title: Slab temperature controls on the Tonga double seismic zone and slab mantle dehydration.
abstract: Double seismic zones are two-layered distributions of intermediate-depth earthquakes that provide insight into the thermomechanical state of subducting slabs. We present new precise hypocenters of intermediate-depth earthquakes in the Tonga subduction zone obtained using data from local island-based, ocean-bottom, and global seismographs. The results show a downdip compressional upper plane and a downdip tensional lower plane with a separation of about 30 km. The double seismic zone in Tonga extends to a depth of about 300 km, deeper than in any other subduction system. This is due to the lower slab temperatures resulting from faster subduction, as indicated by a global trend toward deeper double seismic zones in colder slabs. In addition, a line of high seismicity in the upper plane is observed at a depth of 160 to 280 km, which shallows southward as the convergence rate decreases. Thermal modeling shows that the earthquakes in this "seismic belt" occur at various pressures but at a nearly constant temperature, highlighting the important role of temperature in triggering intermediate-depth earthquakes. This seismic belt may correspond to regions where the subducting mantle first reaches a temperature of ~500°C, implying that metamorphic dehydration of mantle minerals in the slab provides water to enhance faulting.
authors.affiliation: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.; Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093, USA.
authors.forename: S Shawn
authors.identifier: 0000-0002-6217-0545
authors.initials: SS
authors.lastname: Wei
authors.affiliation: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
authors.forename: Douglas A
authors.initials: DA
authors.lastname: Wiens
authors.affiliation: Department of Terrestrial Magnetism, Carnegie Institution for Science, Washington, DC 20015, USA.
authors.forename: Peter E
authors.identifier: 0000-0003-0377-8830
authors.initials: PE
authors.lastname: van Keken
authors.affiliation: Department of Earth and Planetary Sciences, Washington University, St. Louis, MO 63130, USA.
authors.forename: Chen
authors.identifier: 0000-0002-0866-7091
authors.initials: C
authors.lastname: Cai
keywords: Tonga subduction zone; double seismic zone; intermediate-depth earthquakes; slab dehydration