Variation of downdip limit of the seismogenic zone near the Japanese islands: implications for the serpentinization mechanism of the forearc mantle wedge

Abstract

It has been proposed that the downdip limit of interplate thrust-type earthquakes in subduction zones (DLT) is determined either by the brittle–ductile transition around ∼350–450 °C of the crustal material at the plate interface or the Moho of the forearc of the upper plate. The latter is due to possible serpentinization of the forearc mantle wedge above the subducting plate, which makes the thrust zone slide in a stable manner. I examine this criterion in subduction zones near Japan, where the Pacific and Philippine Sea plates are subducting. I show that in most of N. Honshu where the old Pacific plate is subducting, and in Kanto and S. Kyushu where the Izu-Bonin and Kyushu-Palau ridges of the Philippine Sea plate are subducting, the DLT is located at a depth of 60–70 km, which is deeper than the Moho of the upper plate. This shows that the criterion does not hold and the mantle wedge is not likely to be serpentinized in these regions. In contrast, in the region off Iwate in N. Honshu and in Bonin, where the old Pacific plate is subducting, the DLT is located at a depth of 20–30 km around the Moho of the upper plate, suggesting that the mantle wedge is serpentinized in these regions. In SW. Japan where the young Shikoku Basin is subducting, the DLT is around ∼30 km in depth, which coincides with the Moho, but also with the 350 °C temperature at the plate interface. Although seismic tomography indicates that the mantle wedge is serpentinized, the DLT here does not help to examine the criterion. Assuming that hydration of the mantle wedge is conducted through hydro-fracturing by water released from the dehydrating subducted crust, I propose that the stress state of the wedge would determine the extent of serpentinization of the mantle wedge. In the reverse-fault-type stress regime of N. Honshu, Kanto, and S. Kyushu, hydro-fracturing is not favored, and fractures in the thrust zone associated with repeated interplate earthquakes provide conduits for water to flow in the updip direction, resulting in no serpentinization of the mantle wedge. No serpentinization of the mantle wedge in turn causes interplate earthquakes. In contrast, in the normal-fault-type stress regime of the region off Iwate and Bonin, and the strike-slip fault-type stress regime of SW. Japan, hydro-fracturing is favored, resulting in serpentinization of the mantle wedge. The frictional property at the thrust zone is governed by stable sliding, resulting in no interplate earthquakes deeper than the Moho. Thus, the stress regime of the mantle wedge would determine bifurcation whether it is serpentinized or not, and then the depth of DLT.