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.
Assessing the seismic hazard of a fault is usually based on its record of strong earthquakes. Earthquake records with long periods of quiescence for active faults can lead to underestimates of seismic hazards, such as for the Longmenshan fault zone which produced the unanticipated 2008 Mw 7.9 Wenchuan earthquake. The Daliangshan fault zone has a low slip rate and has not produced any strong earthquakes in history. As a result, little is known about its paleoearthquake history, including the behavior of any strong earthquakes it might produce and the seismic hazards posed by the Daliangshan fault zone. To solve this problem, we excavated four trenches across the Jiaojihe and Butuo faults along the southern segment of the Daliangshan fault zone. The paleoseismic investigations revealed six paleoearthquakes on the Jiaojihe fault in ~20,000 years and determined another seven rupturing events on the Butuo fault in ~42,000 years. The strong earthquake history of the Jiaojihe fault has evidence of temporal clustering, while the Butuo fault exhibits a relatively periodic recurrence pattern with intervals of 1,710–2,460 years. Based on its surface rupture length and the magnitude of observed displacement, the southern segment of the Daliangshan fault zone is capable of producing M > 6.5 earthquakes. Furthermore, based on their respective slip rates and the elapsed times since the most recent events along the Jiaojihe and Butuo faults, they have accumulated seismic energy equivalent to M ~ 7.6, suggesting they pose a significant seismic hazard to the southeastern Tibetan Plateau.
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