Using Earthquake Relocation in the Mendocino Region of Northern California to Explore Complex Triple Junction Faulting

Susan Bilek, & Vennessa Maestas

Published 2024, SCEC Contribution #14161

The Mendocino triple junction (MTJ) region is one of the most seismically active areas of California, with least 40 earthquakes of magnitude 6 or larger in the region over the past 100 years, including recent 2021 and 2022 events near the Ferndale, CA region that produced widespread power disruptions and damage. These earthquakes are associated with the interactions of the Pacific, Gorda, and North America plates, bringing together subduction and transform plate boundaries in a complex zone of deformation. Both large and small magnitude earthquakes in this region highlight this fault complexity. Large events occurred on either megathrust or related forearc thrust faults (1992 M7.1 earthquake), combined slab fault and shallow strike slip faults during portions of the more recent 2021 and 2022 M6+ ruptures, and past slip on the San Andreas Fault System faults during the large 1906 rupture. Smaller magnitude seismicity is also complex, and recent studies using data from both onshore and offshore seismic networks suggest activity on several onshore faults as well as within the slab. Here we use earthquake catalogs from these recent studies (Alongi et al., 2021 and Morton et al., 2023) to relocate the seismicity in this complex zone to illuminate the fault structures active during the deployment period (2012-2013 and 2014-2015). We use existing arrival time phase picks with the GrowClust relocation algorithm to refine the event locations for nearly 4.000 possible events in the Mendocino triple junction region offshore northern California. With these relocations, we will explore the relative activity on the different fault structures within this complex zone and any spatiotemporal patterns of slip during the study period.

Citation
Bilek, S., & Maestas, V. (2024). Using Earthquake Relocation in the Mendocino Region of Northern California to Explore Complex Triple Junction Faulting. Poster Presentation at American Geophysical Union Fall Meeting.