Source fault model of the 2018 Mw 5.6 northern Osaka earthquake, Japan, inferred from the aftershock sequence

Abstract

We reconstructed the spatiotemporal evolution of seismicity associated with the 2018 M w 5.6 northern Osaka earthquake, Japan, to discuss the source fault model of the mainshock rupture, the possible link between this rupture and known active faults, and subsequent crustal deformation. We first relocated the hypocenters listed in the earthquake catalog determined by the Japan Meteorological Agency using a double-difference relocation algorithm. We then searched for the earthquake waveforms that closely resembled those of the relocated hypocenters by applying a matched filtering technique to the continuous waveform data. The relocated hypocenters revealed two distinct planar alignments with different fault geometries. A combination of the relocated hypocenters and focal mechanisms suggests that the mainshock rupture initiated on a NNW-SSE-striking thrust fault, dipping ~ 45° to the east, with the rupture propagating to an adjacent sub-vertical ENE-WSW-striking strike-slip fault ~ 0.3 s after the initial mainshock rupture, resulting in the simultaneous propagation of dynamic rupture along the two faults. The strike-slip fault is oblique to the strike of the Arima-Takatsuki Fault, indicating that blind strike-slip faulting occurred. While the eastdipping thrust fault is located deeper than the modeled extent of the Uemachi Fault, a simple extrapolation of the near-surface geometry of the Uemachi fault partially overlaps the mainshock rupture area. Although it is unclear as to whether a blind thrust fault or a deep portion of the Uemachi Fault ruptured during this mainshock-aftershock sequence, a mainshock rupture would have transferred a static stress change of > 0.1 MPa to a portion of the eastdipping thrust fault system. Intensive aftershocks have persisted along the northern and southern edges of the source area, including moderate-magnitude events, whereas the seismicity in the central part of the source area has shown a rapid decay over time. Delayed triggered aftershocks were clearly identified along the northern extension of the rupture area. Because the background seismicity is predominant in this northern area, we interpret that aseismic deformation, such as cataclastic flow lubricated by crustal fluids, triggered this off-fault seismicity.

Seismotectonics of the 2018 northern Osaka M6.1 earthquake and its aftershocks: joint movements on strike-slip and reverse faults in inland Japan

“…It suggests that the mainshock nucleated along the N-S trending reverse fault and then spread also along the dextral strike-slip fault. Our model of the mainshock is basically consistent with model by Kato and Ueda (2019) that was derived from spatiotemporal distribution of aftershocks relocated by double-difference method.…”

Non‐Double‐Couple Microearthquakes in the Focal Area of the 2000 Western Tottori Earthquake (M 7.3) via Hyperdense Seismic Observations

“…Numerous studies have also discussed the physical mechanisms that generate the NDC component in the earthquake focal mechanism, e.g., a combined tensile crack and fault plane caused by high fluid pressure in geothermal and volcanic areas (e.g., Martínez-Garzón et al, 2017;Miller et al, 1998;Šílený & Horálek, 2016) and a complex rupture with multiple subevents separated in space and time (e.g., Frohlich, 1994; Kato & Ueda, 2019;Sipkin, 1986). NDC earthquakes with isotropic components have also been reported in the tectonic fault zone (e.g., Ross et al, 2015;Stierle et al, 2014).…”

Driving stress and seismotectonic implications of the 2013 Mw5.8 Awaji Island earthquake, southwestern Japan, based on earthquake focal mechanisms before and after the mainshock

“…Previous studies reveal that the northern Osaka earthquake consists of two fault segments. Here, the mainshock rupture was initiated on a NNW-SSE-striking reverse fault and then propagated to an adjacent ENE-WSW-striking strike-slip fault (Hallo et al 2019;Kato and Ueda 2019;Li et al 2019). A large nondouble-couple component of the CMT solution (Fig.…”

“…1) exhibits this complex rupture process. Here, we evaluated the on the reverse fault, i.e., the rst ruptured plane, whose fault parameter is (strike, dip, slip) = (345°, 50°, 85°) (Kato and Ueda 2019). Different from the Awaji Island earthquake, coseismic at the hypocenter (34.8443°N, 135.6217°E, 13.0 km) was negative (-0.01 MPa), which delays the occurrence of the earthquake.…”