Rupture Process of the 2018 Northern Osaka Earthquake (Mw 5.6), an Earthquake Involving Both Thrust and Strike-slip Faults Near a Junction of Major Active Fault Systems Surrounding the Osaka Basin, Japan

An M_JMA 6.1 (M_W 5.6) inland earthquake occurred near the northeast edge of the Osaka basin, Japan, at a depth of 13 km, on June 18, 2018 (June 17 in UT). The Osaka basin is an elliptically-shaped sedimentary basin bounded by major active fault systems. The maximum PGA and PGV obtained by the seismic intensity observation network are 900 gal and 44 cm/s, respectively. This event impacted regional community, though observed ground motions were much lower than as predicted for anticipated large earthquakes (M7 or more) along nearby major active faults. Since this event occurred near the Arima-Takatsuki fault system (strike-slip) and the Uemachi fault system (thrust), the relationship between the activated fault and nearby known faults should be investigated in detail.

the source rupture process of this event was analyzed using strong motion waveforms. Since routine CMT solutions by JMA, NIED, Global CMT, and others include large CLVD, we firstly analyzed CMT of this event by the Bayesian full-waveform inversion considering the uncertainty of the Green's functions using the method by Hallo et al. (2017). The inferred CMT had a significant CLVD component and it can be decomposed into major- and minor-DC sources preserving P-axis (WNW-ESE direction). The major- and minor-DCs have strike-slip and thrust type mechanisms, respectively.

The spatiotemporal rupture process was estimated by the finite source inversion using the multiple time-window method (e.g., Hartzell and Heaton, 1983) assuming two fault planes corresponding to the major- and minor-DCs. The earthquake started on the thrust fault with north-south strike, then, the right-lateral strike-slip fault in NE-SW direction ruptured almost simultaneously ( 0.2 s) toward shallow and southwest direction. The southwestward rupture of this strike-slip fault caused the forward directivity effect in the northern part of the Osaka basin. This strike-slip fault overlaps with a belt of micro-earthquakes along parallel to the Arima-Takatsuki fault system, and it is separatedfrom the Arima-Takatsuki fault system by 2-3 km The geometry of the thrust-fault looks similar to that of the Uemachi fault systems, but, it is not clear that this event ruptured a part of the Uemachi fault because aftershock activity does not spread over the Uemachi fault.