Imaging High Pore Fluid Pressure in Subduction Zones Using Ambient Seismic Noise

Abstract:

Mineral dehydration-derived fluids expelled episodically during the earthquake cycle have been implicated in increasing pore fluid pressure and decreasing the effective stress and thus strength of subduction zone forearc crust. Such weakened crust would be more susceptible to damage from dynamic stresses generated by large megathrust earthquakes. We document a seismic velocity reduction in the forearc crust of the Nicoya Peninsula Costa Rica, following the Mw 7.6, 2012 earthquake that we attribute to an increase in fracture density in high pore fluid pressure, weakened crust. Using data from the Nicoya Peninsula broadband seismic network, we perform cross-correlation of two years of ambient seismic noise to retrieve Empirical Green’s Functions (EGFs) between different station pairs. Analysis of short period (1-10 s) data reveal a shallow (1-5 km depth) crustal velocity reduction of ~ 0.5% beneath station pairs within the coseismic rupture zone. This reduction is likely related to non-linear ground acceleration during coseismic deformation of the upper plate. Long-period (3-10 s) analysis, reveals a deeper (5-15 km) and stronger 0.6% coseismic velocity reduction farther from the rupture area, in the southeastern part of the Peninsula. This is the same region where a previous study documented the presence of anomalous forearc fluid pressures. Although pressurized fluids have been linked to weakened crust and an induced seismic velocity reduction beneath volcanic arcs, this is the first observation of a similar process occurring in a subduction zone environment, using ambient seismic noise monitoring. It therefore demonstrates the potential of imaging regions of high pore fluid pressure in subduction zones using temporal variations in ambient noise.