[M-IS04] Interdisciplinary studies on pre-earthquake processes

[MIS04-10] Possible explanation of fluctuation of vertical ground motion before large earthquakes

*Koichiro Oyama^2,3, C.K. Chen¹ (1.Departments of Earth Sciences, National Cheng Kung University, Tainan, Taiwan, 2.Colt. Asia Space Environment Research Consortium, Sagamihara, Japan, 3. International Research Center for Space and Planetary Environmental Science, Kyushu University, )

Keywords:Earthquake, Vertical Ground motion, Tilt meter, Frequency spectrum

Vertical ground motion observed by slant gages are studied. The data acquired (Hi-net data) were downloaded from National Research Institute for Earth Science and Disaster Resilience (www.bosai.go.jp). So far, we picked up 5 earthquakes. 5 earthquakes are:1 Chuetsu offshore EQ, M6.6,2007/07/16 01:13:22UTC, 2.Tohoku-oki EQ M9,2011/03/11 05:46:24UTC, 3.KumamotoEq, M7.0,2016/04/15 16:25:06 UTC 4.Hokkaido Eastern Iburi EQM6.62018/09/05 18:07:58UTC,5.Yamagata M6.4,2019/06/18,13:22:19UTC.
Tilt meter detects the slant of the ground with the accuracy of 10^-8 Radians at the location where the gage is installed. Frequency spectrum is calculated from the raw data, and The Power Spectrum Density (PSD) was calculated. Both frequency spectrum and Power Spectrum density of 5 earthquakes show the similar behavior. We speculate that this behavior reflects the fracturing process of the rocks, water flowing to the fractured rocks, and again strengthening of the rocks (dilatancy hardening) under the stress. Details will be discussed.

Reference
Scholz, C. H., L.R. Sykes, and Y. P. Aggawal, Earthquake Prediction: A physical basis, Science, 181, No 4102, 803-809, 1973.