Kinematic Rupture Process of the 2016 Mw 7.1 Kumamoto Earthquake Sequence

Han Yue, Mark Simons, Cunren Liang, Heresh Fattahi, Eric J. Fielding, Hiroo Kanamori, Donald V. Helmberger, Linjun Zhu, Michael Sylvain, & Jean-Philippe Avouac

Published August 15, 2016, SCEC Contribution #6990, 2016 SCEC Annual Meeting Poster #076

On April 16th, 2016, the Mw 7.1 Kumamoto earthquake ruptured a portion of the Futagawa fault on the Kyushu island. This event was preceded by two (Mw 6.0 and 6.2) foreshocks happened two days earlier. We investigate the kinematic rupture process of the Kumamoto earthquake sequence by integrated analysis of synthetic aperture radar (SAR) interferometry and azimuth-offset images and regional strong motion records. From the SAR azimuth offset images, which covers both the foreshocks and main shock displacement fields, we identify a “bifurcated” fault trace. The main rupture presents approximately 40km lateral extent striking 220 to the southwest. A 10 km long fault branch connected to the southern portion of the main rupture is also identified. Guided by the surface fault trace and aftershock distribution at ~16 km depth, we parameterize the main rupture plane with a curved fault interface. The dip angle of the main fault plane change gradually from 85 (south) to 75 (north). Static GPS, SAR interferometry and azimuth-offset images and near field strong motion records of 7 K-net and KiK-net stations are adopted jointly in both linear and Bayesian inversions. For a portion of SAR images record both foreshocks and the main shock deformation fields, we parameterize the source time function as foreshock (static) and main-shock (kinematic) ruptures and specify their contribution to each dataset. This enables us to invert for the foreshock and main shock ruptures simultaneously in one inversion work. A unilateral rupture process is resolved in both inversions, with the main shock rupture extends for ~30 km from the hypocenter to the northeast. Peak slip of approximately 6 m is resolved at ~5 km depth which locates ~25 km from the hypocenter. Significant shallow slip deficit is resolved at shallow depth with maximum surface rupture of ~2 m. We also resolved significant normal fault slip components, which is consistent with the normal faulting component revealed by GCMT solutions. Both inversion results revealed averaged rupture velocity (Vr) of ~ 2.9 km/s, while the Bayesian inversion resolved several local areas with high rupture velocity. Several sub-parallel off-fault fractures are identified from the InSAR images near the northern end of the main rupture area. These off-fault fractures form a conjugate angle with the main fault plane. Those fractures may be activated by the strong shearing stress of the dynamic waves, while their normal faulting mechanism is consistent with the dilatational back-ground stress field of this volcanic area. Both main shock and aftershocks presents a combination of normal and strike slip components, which indicates the regional stress field is controlled by both the tectonic shearing and the volcano introduced dilatation.

Key Words
Kumamoto EQ, Joint Inversion

Citation
Yue, H., Simons, M., Liang, C., Fattahi, H., Fielding, E. J., Kanamori, H., Helmberger, D. V., Zhu, L., Sylvain, M., & Avouac, J. (2016, 08). Kinematic Rupture Process of the 2016 Mw 7.1 Kumamoto Earthquake Sequence. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)