[S-SS08] Fault Rheology and Earthquake Physics

[SSS08-P14] Initial rupture of two inland earthquakes (Mw 5.9, 5.2) in the northern Ibaraki prefecture, Japan

*Kaho Hashimoto¹, Takuji Yamada² (1.Ibaraki University, 2.Faculty of Science, Graduate School of Science and Technology, Ibaraki University)

Keywords:initial rupture, source process, northern part of Ibaraki Prefecture

In this paper, we analyzed characteristics of initial part of P waves for two different earthquakes that occurred in the northern area of Ibaraki prefecture in December 2016 (Mw5.9) and March 2012 (Mw5.2) by using waveforms of an earthquake with Mw3.8 as an Empirical Green’s functions. Hereafter, we call the smaller and larger earthquakes for each pair of earthquakes as EGF- and Obs-events, respectively. We first picked arrival times of P waves by using Hi-net data, which operated by the National Research Institute for Earth Science and Disaster Resilience, and retrieved 2-s-long waveforms. We then applied band-pass filter from 0.5Hz to 3.0Hz to the waveforms. We finally carried out the waveform inversion and investigated the source process of the two earthquakes under four different analysis conditions. We obtained consistent results for all the conditions (Fig. 1), suggesting that the results are reliable. The seismic moment roughly corresponding to Mw4.5 was released during initial rupture for both of the two earthquakes. In addition, the initial part of the earthquake with Mw5.2 had a greater seismic moment than that of the other earthquake (Mw5.9). This implies that it would be quite difficult to estimate the eventual size from the initial part of the earthquake rupture. We are going to investigate hole rupture process using P waveforms at far stations as well as S waves in the near the future.