Source rupture processes of the 2016 Kumamoto, Japan, earthquakes estimated from strong-motion waveforms

Abstract

The detailed source rupture process of the M 7.3 event (April 16, 2016, 01:25, JST) of the 2016 Kumamoto, Japan, earthquakes was derived from strong-motion waveforms using multiple-time-window linear waveform inversion. Based on the observations of surface ruptures, the spatial distribution of aftershocks, and the geodetic data, a realistic curved fault model was developed for source-process analysis of this event. The seismic moment and maximum slip were estimated as 5.5 × 1019 Nm (M w 7.1) and 3.8 m, respectively. The source model of the M 7.3 event had two significant ruptures. One rupture propagated toward the northeastern shallow region at 4 s after rupture initiation and continued with large slips to approximately 16 s. This rupture caused a large slip region 10-30 km northeast of the hypocenter that reached the caldera of Mt. Aso. Another rupture propagated toward the surface from the hypocenter at 2-6 s and then propagated toward the northeast along the near surface at 6-10 s. A comparison with the result of using a single fault plane model demonstrated that the use of the curved fault model led to improved waveform fit at the stations south of the fault. The source process of the M 6.5 event (April 14, 2016, 21:26, JST) was also estimated. In the source model obtained for the M 6.5 event, the seismic moment was 1.7 × 10 18 Nm (M w 6.1), and the rupture with large slips propagated from the hypocenter to the surface along the north-northeast direction at 1-6 s. The results in this study are consistent with observations of the surface ruptures.

Toward automated directivity estimates in earthquake moment tensor inversion

“…7). However, we emphasize that the Kumamoto earthquake is a relatively large earthquake with complex ruptures across multiple fault segments (Asano & Iwata 2016;Kubo et al 2016;Yagi et al 2016). As a primary estimate, our method well resolves the average behavior of the rupture and provides reasonable and rapid results critical for hazards estimates.…”

“…Preceded by a foreshock (M w 6.0) occurring on the Hinagu fault 28 hr before, the mainshock rupture initiated on the Hinagu fault and propagated north-northeast toward the Futagawa fault (Asano & Iwata 2016;Kubo et al 2016;Yagi et al 2016). The average rupture velocity was around 2.4 km s −1 in the north-northeast direction (Asano & Iwata 2016;Yagi et al 2016).…”

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

“…Indeed, the source models for the mainshock (e.g., Asano and Iwata 2016;Kubo et al 2016) suggest simultaneous right-lateral strike-slip shear movement complemented by normal dip-slip movement of the Futagawa fault segment. The rupture of the mainshock then has to be complex and segmented to two or more fault planes (as suggested by the mainshock source models).…”

“…1), and two aftershocks with M JMA magnitude 4.9 and 4.8. The mainshock itself was omitted from the analysis as it has rather complex earthquake source rupture process that was studied in detail in other studies (e.g., Asano and Iwata 2016;Kubo et al 2016;Kobayashi et al 2017). …”

“…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.…”