Effect of newly refined hypocenter locations on the seismic activity recorded during the 2016 Kumamoto Earthquake sequence

Abstract

We present the results of relocating 17,544 hypocenters determined from data recorded during the 2016 Kumamoto Earthquake sequence, during the interval between April 14, 2016, and August 31, 2016. For this, we used a doubledifference relocation method to constrain high-resolution hypocenter locations by cross-correlation differential times as well as the NIED Hi-net catalog differential times. The sequence included two large events (on 14 April: M JMA 6.5 and on 16 April: M JMA 7.3) that occurred in a complicated region where the Hinagu and Futagawa faults meet. By comparing these high-resolution earthquake locations in three different periods [(P1) between 2001 and 2012; (P2) between M JMA 6.5 and M JMA 7.3; and (P3) between M JMA 7.3 and August 31, 2016], we present the significant seismicity after the mainshock relative to the background seismicity. Events during the Kumamoto Earthquake sequence occurred generally within the same sites of known faults and background seismicity. For an example, the seismicity during period P2 formed a sharp linear shape along the northern part of the Hinagu fault for about 20 km. A series of linear seismicity events occurred during period P3 along the Futagawa fault to the east (for about 28 km), in the northern part of the Aso caldera, and in the Oita region around the Beppu-Haneyama fault zone. These events also extended to the midand southern parts of the Hinagu fault zone and were shaped only after the M7.3 event. Moreover, high-resolution hypocenter locations also allowed us to identify some clusters of events that occurred in regions where background seismicity has not been confirmed. For instance, activity on the northwestern edge of the Aso caldera and in small areas within the Beppu-Haneyama fault zone became apparent with new seismic activity. We also demonstrate herein the absence of seismicity between the northeast extension of the Futagawa fault zone and the Aso caldera region, which became clearly shown after the M7.3 event. This low-seismicity region is located at the boundary of the low-and high-velocity structures and different focal mechanisms, but is also close to the maximum slip area of the M7.3 event.

Coseismic Groundwater Drawdown Along Crustal Ruptures During the 2016 Mw 7.0 Kumamoto Earthquake

“…The 2016 Kumamoto crustal earthquake sequence occurred with a large inelastic strain‐rate (>10 ‐7 year ‐1 ; Matsumoto et al, ), resulting in an enormous number of aftershocks along the Futagawa and Hinagu active faults at a depth of ~17 km and under the Suizenji fault area at ~10 km (Goto et al, ; Yano & Matsubara, ). These seismotectonic features imply that the crustal materials are inelastic and deformation extends from near the surface down to several kilometers in the vicinity of faults.…”

An Ancient >200 m Cumulative Normal Faulting Displacement Along the Futagawa Fault Dextrally Ruptured During the 2016 Kumamoto, Japan, Earthquake Identified by a Multiborehole Drilling Program

“…In addition, one to four paleoseismic events prior to the 2016 earthquake in the past ∼10,000 years were detected on the Futagawa fault from trench excavation observations and radiocarbon dating results, and a recurrence interval of ∼2,000-4,000 years was inferred. On the other hand, Yano and Matsubara (2017) determined a dip of ∼75°NW for the Futagawa fault based on the locations of seismicity that occurred from 14 April to 31 August 2016. This steep dip also corresponds to the fault dip through the finite difference method using radon-222 (Rn) concentration data in soil gas obtained on survey lines mostly orthogonal to the fault strike (Koike et al, 2009).…”

“…During the earthquake, surface ruptures that were approximately 28 km long appeared along the ENE-WSWtrending Futagawa fault zone on the western side of Aso Volcano (Figure 2a). Large dextral slip with a maximum displacement of 2.2 m was measured throughout the central section of the rupture zone along the Futagawa fault, which is nearly vertical near the surface and dips northwestward at depth (Shirahama et al, 2016;Yano & Matsubara, 2017). Moya et al (2017) calculated the coseismic displacement along the Futagawa fault using a pair of digital surface models obtained from LiDAR data before and after the mainshock.…”

The 2016 Gyeongju earthquake sequence revisited: aftershock interactions within a complex fault system

“…The detailed spatial distribution of the Gyeongju earthquakes represents the occurrence of E1 and E2 on two parallel fault planes of F1 and F2, with an offset of 600-700 m. Complex fault systems with two or more parallel faults can also be found in other regions (Durá-Gómez & Talwani 2009;Rabak et al 2010;Yano & Matsubara 2017). Yano & Matsubara (2017) reported that part of the 2016 Kumamoto earthquake sequence was aligned on two vertical fault planes in the northeastern area of Mt Aso with two moderatesized aftershocks (M ≥ 5) located on either side of the fault plane. This is consistent with our results that major events in a sequence separately occurred on visually identified faults.…”