5:15 PM - 7:15 PM
[SSS12-P01] Comparison of stress fields between western and eastern Aomori Prefecture
Aomori Prefecture, located around the junction between the northeastern Japan arc and the Kurile arc, is characterized by a complex geological and tectonic setting and active faults with a potential to cause magnitude-7 class earthquakes are distributed (Headquarters for Earthquake Research Promotion, 2021). To understand the tectonic setting of the region and to evaluate these active faults, it is important to investigate the physical process of the seismicity in Aomori. For this purpose, we have examined the seismicity in the eastern part of the Aomori (Maeda et al., 2020), but the seismicity in the western Aomori has not been studied well. In this study, we examine the seismicity in the western part of Aomori to understand the stress field and discuss the spatiotemporal variations in Aomori Prefecture.
We first picked P-wave first-motion polarities of 179 micro and small earthquakes in western Aomori with magnitude 2.0 or greater and with depths between 0 km and 25 km for the period from June 2002 to December 2017, using the seismograms of NIED Hi-net. We then determine the focal mechanisms of these earthquakes using the software pick2mec (Maeda 1992; Katao and Iio 2004), with the velocity structure of Hasegawa et al. (1978). Among them, we used the reliable 40 mechanisms to determine the stress field using the method developed by Michael (1984, 1987). We further determine the stress field using the focal mechanisms before (20 events) and after (20 events) the 2011 Tohoku earthquake to examine the impact of the megathrust earthquake.
Our results indicate the western part of Aomori Prefecture is characterized by a reverse faulting stress regime with the σ1 axis being sub-horizontal and trending WNW–ESE, which is a typical stress field in northeastern Japan, expected from the horizontal compression due to the subduction of the Pacific plate. This feature is also observed in the results of analyses using focal mechanisms for the periods before and after the 2011 Tohoku-Oki earthquake separately. This result indicates the western Aomori Prefecture was not affected by the static stress change caused by the Tohoku-Oki earthquake. This is in contrast to the stress field in the eastern Aomori Prefecture, which is considered to be affected not only by the plate convergence but also by the bending of the upper plate, and was significantly changed after the Tohoku-Oki earthquake (Maeda et al. 2020). The estimated stress field may reflect a local stress around Mt. Iwaki, because the earthquakes used for determining the stress field were concentrated there. However, this insignificant change in the stress field in western Aomori after the Tohoku-Oki earthquake may be reasonable because western Aomori is located far from the Japan trench as well as the source region of the Tohoku-Oki earthquake; the background WNW-ESE compressional stress is expected to be large because the effect of the upper-plate bending should be very weak, and the stress change due to the Tohoku-Oki earthquake is as small as less than 0.5 MPa there (Yoshida et al., 2012).
A part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (Secretariat of NRA), Japan.
We first picked P-wave first-motion polarities of 179 micro and small earthquakes in western Aomori with magnitude 2.0 or greater and with depths between 0 km and 25 km for the period from June 2002 to December 2017, using the seismograms of NIED Hi-net. We then determine the focal mechanisms of these earthquakes using the software pick2mec (Maeda 1992; Katao and Iio 2004), with the velocity structure of Hasegawa et al. (1978). Among them, we used the reliable 40 mechanisms to determine the stress field using the method developed by Michael (1984, 1987). We further determine the stress field using the focal mechanisms before (20 events) and after (20 events) the 2011 Tohoku earthquake to examine the impact of the megathrust earthquake.
Our results indicate the western part of Aomori Prefecture is characterized by a reverse faulting stress regime with the σ1 axis being sub-horizontal and trending WNW–ESE, which is a typical stress field in northeastern Japan, expected from the horizontal compression due to the subduction of the Pacific plate. This feature is also observed in the results of analyses using focal mechanisms for the periods before and after the 2011 Tohoku-Oki earthquake separately. This result indicates the western Aomori Prefecture was not affected by the static stress change caused by the Tohoku-Oki earthquake. This is in contrast to the stress field in the eastern Aomori Prefecture, which is considered to be affected not only by the plate convergence but also by the bending of the upper plate, and was significantly changed after the Tohoku-Oki earthquake (Maeda et al. 2020). The estimated stress field may reflect a local stress around Mt. Iwaki, because the earthquakes used for determining the stress field were concentrated there. However, this insignificant change in the stress field in western Aomori after the Tohoku-Oki earthquake may be reasonable because western Aomori is located far from the Japan trench as well as the source region of the Tohoku-Oki earthquake; the background WNW-ESE compressional stress is expected to be large because the effect of the upper-plate bending should be very weak, and the stress change due to the Tohoku-Oki earthquake is as small as less than 0.5 MPa there (Yoshida et al., 2012).
A part of this research project has been conducted as the regulatory supporting research funded by the Secretariat of Nuclear Regulation Authority (Secretariat of NRA), Japan.