Abstract
On June 18, 2018, an MJMA6.1 inland crustal earthquake occurred on the northeast edge of the Osaka basin, Japan. This event impacted the region by the maximum PGA larger than 0.9 g, and it was followed by a series of weaker aftershocks. The earthquakes were located near the Arima-Takatsuki Tectonic Line (ENE–WSW dextral strike-slip faults) and the Uemachi fault system (N–S reverse faults), hence the seismotectonic interpretations we assumed to be rather complex. Here we propose a seismotectonic model of this sequence based on seismological data and stress field considerations. In particular, we infer to a centroid moment tensor for the mainshock using Bayesian full-waveform inversion from strong motion records. The solution of Mw5.6 involved a significant CLVD component, which we interpreted as being due to rupture process on a complex fault geometry. Decomposition of the non-DC moment tensor into major and minor pure-shear moment tensors suggests a combination of strike-slip and reverse faulting mechanisms. We also analyzed the 108 strongest aftershocks with MJMA between 2.0 and 4.1 using records from broadband and short-period stations. Aftershocks’ moment tensors inverted from P-wave amplitudes exhibit mainly strike-slip and reverse faulting mechanisms, having significant spatial variations. The local stress field inverted from these mechanisms had a dominant maximum (compressional) principal stress σ1 in ESE–WNW direction, while σ2 ≅ σ3. Both ENE–WSW dextral strike-slip and N–S reverse faults can be active in such stress field as observed in the mainshock (without any need for stress spatial inhomogeneity). To conclude, the activated strike-slip fault is parallel to the Arima-Takatsuki Tectonic Line. The activated N–S reverse fault is dipping to east by 50° similarly as the Uemachi fault system. Joint shear movements on both of these faults contributed significantly to the total seismic moment of the mainshock.
Highlights
The 2018 northern Osaka earthquake mainshock–aftershocks sequence was started on June 18, 2018 (07:58 of Japan Standard Time), by an MJMA6.1 mainshock
Bayesian inference of CMT In the low frequency point source approximation, the earthquake source model can be described by the centroid moment tensor (CMT, e.g., Aki and Richards 2002, pp. 49–52)
The CMT is inferred from observed waveforms by inverse modeling, which is subject to uncertainty (e.g., Zahradník and Custódio 2012)
Summary
The 2018 northern Osaka earthquake mainshock–aftershocks sequence was started on June 18, 2018 (07:58 of Japan Standard Time), by an MJMA6.1 mainshock. The hypocenter was located by the Japan Meteorological Agency (JMA) at a depth of 13 km under Takatsuki city (34.844°N, 135.622°E) in the Osaka metropolitan area, Kansai, Japan (see Fig. 1). This event generated strong ground motions in the near-source region, causing damage on a local scale and four casualties. The earthquake damage and response are briefly summarized in Hirata and Kimura (2018). The maximum measured peak ground acceleration (PGA) exceeded 0.9 g, and the instrumental intensity in the epicentral area reached a degree 6-lower of the 7-degree JMA scale. The source mechanism of this earthquake has been routinely investigated by JMA, National Research Institute for Earth
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