Characteristics of the surface ruptures associated with the 2016 Kumamoto earthquake sequence, central Kyushu, Japan

Abstract

The 2016 Kumamoto earthquake sequence started with a M J (Japan Meteorological Agency magnitude) 6.5 event on April 14, and culminated in a M J 7.3 event on April 16. Associated with the sequence, approximately 34-km-long surface ruptures appeared along the eastern part of the Futagawa fault zone and the northernmost part of the Hinagu fault zone. We carried out an urgent field investigation soon after the earthquake to map the extent and displacement of surface ruptures with the following results. (1) The rupture zone generally consisted of a series of left-stepping en echelon arrays of discontinuous fault traces of various lengths. (2) Slip exceeding 100 cm occurred on previously unrecognized fault traces in the alluvial lowland of the Kiyama plain and on the western rim of the Aso volcano caldera. (3) Large slip with maximum dextral slip of 220 cm was measured throughout the central section of the rupture zone along the Futagawa segment, and the slip gradually decreased bilaterally on the adjoining northeastern and southwestern sections. (4) The surface rupture mostly occurred along fault traces mapped in previous active fault investigations. (5) Most of the surface ruptures were produced by the mainshock, and significant postseismic slip occurred after the mainshock.

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

“…These long-duration subfaults are expected to cause weak short-period ground motions. The location of the shallow asperity with large slip corresponds to the locations of the observed surface rupture (e.g., Shirahama et al 2016). The overall patterns of vertical static displacements calculated from the derived source model agree with those of the observed ones acquired by InSAR.…”

“…Another minor asperity, the A2 asperity, was identified on the deep part of the F3 segment. The A1 asperity, at the depth of 0.5 km, corresponded to the location of the surface ruptures during the earthquake (e.g., Shirahama et al 2016). The proper rupture area S, at 792 km 2 , and the combined asperity area S a , at 240 km 2 , agree with the scaling relations of S-M 0 and S a -M 0 Miyake 2001, Miyakoshi et al 2015) for the seismic moment M 0 .…”

“…These abundant nearfault strong-motion records have enabled us to analyze detailed source characteristics. Many surface ruptures (e.g., Shirahama et al 2016) and significant crustal deformations (e.g., Geospatial Information Authority of Japan 2016a) have been investigated for these earthquakes.…”

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

“…The field surveys discovered surface ruptures with a length of approximately 30 km along the surface traces of the Hinagu and Futagawa fault zones after the M 7.3 event (e.g., GSJ/AIST 2016; Kumahara et al 2016;Shirahama et al 2016). They reported that the surface ruptures near the epicenter were not very large (<0.5 m), and that large surface ruptures of more than 1 m were observed from approximately 5 km to approximately 30 km northeast of the epicenter along the Futagawa fault zone.…”

“…This observation implies that the source area of the M 7.3 event spread over the Hinagu and Futagawa fault zones. After the M 7.3 event, various groups conducted field surveys of surface ruptures (e.g., GSJ/AIST 2016; Kumahara et al 2016;Shirahama et al 2016). The surveys revealed surface ruptures along the known surface traces of the Hinagu and Futagawa fault zones.…”