicity at the northern Ibaraki prefecture and Fukushima Hamadori areas are active after the 2011 Tohoku-oki Earthquake. In these areas, active seismicity has been observed in the upper crust and at depths of 15-20 km. The depths of the deep crustal seismicity corresponded to that of a crustal reflector estimated from the reflected S-waves [Iidaka et al. 2019, JpGU; Usuda et al. 2020, JpGU]. The observations of reflected waves implied a presence of fluid. Therefore, it was considered that the crustal fluids were related to the activity of crustal earthquakes in and around the northern Ibaraki area.
The Geological Survey of Japan (GSJ), AIST, has maintained the off-line seismograph array at the northern Ibaraki area from June 2016. The GSJ array employs seven stations with ~5 km spacing. We identified reflected S waves on the seismic records observed in the GSJ array.
We, therefore, investigated features of the reflected S waves and their sources using the seismic records observed at the GSJ array and the surrounding permanent stations. First, we determined hypocenters of the crustal earthquakes and the one-dimensional velocity model in and around the north Ibaraki area. The arrival time data in the JMA unified catalog was adapted to the approach of Shiina and Kano [accepted]. Second, we identified reflected S waves in the GSJ array and three Hi-net stations (N.THGH, N.KIBH (N.KI2H), and N.JUOH) by visual inspections. Then, we estimated the reflector (or scattering point) using the delay times of the reflected S waves from the direct S waves. The previously determined hypocenters and the one-dimensional velocity model were used to compute the theoretical delay times.
The reflectors were estimated at the depths of 10-15 km beneath the northern Ibaraki area, which was almost consistent with those estimated by Iidaka et al. [2019]. The possible crustal reflectors showed the complementary distribution to the crustal earthquakes. These results suggest that characteristics of crustal structure in the excitation zone of the reflected S waves will be different from those in areas that activated the crustal seismicity were different.
Acknowledgment:
We used Hi-net data from NIED and the JMA Unified Earthquake Catalog. This study was supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) Grant Number JPJ010217.
The Geological Survey of Japan (GSJ), AIST, has maintained the off-line seismograph array at the northern Ibaraki area from June 2016. The GSJ array employs seven stations with ~5 km spacing. We identified reflected S waves on the seismic records observed in the GSJ array.
We, therefore, investigated features of the reflected S waves and their sources using the seismic records observed at the GSJ array and the surrounding permanent stations. First, we determined hypocenters of the crustal earthquakes and the one-dimensional velocity model in and around the north Ibaraki area. The arrival time data in the JMA unified catalog was adapted to the approach of Shiina and Kano [accepted]. Second, we identified reflected S waves in the GSJ array and three Hi-net stations (N.THGH, N.KIBH (N.KI2H), and N.JUOH) by visual inspections. Then, we estimated the reflector (or scattering point) using the delay times of the reflected S waves from the direct S waves. The previously determined hypocenters and the one-dimensional velocity model were used to compute the theoretical delay times.
The reflectors were estimated at the depths of 10-15 km beneath the northern Ibaraki area, which was almost consistent with those estimated by Iidaka et al. [2019]. The possible crustal reflectors showed the complementary distribution to the crustal earthquakes. These results suggest that characteristics of crustal structure in the excitation zone of the reflected S waves will be different from those in areas that activated the crustal seismicity were different.
Acknowledgment:
We used Hi-net data from NIED and the JMA Unified Earthquake Catalog. This study was supported by MEXT Project for Seismology toward Research Innovation with Data of Earthquake (STAR-E) Grant Number JPJ010217.