SEISMIC EVIDENCE FOR HYDROLOGIC CONTROLS OF EPISODIC SLOW EARTHQUAKES AND UPPER PLATE STRENGTH IN SUBDUCTION ZONE FOREARCS

Water and hydrous minerals play a key part in geodynamic processes at subduction zones by weakening the plate boundary, aiding slip and permitting subduction - and indeed plate tectonics - to occur. Recent discoveries of slow slip events (i.e., day- to week-long ruptures) on the subduction zone thrust fault have elucidated a down-dip transition in slip behavior from frictionally-controlled slip to continuous plastic creep. In this presentation I will review seismic evidence for the role of water on the seismogenic behavior at the Cascadia and Costa Rican subduction zones. In the slow slip region in both Cascadia and Costa Rica we find evidence for low-velocity oceanic crust with extremely high Poisson's ratios that are interpreted to manifest elevated pore-fluid pressures generated through the release of water from pro-grade metamorphic dehydration reactions within the subducting oceanic crust. Elevated pore pressures play a key role by weakening the fault and allowing slip to occur at low differential stress. Accordingly, the plate interface likely represents a low-permeability boundary that hampers vertical migration of fluids into the overlying crust. In Costa Rica, the along-strike change in the origin of the subducting plate appears to influence the seismogenic behaviour, from frequent regular earthquakes in the northwest part of the Nicoya Peninsula to large slow slip events in the southeast. Seismic evidence suggests that variations in pore-fluid pressure and/or fluid content within the overlying continental fore arc, due to along-strike differences in the permeability structure of the subducting oceanic crust, is likely responsible for variations in the strength of the upper plate. We therefore suggest that the spatial gradient in fluid content controls the segmentation in seismogenic behavior through corresponding variations in upper plate strength, in agreement with patterns of interseismic locking and seismogenic behavior along the Nicoya Peninsula.