High Resolution Catalogs of Template Earthquakes and Focal Mechanisms for Resolving Fine-Scale Fault Structures and Crustal Rheology in Southern California

Egill Hauksson

Submitted September 8, 2024, SCEC Contribution #13812, 2024 SCEC Annual Meeting Poster #037

We continue to refine and update the catalogs of earthquake hypocenters (Hauksson et al., 2012) and focal mechanisms (Yang et al., 2012) from 1981 to present in southern California. We present the latest versions of these catalogs that are publicly available through the SCEDC.

The next generation of the SCSN relocated catalog, expected to be available in early 2025, will apply GrowClust3D.jl using three-dimensional (3d) velocity model (Trugman et al., 2023) and a travel time grid produced by SIMUPLS or NonLinLog (Lomax et al., 2000). This will improve the quality of the SCSN catalog and enable easy adaptation of new technologies as they are added to GrowClsut3D.jl or other compatible location algorithms.

Currently our data processing relocation workflow includes three major steps. First, as part of initial data quality control, we collect hand-picked P and S phase arrivals from the Southern California Seismic Network (SCSN) and relocate the events using HYPOINVERSE with a 1d velocity model (Kein, 2000). All of the events in the catalog qualify for this step. Second, we relocate the events with SIMULPS (Thurber, 1983) to account for the complex 3d crustal structure in southern California. In both steps, we include station elevations to refer the focal depths to mean-sea level. Third, we cross-correlate the seismic waveforms to determine both P and S differential travel times. As part of this final step, we apply GrowClust (gc) to determine new locations for ~82% of the events that had sufficient data to qualify for relative relocations (Trugman and Shearer, 2017). The final complete catalog is built by using the 1d HYPOINVERSE catalog as the base and inserting 3d or preferably gc solutions when available. The catalog consists of ~800,000 earthquakes and ~40,000 quarry blasts that are available separately. We used the HASH method of Hardebeck and Shearer (2002 &2003) to determine corresponding ~185,000 focal mechanisms of A, B, C, and D quality using P-polarities and S/P amplitude ratios.

Most recently, we have used these catalogs to analyze the seismicity along the southern San Andreas fault (S-SAF), which extends for a ~450 km distance, from Parkfield to the Salton Sea. Because the S-SAF is the longest and fastest slipping fault in southern California, understanding the characteristics of the nearby high-precision relocated background seismicity in light of past and possible future M7.9+ events is of crucial importance.

Key Words
Seismicity, Seismotectonics, Earthquake Predictability

Citation
Hauksson, E. (2024, 09). High Resolution Catalogs of Template Earthquakes and Focal Mechanisms for Resolving Fine-Scale Fault Structures and Crustal Rheology in Southern California. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology