Shallow slow slip events can nucleate on velocity-strengthening thrust faults

Abstract

Recent observations of faults in the shallow regions of subduction zones have revealed slow slip events that nucleate up-dip of the locked zone. Clay-rich sediments are prevalent at shallow depths and a large body of experimental work has shown that these sediments have a tendency towards velocity-strengthening frictional behavior, although velocity-weakening behavior is observed as well. Models of deeper slow slip, down-dip of the locked zone, generally require velocity-weakening behavior for events to nucleate. Here I show that slow slip events can nucleate and propagate on shallow, velocity-strengthening thrust faults, in a numerical model of a thrust fault dipping in a homogeneous, elastic half-space. This behavior is due to the broken symmetry of the thrust fault geometry, and is similar to behavior previously reported on bi-material, and poro-elastic faults. The interaction of the fault with the free surface (i.e. the sea floor) creates a coupling between normal stress on the fault and fault slip. This coupling allows velocity-strengthening slow slip events to nucleate, and becomes stronger at shallower depths. Here I conduct a parameter analysis, and show how this behavior is limited to certain values of the frictional and elastic parameters on the fault.