Source process of the 2016 Kumamoto earthquake (Mj7.3) inferred from kinematic inversion of strong-motion records

Abstract

In this study, we estimated source process of the 2016 Kumamoto earthquake from strong-motion data by using the multiple-time window linear kinematic waveform inversion method to discuss generation of strong motions and to explain crustal deformation pattern with a seismic source inversion model. A four-segment fault model was assumed based on the aftershock distribution, active fault traces, and interferometric synthetic aperture radar data. Three western segments were set to be northwest-dipping planes, and the most eastern segment under the Aso caldera was examined to be a southeast-dipping plane. The velocity structure models used in this study were estimated by using waveform modeling of moderate earthquakes that occurred in the source region. We applied a two-step approach of the inversions of 20 strong-motion datasets observed by K-NET and KiK-net by using band-pass-filtered strong-motion data at 0.05-0.5 Hz and then at 0.05-1.0 Hz. The rupture area of the fault plane was determined by applying the criterion of Somerville et al. (Seismol Res Lett 70:59-80, 1999) to the inverted slip distribution. From the first-step inversion, the fault length was trimmed from 52 to 44 km, whereas the fault width was kept at 18 km. The trimmed rupture area was not changed in the second-step inversion. The source model obtained from the two-step approach indicated 4.7 × 10 19 Nm of the total moment release and 1.8 m average slip of the entire fault with a rupture area of 792 km 2 . Large slip areas were estimated in the seismogenic zone and in the shallow part corresponding to the surface rupture that occurred during the Mj7.3 mainshock. The areas of the high peak moment rate correlated roughly with those of large slip; however, the moment rate functions near the Earth surface have low peak, bell shape, and long duration. These subfaults with long-duration moment release are expected to cause weak short-period ground motions. We confirmed that the southeast dipping of the most eastern segment is more plausible rather than northwest-dipping from the observed subsidence around the central cones of the Aso volcano.

Applicability of source scaling relations for crustal earthquakes to estimation of the ground motions of the 2016 Kumamoto earthquake

“…High acceleration values were observed, such as 1791 cm/s 2 (synthesis of the three components) at the municipal observatory of Ohzu and 1362 cm/s 2 at KMMH16 (KiK-net Mashiki). Slip distributions obtained from waveform inversion of the strong-motion data from this event have previously been published (e.g., Asano and Iwata 2016;Kubo et al 2016;Yoshida et al 2016). Kubo et al (2016) performed waveform inversion with strong-motion data from 27 stations of K-NET, KiK-net, and F-net based on an assumed single fault plane with a strike of 226° and a dip of 65° and a fault area 56 km in length and 24 km in width.…”

“…1), which reduces the rupture area and seismic moment by 10% and 3%, respectively. Secondly, the slip distribution was reanalyzed based on the 0.05-1.0 Hz waveform inversion of the strong-motion data for this reduced Yoshida et al (2016). The entire rupture area was trimmed following the criteria defined by Somerville et al (1999).…”

“…Asano and Iwata (2016) used strong-motion data from 15 stations of K-NET, KiK-net, and F-net and assumed a fault model with two segments along the Futagawa and Hinagu fault zones. Yoshida et al (2016) used strong-motion data from 22 stations of K-NET and KiK-net, and an assumed fault model with four fault segments of different strikes and dips: one (Seg. 1) along the Hinagu fault zone, another (Seg.…”

“…1a. Yoshida et al (2016) conducted a two-step approach for multiple time-window kinematic waveform inversions of strong-motion data to estimate detailed slip distributions on these four fault segments. First, the slip distribution was inverted using 0.05-0.5 Hz band-pass filtered strongmotion data, as shown in Fig.…”

Bayesian inference and interpretation of centroid moment tensors of the 2016 Kumamoto earthquake sequence, Kyushu, Japan

“…The normal faulting was dominant especially in the northeast part of the rupture zone (e.g., Toda et al 2016). Finite source models for the mainshock were inverted from strong motion records (e.g., Asano and Iwata 2016;Kubo et al 2016;Hao et al 2017;Kobayashi et al 2017;Yoshida et al 2017). The inferred models suggest that the M JMA 7.3 event started near the intersection of the Futagawa and Hinagu faults by right-lateral strike-slip movement; then, the rupture propagated to the NE along the Futagawa fault as strike-slip with a normal faulting component.…”