After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata-Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 À15 (m 2 ). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N-S or NE-SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.

[1] Small repeating earthquake (RE) analysis is a useful method for estimating interplate quasi-static slip, which is a good indicator of interplate coupling. We detected 170 continual-type interplate RE groups and then estimated the spatial variation in quasi-static slip in the Hyuga-nada over the past 17 years. The RE activity in this region has different characteristics compared with that in the northeast Japan subduction zone, presumably reflecting differences in the subduction properties. Our results revealed that interplate coupling spatially changes along the trench-axis and dip-direction-a phenomenon that cannot be resolved by land-based Global Positioning System (GPS) analysis. By comparing seismicity, the low-slip-rate areas correspond with the location of hypocenters and asperities for largeand moderate-sized interplate earthquakes, suggesting strong interplate coupling at these sites. These results indicate that the slip rate distribution estimated from RE activity is reliable and useful for assessing the potential of future large earthquakes. Citation: Yamashita, Y., H. Shimizu, and K. Goto (2012), Small repeating earthquake activity, interplate quasi-static slip, and interplate coupling in the Hyuga-nada, southwestern Japan subduction zone, Geophys. Res. Lett., 39, L08304,

The stress field due to olivine‐spinel phase transition in and around a descending plate is studied based on the calculations for an equilibrium phase transition with a constant Clapeyron's slope and for a nonequilibrium transition taking a kinetic effect into consideration. The material concerned is assumed to be a viscoelastic body whose physical coefficients are functions of temperature and pressures. The calculated stress distribution for the equilibrium transition is characterized by a compressional stress near the upper and/or lower surfaces of the plate and by a tensional stress at the central part in the depth range from 200 to 550 km, the magnitude of principal stress being greater than 0.5 GPa. However, for the nonequilibrium transition a tensional stress predominates near the upper and lower surfaces, and the central part is compressional in the depth range from 300 to 600 km, unlike the case of equilibrium transition. The maximum principal stress in the latter case is greater than 2 GPa, which is much more than that for the equilibrium transition. The principal axes in the high stressed region are oriented almost parallel to the descending direction in both cases. Comparison of the calculated results to the actually observed seismicity‐depth relation and to focal mechanisms of deep focus earthquakes in descending plates beneath island arcs shows that the nonequilibrium phase transition agrees better with the observations.

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three‐dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre‐condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low‐pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations.

[1] Intense swarm-like seismicity associated with shallow normal faulting was induced in Ibaraki and Fukushima prefectures, Japan, following the 2011 Tohoku-Oki earthquake. This seismicity shows a systematic spatiotemporal evolution, but little is known of the heterogeneity in crustal structure in this region, or its influence on the evolution of the seismicity. Here, we elucidate a high-resolution model of crustal structure in this region and determine precise hypocenter locations. Hypocenters in Ibaraki Prefecture reveal a planar earthquake alignment dipping SW at~45 , whereas those in Fukushima Prefecture show a more complex distribution, consisting of conjugate sets of aligned small earthquakes. On the north of the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), we imaged a high-velocity body at shallow depths that lacks aftershock seismicity. Based on fault source models, the large-slip region of the Iwaki earthquake is situated along a zone that roughly coincides with this high-velocity body. We delineated a separate low-velocity anomaly directly beneath the hypocenter of the Iwaki earthquake, indicating crustal fluids in this region. We hypothesize that strong crust underwent structural failure due to the infiltration of crustal fluids into the seismogenic zone from deeper levels, causing the Iwaki earthquake.