[S-IT20] MAGMA, FLUID TRANSPORT, AND SEISMICITY IN THE EARTH'S INTERIOR

[SIT20-03] Intra-slab earthquakes and slab dehydration in the Tonga subduction zone

★Invited Papers

*Songqiao Shawn Wei¹、Dongdong Tian¹、Fan Wang¹、Wei Wang² (1.Michigan State University、2.University of Southern California)

キーワード：Subduction, Deep earthquakes, Tonga

Intermediate-depth intra-plate earthquakes are related to fluids within the subducted slab, as increased pore fluid pressure can reduce the confining pressure to allow brittle failure. The source of the free fluids is usually assumed to be either in-situdehydration of hydrous minerals or fluids from such a source migrating for some distance. More intriguingly, double seismic zones, in which intermediate-depth earthquakes occur along two layers parallel to the dip of the subducting slab and separated by 20-40 km, have been discovered in several subduction zones, possibly also indicating fluids in the slab mantle. We investigate the subducted Tonga slab by analyzing seismic data from a local amphibious temporary network. Earthquake relocation shows a double seismic zone extending to about 300 km depth, deeper than in any other subduction system. A “seismic belt” occurs at various depths (160−280 km) but a nearly constant temperature. The source spectral analysis suggests that intra-slab earthquakes have relatively stable corner frequencies throughout a wide depth range from 50 to 700 km. We also image a low-velocity layer within the Tonga slab down to at least 180 km depth, indicating hydrous minerals in the subducted slab crust and uppermost mantle.