JpGU-AGU Joint Meeting 2020

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS14] Crustal Deformation

convener:Tadafumi Ochi(Institute of Earthquake and Volcano Geology, Geological Survey of Japan, The National Institute of Advanced Industrial Science and Technology), Masayuki Kano(Graduate school of science, Tohoku University)

[SSS14-P11] Strain changes due to the co-seismic fault slips observed in Kobe

*Atsushi Mukai1, Shigeaki Otsuka2, Yoichi Fukuda3 (1.Faculty of Urban Management, Fukuyama City University, 2.Former Professor at Kobe Gakuin University, 3.Graduate School of Science, Kyoto Univ.)

Keywords:strain changes, fault slip

Significant strain changes due to the 2018 northern Osaka earthquake were observed at Rokko-Takao station about 45 km away from the focus. These strain changes showed the step-like extension in the east-northeast direction for a few seconds just after arrival of the seismic waves. The direction of the maximum principal strain agreed with the calculated one based on the source process reported by Disaster Prevention Research Institute, Kyoto University (2018). We can reproduce the characteristics of the time development of the observed principal strains, if the rupture on the seismic fault had been expanding at 3 km/s from the focus.

Rokko-Takao station in Kobe passes through the fracture zone of Manpukuji fault. We have performed the continuous observation of crustal movements by using the Ishii-type three-component strainmeter and so on. When the 2018 northern Osaka earthquake (M6.1) occurred, we observed the strain changes with time constant of a few minutes and the extension of the order of 0.1 μstrain in the north-northwest direction. This maximum principal strain were far from the calculated one (+0.03 μstrain in the direction of N56oE) based on the source process, and was considered to be caused by the contraction of the fracture zone and the pore pressure increasing (Mukai et al., 2018). As shown by this case, the observation of strain changes at this station is significantly affected by the groundwater migration. However, we can expect to detect the strain step due to the source process just after the arrival of seismic wave, because the groundwater migration starts with some delay.

We observed the linear strain changes just after the arrival of the seismic waves and, as a result, the strain steps of the order of 0.01 μstrain. The maximum and minimum principal strains were +0.06 and -0.03 μstrain, respectively. The direction of the maximum principal strain was N61oE. This direction almost agreed with the calculated one based on the source process, although the magnitudes of the principal strains were about double of the calculated ones. It is considered that these discrepancies in the magnitude were caused by the dynamic properties of the surrounding crust with the fracture zone, and major part of the observed strain steps was caused by the co-seismic fault slips.
It is considered that the linear strain changes just after the arrival of seismic motions showed the crustal movements due to the time development of co-seismic fault slips, that is, the source process. In fact, we can reproduce the characteristics of the observed principal strain changes by using the source process with the rupture velocity of 3 km/s. However, there also remained some negligible discrepancies between the observed and the calculated principal strains. These discrepancies might be caused by the effect of pore pressure changes in the surrounding crust. In this study, we report the investigated results on the strain steps observed at the 2011 off the Pacific coast of Tohoku Earthquake as well.