Characteristics of secondary-ruptured faults in the Aso Caldera triggered by the 2016 Mw 7.0 Kumamoto earthquake

Yo Fukushima,Daisuke Ishimura

doi:10.1186/s40623-020-01306-y

Yo Fukushima, Daisuke Ishimura

Open Access

https://doi.org/10.1186/s40623-020-01306-y

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Abstract

The 16 April 2016 Mw 7.0 Kumamoto earthquake caused prominent fault displacements and crustal deformation, not only around the main rupture faults but also around numerous secondary-ruptured faults. The physics and characteristics of such secondary faulting have not yet been studied in detail. We investigated a set of two secondary faults that appeared at the timing of the Mw 7.0 quake in the Aso Caldera by mainly using synthetic aperture radar interferometry and fault slip modeling. The two faults were found to be associated with surface displacement offsets of several centimeters or more, in the oblique sense of right-lateral and vertical motion. Fault slip inversions found that the slip was dominantly in normal sense with smaller contribution from the right-lateral component. The deeper limit of the slips was estimated to be around 1.3 km, which may coincide with the boundary between the superficial sediment layer and the basement rock. The shallowness of the slip and the difference in the dip angles of the main secondary fault and the Mw 7.0 seismogenic fault suggest separation of the two fault systems, although the fault strike and sense of motions were similar. The amount of slip on the two secondary faults was larger than that expected from the scaling law derived from seismogenic faults, which may indicate the difference in the physics of seismogenic and secondary faultings.

Highlights

The 16 April 2016 Mw 7.0 Kumamoto earthquake, the largest event of the 2016 Kumamoto earthquake sequence, caused prominent fault displacements and crustal deformation
What makes the Kumamoto earthquake unique was numerous triggered secondary faultings captured by Interferometric Synthetic Aperture Radar (InSAR) (Fujiwara et al 2016; Ozawa et al 2016)
Our main interests on the secondary faulting can be summarized by two questions: (1) what are the depths of the slipped portion of the faults? (2) How are the two secondary faults related to each other? To answer these questions, we estimated the fault slip models of the two secondary faults using the inversion procedure developed and conducted in previous studies (Fukushima et al 2013, 2018; Ghayournajarkar and Fukushima 2020)

Summary

Introduction

The 16 April 2016 Mw 7.0 Kumamoto earthquake, the largest event of the 2016 Kumamoto earthquake sequence, caused prominent fault displacements and crustal deformation. Complex rupture patterns associated with the earthquake were revealed by InSAR (e.g., Fujiwara et al 2016; Ozawa et al 2016), SAR pixel offset analysis (e.g., Himematsu and Furuya 2016; Yue et al 2017), differential Lidar (Scott et al 2018), and field surveys (Goto et al 2017; Kumahara and Research Group of Inter-University 2016; Shirahama et al 2016). What makes the Kumamoto earthquake unique was numerous triggered secondary faultings captured by InSAR (Fujiwara et al 2016; Ozawa et al 2016). Since the deformation around each of the secondary faults did not extend over a broad area, Ozawa et al (2016) speculated that the fault slips must have been limited to shallow depths

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