Earthquake clustering and stress inversion in the Mendocino Triple Junction region

SUMMARY The Mendocino Triple Junction (MTJ), where the Gorda, North American and Pacific plates meet, is one of the most seismically active regions in California. The tectonic movements along the Mendocino transform fault zone (MTFZ), Gorda slab (GS) and northern San Andreas Fault systems (NSAF) lead to high background seismicity rates but relatively low aftershock productivity. To improve the understanding of earthquake processes in the area, we analyse relations between background seismicity, aftershock productivity and stress parameters. We apply the nearest-neighbour approach to investigate the spatial distributions and properties of background and clustered seismicity, and invert focal mechanisms of events in Voronoi cells for features of the deviatoric stress field. The results indicate that the intensity of background seismicity and aftershock productivity decrease with distance from the MTJ, defined here for simplicity as the hypocentre of the 1992 Mw 7.2 main shock. We also find that the stress regime is the most compressive in the area directly surrounding the MTJ. In the MTFZ and GS, the compressive stress decreases with increasing distance from the MTJ, correlating with the reduced aftershock productivity and background seismicity. In the NSAF, the observed relations between the stress, aftershock productivity and background seismicity are not clear, possibly due to crustal extension related to the slab window and elevated heat flow. Compared to the MTFZ and GS, the NSAF has a higher foreshock proportion, lower aftershock proportion and small-to-medium main shock magnitudes, indicating more swarm-like clusters in this region. The inverted stress regimes in the MTFZ and NSAF are dominated by strike-slip faulting. The GS exhibits mostly strike-slip and normal mechanisms despite the subduction environment, which may reflect slab bending and reactivation of pre-existing normal faults.