A high-density nodal array to study the structure and seismogenic behavior of the southern Cascadia forearc

Abstract:

Details of the seismic behavior and structure of forearcs that experience episodic tremor and slip (ETS) remain difficult to quantify due to significant heterogeneity in the seismic properties of these regions and sparse seismic station distributions. Along the southernmost portion of the Cascadia margin’s forearc, inferences about the detailed geometric and structural configuration appear to show significant discrepancies with regional-scale models of slab geometry, further complicating our understanding of ETS and associated processes. In an attempt to better characterize the structure and seismogenic behavior in this particularly productive region of ETS, a passive array of 60 nodal seismometers was deployed throughout the forearc in northern California, USA.

The goals of this array are multifold, including: 1) quantifying low frequency earthquake distributions through detailed earthquake locations, 2) constraining the depth to the plate interface between the subducting Gorda and overriding North American plate, 3) imaging the structure of the overriding forearc crust, and 4) characterizing the complex nature of the crustal seismicity in the region. Currently, a relatively small number of well-located forearc LFEs in the region suggest that the Gorda slab subducts at a much shallower angle than existing slab models predict, in agreement with new seismic images of the margin. In addition, observations from thermochronological analyses and seismic velocity models from a relatively sparse station distribution suggest surficial patterns of exhumation and deep tremor distribution may be related through subduction underplating. To further understand the possible linkages of these phenomena, we attempt to obtain higher resolution images of the forearc lower crustal structure in this tectonically complex forearc. Lastly, this array densifies station coverage in the region by 8-fold, allowing for better earthquake locations to be obtained in this region of broadly distributed crustal seismicity as it responds to subduction from the west and the impingement transform motion from the south.