The 2016 Kumamoto M_w = 7.0 Earthquake: A Significant Event in a Fault–Volcano System

Abstract

The 2016 Kumamoto earthquake sequence occurred on the Futagawa–Hinagu fault zone near the Aso volcano on Kyushu island. The sequence was initiated with two major (Mw ≥ 6.0) foreshocks, and the mainshock (Mw = 7.0) occurred 25 h after the second major foreshock. We combine GPS, strong motion, synthetic aperture radar images, and surface offset data in a joint inversion to resolve the kinematic rupture process of the mainshock and coseismic displacement of the foreshocks. The joint inversion results reveal a unilateral rupture process for the mainshock involving sequential rupture of four major asperities. The slip area of the foreshocks and mainshock and the aftershock loci form a detailed complementary pattern. The mainshock rupture terminates near the rim of the caldera, leaving a ~10 km long gap of aftershocks. This area is characterized by high temperature and low shear wave velocity, density, and resistivity, which may be related to the partially melted geothermal condition. Ductile material property near the volcano may act as a “material barrier” to the dynamic rupture. Topographic weight of the caldera increases compressional normal stress on the fault plane, which may behave as a “stress barrier.” Long‐term seismic hazard and deformation behaviors related to these two types of barriers are discussed in terms of the associated frictional mechanism. Significant postseismic creeps observed near the volcano area indicates a velocity strengthening frictional behavior near the rupture termination, which confirms that the “material barrier” mechanism is likely the dominant rupture termination mechanism.

Modeling and Forecasting Aftershocks Can Be Improved by Incorporating Rupture Geometry in the ETAS Model

“…At the northeast end of rupture fault, there is a high productivity asperity where the coseismic rupture terminates and a gap of ∼8 km length, with low productivity values, in between. This gap is consistent with the 10‐km‐long gap of aftershocks reported in Yue et al () who attribute this phenomenon to partial melting in this volcanic region.…”

“…(b1-c3) Comparison between aftershock productivity distribution and coseismic slip distribution. (b1-b3) Interpolated productivity distributions on the mainshock fault planes from results ofYagi et al (2016),Asano and Iwata (2016), andYue et al (2017). (c1-c3) Slip distributions from the three models.…”

The 2017 Jiuzhaigou Earthquake: A Complicated Event Occurred in a Young Fault System

“…From this sense, the Huya and Minjiang are splay faults associated with the Tazang fault, which is a direct extension of the East Kunlun fault. Similar splay faults were often resolved as hosting the initial rupture of great earthquakes such as the 2001 Kokoxili earthquake (Klinger et al, 2005) or foreshocks such as the 2016 Kumamoto earthquake (Yue et al, 2017). Therefore, the occurrence of the Jiuzhaigou earthquake calls for attention for the future seismic hazard on the Tazang or the East Kunlun fault.…”

Stress Field in the 2016 Kumamoto Earthquake (M 7.3) Area

“…Many studies have demonstrated that fluids and arc magma exist under the Aso active volcano near the Kumamoto source zone: for example, a high‐temperature (over ~500 °C) anomaly at ~3‐km depth beneath the Aso caldera (Okubo & Shibuya, ), a prominent low‐velocity anomaly indicating a magma chamber (H. Wang et al, ; Xia et al, ; Yu et al, ; Zhao et al, ), a broad low‐viscosity anomaly in the lower crust (Moore et al, ), a large velocity reduction caused by pressurized volcanic fluids (Nimiya et al, ), and volcanic rocks containing components of slab‐derived fluids (Kita et al, ). Coulomb stress changes indicate that the high‐temperature and widespread fluids around the Aso volcano also stopped the rupture of the Kumamoto mainshock (Yue et al, ; Zhang et al, ). It is generally considered that arc magma and fluids can affect the generation of large crustal earthquakes in Japan (e.g., Liu & Zhao, ; Tong et al, ; Zhao, ; Zhao et al, ).…”

“…Journal of Geophysical Research: Solid Earth high-temperature and widespread fluids around the Aso volcano also stopped the rupture of the Kumamoto mainshock (Yue et al, 2017;Zhang et al, 2018). It is generally considered that arc magma and fluids can affect the generation of large crustal earthquakes in Japan (e.g., Liu & Zhao, 2015;Tong et al, 2012;Zhao, 2015;Zhao et al, 1996).…”