Frictional Properties of Subduction Input Sediments at an Erosive Convergent Continental Margin and Related Controls on Decollement Slip Modes: The Costa Rica Seismogenesis Project

The spectrum of slip modes occurring along shallow portions of the plate boundary decollement in subduction zones includes aseismic slip, slow slip, and seismogenic slip. The factors that control slip modes directly influence the hazard potential of subduction zones for generating large-magnitude earthquakes and tsunamis. We conducted an experimental study of the frictional behavior of subduction input sediments, recovered from two Integrated Ocean Drilling Program expeditions to the erosive subduction margin offshore Costa Rica (Expeditions 334 and 344), employing rotary shear under hydrothermal conditions. The velocity dependence of friction was explored, using simulated gouges prepared from all major lithologies, covering a wide range of conditions representative for the initial stages of subduction. Temperature, effective normal stress, and pore fluid pressure were varied systematically up to 140 degrees C, 110 MPa, and 120 MPa, respectively. Sliding velocities up to 100 mu m/s, relevant for earthquake rupture nucleation and slow slip, were investigated. The only sediment type that produced frictional instabilities (i.e., laboratory earthquakes) was the calcareous ooze carried by the incoming Cocos Plate, which by virtue of its slip-weakening behavior is also a likely candidate for triggering slow slip events. We evaluate this mechanism of producing unstable slip and consider alternatives. Therefore, locking and unlocking of plate boundary megathrusts are not only related to variations in pore fluid pressure but may also depend on the presence of pelagic carbonate-rich lithologies. Subduction systems containing such input are likely low latitude, where extensive deposition of carbonates takes place above the carbonate compensation depth.