[S-SS05] Fault Rheology and Earthquake Physics

[SSS05-P07] Rupture directivity of moderate-sized aftershock of the 2016 Kumamoto Earthquake estimated by dense observation

*Taiki Kono¹, Satoshi Matsumoto², Kentaro Emoto², Takeshi Matsushima², Hiroshi Shimizu³, Kazuya Uchida², Azusa Mori⁵, Takuji Watanabe¹, Yota Suzuki¹, Ryunosuke Takayama¹, Shungo Tasa¹, Shinichi Sakai⁴ (1.Kyushu University, 2.Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, 3.Association for the Development of Earthquake Prediction, 4.The University of Tokyo III/GSII, 5.Graduate School of Environmental Studies, Nagoya University)

Keywords:Rupture directivity

Earthquakes are caused by fault slip, and how rupture propagates along the fault plane is an important issue in understanding structure of earthquake. For large events, the direction of rupture propagation has been required as one of the results in the analysis of source process for a relatively long period. In recent years, there have been attempts to estimate the direction of rupture not only for large inland earthquakes but also for small- and medium-scale inland earthquakes in Japan by dense observations. However, there are only a few cases in which the direction has been studied in detail in the region where strike-slip earthquakes occur. In this study. Thus, we estimate the direction of rupture propagating for one aftershock of the Kumamoto earthquake (2016).
It is known that apparent rupture duration time for a case of unilateral rupture along a fault depends upon the fault geometry relative to the station. Based on unilateral rupture assumption, we attempted to measure widths of source time function. Distribution of the widths from various stations enable us to estimate the direction of rupture propagation. A target earthquake we analyzed was MJMA4.7 earthquake that occurred on June 26, 2022, which was located at around the center of dense seismic observations by Kyushu University. We adopted MJMA1.4 earthquake on June 27, 2022, at distance of 0.5 km from the target earthquake, as a reference event. The widths of the target earthquake at various station are obtained by iterative deconvolution method with regarded the reference earthquake as EGF of the target earthquake and their characteristics were investigated. This method can apply to a complex rupture process observed in a records limited frequency band width.
On the other hand, as a simplest method, pulse width analysis measuring first P-wave width in displacement record, which was normalized by it for reference event show shorter pulse widths observed in direction parallel to the strike of the fault of this target earthquake. In addition, as a standard method, corner frequencies estimated from spectral ratio of P-waves of the target event to the reference one showed similar tendency to the pulse width analysis. We discuss rupture direction estimated from EGF are compared with the directions estimated with other method. Optimal selection for the methods can be used to obtain more accurate rupture direction for a larger number of earthquakes.