Slow Slip and Tremor: A Review of the Role of Water Expelled From Subducting Plate

Date: 4/24/2019

Time: 02:15 PM

Room: Cascade II

Sporadic seismic tremor and slow slip (ETS) occur in many subduction zones; I deal with the ~35 km deep continuous band that occurs in hot subduction zones, including Cascadia and SW Japan. Important conclusions are: (a) there is a gap on the thrust between the downdip limit of the seismogenic zone and the ETS, not a continuous transition, (b) the ETS temperatures are 500-550C, too hot for normal earthquake rupture. What is the process of ETS and what controls the location? A critical observation is the close spatial coincidence between ETS and the forearc mantle corner. We have proposed that downdip of the corner, fluid dehydrated from the oceanic plate is blocked from rising vertically by the low permeability forearc mantle serpentinite inferred by very high Poisson’s Ratio. The fluid is channelled updip in the high permeability oceanic crust until it is released upward at the corner. Concentrated fluid expulsion above the corner is indicated by: (a) Poisson’s Ratio evidence for quartz deposition in the overlying forearc crust; (b) A high electrical conductivity plume above the corner from magnetotelluric data. Slip and tremor may occur in spite of the high temperatures because of “fault valve” buildup and simultaneous release of high pore pressure and shear stress. Buildup and release of pore pressure may give the slow slip and tremor, mainly at the thrust (LFEs) but also in the overlying crust. Near-lithostatic pore pressures are inferred by: (a) high Poisson’s ratio in a dipping low velocity zone in the upper oceanic crust, (b) tremor sensitivity to small tidal and surface wave stresses, (c) Evidence for crack-seal and quartz deposition structures in exhumed subduction thrust zones.

Presenting Author: Roy D. Hyndman

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