Slow Slip to Trench Associated With Deeper Seismic and Aseismic Events Inferred From Seafloor and Formation Fluid Pressure Observations at Costa Rica and Nankai

Abstract:

Slow slip along the subduction plate interface -- as opposed to seismic rupture during a subduction earthquake -- can occur in the interseismic period and/or follow a thrust earthquake as afterslip. At Costa Rica, examples for the propagation of slow slip to the trench have been documented with seafloor and formation fluid pressures recorded since 2003 at two CORK sites < 1 km apart on the seaward and landward sides of the subduction prism toe. The inter-site difference in seafloor pressure did not change at the time of the 2012 M 7.6 Nicoya earthquake, but changes beginning 1.5 days after the earthquake revealed both episodic and gradual postseismic uplift of the prism toe relative to the incoming plate. By inference, trench-breaching afterslip occurred on the shallow plate interface. The temporal evolution of the differential pressure at the trench and the postseismic GPS time series on the coast share common time scales, implying that the onshore and offshore deformations are caused by the same afterslip process. The slow motion of the outermost prism is probably passively driven. Several other “paired” onshore and offshore deformation events have been documented. In the case of a 2009 slow slip event, the inferred slip at the trench (~ 11 cm) is larger than the slip further landward estimated from GPS data by a factor of ~ 5. This suggests that some shallow slow slip must be driven by local stress stored in the outermost prism. At Nankai trough where a similar pair of CORK borehole observatories have been operating since 2002, a clear relationship is seen between episodic slip to the trench (constrained by the borehole data) and deformation further landward (constrained by VLFE activity along the plate interface and/or within the prism < 10 km below seafloor). In this case, the link appears to be limited to the outer prism seaward of the seismogenic zone. Near-trench seafloor and formation pressure observations at these two subduction zones provide important clues for studying the shallow slow slip and its connection to deeper seismogenic or slow slip and tremor events. These, and future comparative studies of shallow fast and slow slip events at different subduction zones, will help us understand the variations of shallow megathrust behavior geographically and through subduction earthquake cycles.