Onshore-offshore Velocity Models North of the Mendocino Triple Junction in Northern California

The Mendocino Triple Junction (MTJ) marks the boundary where the Gorda plate, North American Plate, and Pacific Plate meet, separating the Cascadia subduction zone to the North from transform tectonics of the San Andreas plate boundary. This actively deforming region experiences high seismic activity and young geological features. Here we examine offshore and onshore seismic wide-angle reflection and refraction data north of the MTJ crossing the Cascadia subduction zone from offshore northern California to the Western edge of the Sierra Nevada. We image subsurface structures in this region by creating P-wave velocity models using datasets collected in 1993 to 1994 during the MTJ seismic experiment (Trehu et al., 1995). Specifically, we use wide-angle reflection and refraction data collected along line WA6. The survey consisted of 220 km of onshore seismic refraction and reflection data and 140 km of offshore data. Previous studies using these data have created separate offshore P-wave velocity models using ocean bottom seismometer (OBS) data and onshore P-wave velocity models using onshore wide-angle, explosive-source seismic data. We have merged these datasets to create an integrated offshore-onshore P-wave velocity model. We use RAYINVR to construct a 2-dimensional (2-D) P-wave velocity model using forward modeling and TOMO2D to construct a 2-dimensional P-wave tomographic image of the subducting slab (Korenaga et al., 2000; Zelt and Smith, 1992). Our new velocity models offer insight into the structure and composition of both the Gorda and North American plates as well as the plate boundary megathrust in the region. We find P-wave velocities within the subducting 5-km-thick Juan de Fuca slab of ∼4.5-5.5 km/s, which is consistent to other previous studies. We also observed California’s Central Valley has low velocities of ∼3.0-4 km/s in the top ∼4-km-depth and is underlain by mid-crustal velocities of ∼6-7.0 km/s. The Klamath terrane is another area of interest, and it appears to be underlain by material with velocities of ∼5.5-6.0 km/s (Trehu et al., 1993). Our velocity model appears to show the Gorda plate dipping ∼13° at depths of ∼10-25 km. This study contributes to the understanding of earthquake hazards and tectonics in Northern California