Improving 3-D Seismic Velocity Structure of the Mendocino Triple Junction Region in Southern Cascadia with Densified Permanent Seismic Network Data

Hao Guo

Submitted September 7, 2025, SCEC Contribution #14848, 2025 SCEC Annual Meeting Poster #TBD

The Mendocino Triple Junction (MTJ) region in northern California, the southernmost segment of the Cascadia subduction zone, is one of the most seismically active regions in the U.S. and poses a high risk of earthquake hazards. In the MTJ region, the Gorda plate interface, i.e., megathrust fault, is segmented into an interseismically locked zone at depths shallower than ~25 km, a region of episodic tremor and slow slip at ~40-60 km depths, and in a transition state in between those depth ranges. A 3-D seismic velocity model is essential for understanding downdip variations in megathrust fault-zone material properties and locking state, which are key to constaining the onshore extent of megathrust earthquake rupture and assessing seismic hazard. Guo et al. (2021) developed a high-resolution 3-D velocity model for both P and S waves (Vp & Vs) in the MTJ region through local earthquake tomography using the triple-difference tomography method with combined onshore and offshore datasets. This model is well resolved in the vicinity of the megathrust from the locked zone to the upper part of the transition zone. More importantly, it shows a substantial decrease in Vp/Vs from the locked zone (~1.85) to the transition zone (1.7-1.75), which likely indicates a significant decrease in porosity and fluid content and coincides with the brittle-to-ductile transition. Due to limited onshore data coverage, however, the model loses resolution in the lower part of the transition zone. Permanent seismic networks in the MTJ region have been densified in recent years in support of the ShakeAlert Earthquake Early Warning system, providing better coverage in the low-resolution regions of the Guo et al. (2021) model. We present an updated tomographic model that incorporates post-2016 earthquake data from these densified permanent seismic networks into the Guo et al. (2021) inversion, aiming to improve resolution in the lower part of the transition zone.

Key Words
Southern Cascadia Subduction Zone, Mendocino Triple Junction, Seismic Tomography, Seismic Velocity Model

Citation
Guo, H. (2025, 09). Improving 3-D Seismic Velocity Structure of the Mendocino Triple Junction Region in Southern Cascadia with Densified Permanent Seismic Network Data. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)