Interseismic Slip Rates on Quaternary Faults in the Southern Cascadia Forearc Based on Block Modeling of Geodetic Data

The Mendocino Triple Junction (MTJ) represents a region of complex deformation in which the Cascadia subduction zone and the San Andreas strike-slip systems converge. North of the MTJ, the southern Cascadia forearc accommodates strain from both of these tectonic regimes along thrust and strike-slip fault structures. However, the interseismic slip rate on several of these major fault systems, including the Mad River fault zone (MRFZ), Little Salmon fault and Grogan fault, are not well constrained. Additionally, regional strain rate maps indicate that these structures may accommodate some degree of oblique slip instead of pure thrust or strike-slip motion. Paleoseismic work reveals that the Little Salmon fault has hosted several large (Mw >7) earthquakes in the past ~1700 years, while the MRFZ has also hosted more frequent, smaller events. Therefore, constraining the slip rate and sense of slip on these faults is crucial for evaluating regional seismic hazards and for gaining a better understanding of how crustal strain is accommodated near the triple junction. This work focuses on using elastic block models to investigate the overall nature of these three fault systems and to explore how each fault contributes to deformation in the forearc. While separate large-scale interseismic block models have been constructed for the Pacific Northwest and the Southwestern US, the faults surrounding the MTJ have not previously been studied using a local-scale block model approach. We test eight different block geometries in order to explore various forearc fault configurations and constrain the partitioning of strain. In addition to crustal faults, we also include the Cascadia megathrust to account for tradeoffs between subduction and crustal thrusts, and to test different locking scenarios. Block rotation, internal strain, and interseismic slip on block edges are constrained by interseismic horizontal GPS velocities provided by PANGA. After assessing all eight models, we conduct a bootstrapping analysis on the four best-performing block geometries to obtain histograms of fault slip rates. The preliminary bootstrapping results indicate that the MRFZ, Little Salmon, and Grogan faults all experience some degree of oblique slip, with the proportion of dip-slip to strike-slip motion and overall slip rate varying along strike.