Highly Faulted Upper Crust Caused by Continental Rifting: Evidence from Seafloor Compliance Inversion in the Okinawa Trough

Abstract:

Seafloor compliance measures the elastic response of the ocean bottom to water pressure fluctuation, and is a function of the elasticity of the sediment/crust. We have measured seafloor compliance from OBS data at two sites on the Okinawa trough (OT), which results from continental rifting, and one site on the Ryukyu arc (RA), a crustal block separated by the OT from the Eurasian plate, in the vicinity of Taiwan. Because the water depth is known and the sediment thickness can be estimated from seismic studies, this study offers a comparison in crustal properties between rifting and non-rifting environment. We performed both grid search and minimum-structure inversion for Vs. The models that best explain the compliance data indicate that the shear velocity of the upper crust of the RA is higher than that in the OT. This implies that the extension of the OT may cause significant faulting/fracturing of the brittle part of the crust and lower its shear modulus. Between the 2 OT sites, the Vs is lower for the OBS more distant from Taiwan. This observation is consistent with the scenario in which the crust is faulted and weakened more as rifting process intensifies from the edge of the Okinawa trough.