Local earthquake tomography of the Central Oregon forearc using a large-N, short duration, nodal array

Abstract

The crustal structure of the overriding plate in a subduction zone can have significant effects on subduction processes and therefore hazard along the margin. To investigate the controls of the overriding North American crust on the Cascadia Subduction Zone, a dense, trench perpendicular line of nodal seismometers was deployed in the Central Oregon forearc. The location of this deployment was chosen to fulfill two main goals (1) to examine the utility of large-N, short duration, nodal deployments in passive source seismic imaging (2) and to gain a better understanding of the structure of the Central Oregon forearc. With ∼40 days of continuous data recorded on 174, 3-component, 5 Hz instruments in a 130 km line with 500 m station spacing, 116 earthquakes and explosions are detected and located, a ∼3.5-fold increase from the number reported by the regional Pacific Northwest Seismic Network (PNSN) catalog. P, S, and PmP travel times from earthquakes located near the line of stations are used in a local earthquake tomography study to model Vp/Vs in the upper/middle crust and Vp throughout the crust of central Oregon down to 40 km depth. In conjunction, these models show a clear compositional discontinuity between the upper and lower Siletzia Terrane at ∼10 km depth as well as a zone of low Vp (6.5–6.8 km/s) in the lower crust that is attributed to slab derived fluids. This region of low Vp is juxtaposed with high Vp (6.9–7.2 km/s) to the east, interpreted as a strong, unfractured, block of Siletzia. High pore fluid pressures in the lower crust have been imaged to varying degrees across the Cascadia subduction zone. These features may be related to both the occurrence of tremor near the plate interface and the reduction of upper plate crustal earthquakes.