Fault source model for the 2016 Kumamoto earthquake sequence based on ALOS-2/PALSAR-2 pixel-offset data: evidence for dynamic slip partitioning

Abstract

Series of earthquakes including three M w > 6 earthquakes occurred in Kumamoto prefecture in the middle of the Kyushu island, Japan. In order to reveal the associated crustal deformation signals, we applied offset tracking technique to ALOS-2/PALSAR-2 data covering three M w > 6 earthquakes and derived the 3D displacements around the epicenters. We could identify three NE-SW trending displacement discontinuities in the 3D displacements that were consistent with the surface location of Futagawa and Hinagu fault system. We set three-segment fault model whose positions matched the displacement discontinuities, and estimated the slip distributions on each segment from the observed pixel-offset data. Whereas right-lateral slip was dominant in the shallower depth of the larger segments, normal fault slip was more significant at a greater depth of the other segment. The inferred configuration and slip distribution of each segment suggest that slip partitioning under oblique extension stress regime took place during the 2016 Kumamoto earthquake sequence. Moreover, given the consistent focal mechanisms derived from both the slip distribution model and seismology, the significant non-double couple components in the focal mechanism of the main shock are due to simultaneous ruptures of both strike-slip and normal faulting at the distinct segments.

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

“…Several source models of the Mj7.3 of the 2016 Kumamoto earthquake have been proposed on the basis of the Open Access *Correspondence: yoshida@geor.or.jp 1 Geo-Research Institute, 2-1-2 Otemae, Chuo-ku, Osaka 540-0008, Japan Full list of author information is available at the end of the article strong motions (e.g., Asano and Iwata 2016), teleseismic waveforms (Yagi et al 2016), and crustal deformation (e.g., Himematsu and Furuya 2016). Most of these studies based on seismic data assume northwest-dipping fault planes.…”

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

“…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. Finite source models and also static slip models from geodetic data (Himematsu and Furuya 2016;Fukahata and Hashimoto 2016) are consistent with field measurements of co-seismic surface ruptures. The source models introduce two or three segments of the mainshock rupture, but their physical relation to the foreshock ruptures remains unclear because of the complex tectonic settings of the intersection of the Futagawa and Hinagu faults.…”

“…Co-activation of the right-lateral strike-slips with normal faulting ruptures through the sequence was introduced also by static slip model from geodetic data (Himematsu and Furuya 2016;Kobayashi 2017). Field investigation by Toda et al (2016) shows that surface displacements along the previously mapped active fault traces of the Hinagu-Futagawa fault zone are dominated by right-lateral strike-slip surface displacement up to 2 m. A normal surface rupture zone of about 10 km in length dipping to northwest, which is parallel to the Futagawa fault outside the Aso caldera, was also reported by Toda et al (2016), and its maximum co-seismic displacement is also up to 2 m. The normal dip-slip aftershocks that occurred along the NW edge of the mainshock rupture had no clear relationship with co-seismic surface ruptures; however, minor surface ruptures in downtown of Kumamoto City have been mapped by InSAR (Himematsu and Furuya 2016) and field survey (Goto et al 2017).…”

“…Adopted from Matsumoto et al (2015) fault plane of the M JMA 7.3 mainshock (see Fig. 10), where the surface subsidence occurred as documented by InSAR (Himematsu and Furuya 2016). Aftershock's ruptures are situated at depths greater than 5 km, close to the assumed fault plane of the mainshock (adopted from Asano and Iwata 2016), and three of them intersect the mainshock fault plane.…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…Using the data from ALOS-2, several groups present models of coseismic deformation on faults. Himematsu and Furuya (2016) exploit pixel offsets as well as phase changes to reveal deformation. Ozawa et al (2016) present a model with four fault segments by fitting the observed interferograms of ALOS-2.…”