Imaging the distribution of transient viscosity after the 2016 M _w 7.1 Kumamoto earthquake

Abstract

The deformation of mantle and crustal rocks in response to stress plays a crucial role in the distribution of seismic and volcanic hazards, controlling tectonic processes ranging from continental drift to earthquake triggering. However, the spatial variation of these dynamic properties is poorly understood as they are difficult to measure. We exploited the large stress perturbation incurred by the 2016 earthquake sequence in Kumamoto, Japan, to directly image localized and distributed deformation. The earthquakes illuminated distinct regions of low effective viscosity in the lower crust, notably beneath the Mount Aso and Mount Kuju volcanoes, surrounded by larger-scale variations of viscosity across the back-arc. This study demonstrates a new potential for geodesy to directly probe rock rheology in situ across many spatial and temporal scales.

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

“…These aftershocks may be attributed to the brittle creeping fault zone mechanism, because the depth is consistent with the brittle and ductile transition (Perfettini & Avouac, ). Areas at the southwestern end of the main fault plane (segment 3, Figure b) present limited postseismic deformation or after slips (Moore et al, ), and the microseismicity are widely distributed from 5 km to 18 km, which indicates that the coseismic stress triggering (first mechanism) may be the dominant triggering mechanism. Two areas with substantial Coulomb stress increase present limited aftershocks, that is, the shallow slip deficit area and the downdip edge of the last asperity (Seg 1 in Figure b).…”

“…We processed a SAR interferogram over 6 months after the mainshock showing postseismic ground deformation near the source area (Figure ). This was also revealed by SAR time series analysis (Moore et al, ). The postseismic deformation field presents localized deformation in comparison with the coseismic deformation field (Figure ).…”

“…Subsidence or slip happened close to the surface fault traces, which may be due to after slip at shallow depths related to the shallow slip deficit. Significant deformation is observed near the northeastern end of the fault plane inside the caldera, which is likely related to both the after slip on the main fault plane and ductile deformation under the Aso volcano (Moore et al, ). Although the SW end of the fault plane presents dense aftershock activities, the observed ground deformation is much smaller than that near the NE end.…”

“…The observation of significant postseismic deformation demonstrates that a material barrier is likely the mechanism that stops the coseismic rupture. The after slip related to the deformation near the volcano appears to be complementary to the mainshock slip pattern (Moore et al, ), which is also consistent with the behavior of a material barrier (Figure ). Viscoelastic relaxation and poroelastic diffusion need to be considered to model the postseismic ground deformation properly, which should be investigated in further studies.…”

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

“…Recent tomographic studies show that the 2016 Kumamoto earthquake occurred in a strong patch (with a low attenuation and a high velocity) in the upper crust but was underlain by arc magma and fluids in the lower crust and upper mantle wedge (with a high attenuation and a low velocity; H. Wang et al, ; Z. W. Wang et al, ; Zhao et al, ). 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, ).…”