Abstract
On April 16, 2016, Kumamoto prefecture in Kyushu region, Japan, was devastated by a shallow MJMA7.3 earthquake. The series of foreshocks started by MJMA6.5 foreshock 28 h before the mainshock. They have originated in Hinagu fault zone intersecting the mainshock Futagawa fault zone; hence, the tectonic background for this earthquake sequence is rather complex. Here we infer centroid moment tensors (CMTs) for 11 events with MJMA between 4.8 and 6.5, using strong motion records of the K-NET, KiK-net and F-net networks. We use upgraded Bayesian full-waveform inversion code ISOLA-ObsPy, which takes into account uncertainty of the velocity model. Such an approach allows us to reliably assess uncertainty of the CMT parameters including the centroid position. The solutions show significant systematic spatial and temporal variations throughout the sequence. Foreshocks are right-lateral steeply dipping strike-slip events connected to the NE–SW shear zone. Those located close to the intersection of the Hinagu and Futagawa fault zones are dipping slightly to ESE, while those in the southern area are dipping to WNW. Contrarily, aftershocks are mostly normal dip-slip events, being related to the N–S extensional tectonic regime. Most of the deviatoric moment tensors contain only minor CLVD component, which can be attributed to the velocity model uncertainty. Nevertheless, two of the CMTs involve a significant CLVD component, which may reflect complex rupture process. Decomposition of those moment tensors into two pure shear moment tensors suggests combined right-lateral strike-slip and normal dip-slip mechanisms, consistent with the tectonic settings of the intersection of the Hinagu and Futagawa fault zones.Graphical abstract.
Highlights
The 2016 Kumamoto earthquake sequence started on April 14, 2016, with an MJMA6.5 shallow earthquake in the central part of Kyushu, Japan
The seismic activity continued by weaker events, including MJMA5.8 and MJMA6.4 earthquakes 41 min and 2.6 h after the first shock, respectively
The mainshock of the sequence of MJMA7.3 occurred on April 16, 2016
Summary
The 2016 Kumamoto earthquake sequence started on April 14, 2016, with an MJMA6.5 shallow earthquake in the central part of Kyushu, Japan (see Fig. 1). The seismic activity continued by weaker events, including MJMA5.8 and MJMA6.4 earthquakes 41 min and 2.6 h after the first shock, respectively. The mainshock of the sequence of MJMA7.3 occurred on April 16, 2016 The following aftershocks had wide spatial distribution across the whole Kyushu with the strongest event of MJMA5.9 that occurred 20 min after the mainshock. Increased seismic activity in the area was notable even 2 weeks after the first foreshock, and it was daily reported by the Japan Meteorological Agency (JMA). In the 2016 Kumamoto sequence, seven earthquakes reached or exceeded instrumental intensity 6− of the JMA intensity scale which consists of degrees 0, 1, 2, 3, 4, 5−, 5+, 6−, 6+, 7
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