Cluster analysis and stress inversion in the Mendocino Triple Junction

Yu-Fang Hsu, Daniel T. Trugman, Patricia Martínez-Garzón, & Yehuda Ben-Zion

Published September 8, 2024, SCEC Contribution #13504, 2024 SCEC Annual Meeting Poster #003

The Mendocino Triple Junction (MTJ) defines where the Gorda, North American, and Pacific plates meet and is one of the most seismically active regions in the contiguous United States. Tectonic movements along continental and oceanic transform faults and subduction zones lead to high background seismicity rates, but paradoxically, the productivity of earthquake clusters in this region is anomalously low, meaning that relatively few aftershocks are observed within any given earthquake sequence. Here we analyze earthquake-triggering processes near the MTJ by combining cluster analysis with characterization of the tectonic stress field. We apply the nearest-neighbor approach (Zaliapin & Ben-Zion, 2013) to investigate the spatiotemporal features of earthquake clusters and use the MSATSI algorithm (Martínez‐Garzón et al., 2014) to examine stress field orientations in the Mendocino transform fault (MTF), Gorda slab (GS), and northern San Andreas Faults (NSAF). The results indicate that the aftershock productivity (a), defined as the exponent quantifying the scaling between the number of aftershocks and the magnitude of mainshocks, of clusters in different regions vary from a=0.38 for the NSAF to a=0.27 for the MTF and a=0.25 for the GS region, all much lower than northern and southern California (a= 0.72 and 0.86, respectively). The cluster properties are stable with time and most clusters are spatially confined and do not extend across different plates, with the notable exception of the 1992 M7.1 Mendocino mainshock. The stress regime is the most compressive at the triple junction. The maximum horizontal compressional stress orientations (SHmax) form an included angle of 30-40° with the fault structures in the NSAF, suggesting that the faults are optimally oriented for failure. The SHmax orientations are variable around the boundary between the MTF and GS. The continuing study can provide additional insights into the tectonic regime, dynamics of earthquakes, and seismic hazards in the MTJ.

Citation
Hsu, Y., Trugman, D. T., Martínez-Garzón, P., & Ben-Zion, Y. (2024, 09). Cluster analysis and stress inversion in the Mendocino Triple Junction. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology