Brittle-ductile Transition Depth and Depth of Seismicity in Southern California: Agreement in Active Regions but Discrepancy Elsewhere

Megan Perry, & Laurent G. Montesi

Published September 8, 2024, SCEC Contribution #13523, 2024 SCEC Annual Meeting Poster #224

Earthquakes at active continental plate boundaries typically occur at depths less than 12-15 km, possibly linked to a critical temperature or a transition from brittle to ductile behavior. In southern California, earthquake depth varies with activity and potentially correlates with heat flow and strain rate, suggesting the brittle-ductile transition depth (BDT) may control seismicity depth. To further examine this hypothesis, we compare here predictions of BDT from the Statewide California Earthquake Center (SCEC) Community Rheology Model (CRM) in Southern California south of ~37.7°N with observed seismicity cutoff depth. Earthquake locations and depths come from the Southern California Earthquake Data Center (SCEDC), declustered following the Gardner and Knopoff (1974) algorithm. We identified all the earthquakes in that catalog between 2000 and 2020 located within 20 km of target points distributed throughout the study area to obtain D95 estimates. Alternative depth of seismicity estimates in each block defined in the CRM are provided either by averaging the local D95 values in each block or calculating the 95th percentile depth of all the earthquakes occurring in that block. Then, SCEC community models provide estimates of temperature, lithology, and ductile flow law at the depth of seismicity. Finally, we deduce the strain rate necessary to match the depth of seismicity and compare it with separately estimated strain rates from the Community Geodetic Model (CGM). The D95/CRM-derived strain rates are consistent with the CGM in active regions but show discrepancies in inactive regions. Adding diffusion creep flow laws or considering localized temperature anomalies may help resolve these discrepancies. However, this solution is best justified in ductile shear zones and may be less relevant for low-activity regions. Alternatively, the stress level may not be sufficient in these locations to allow earthquake activity to the rheologically-controlled BDT, or deep earthquake activity may be infrequent enough that the 20-year catalog used here is too short. Looking at variations in D95 and thermal structure within lithotectonic blocks may help differentiate between these hypotheses.

Citation
Perry, M., & Montesi, L. G. (2024, 09). Brittle-ductile Transition Depth and Depth of Seismicity in Southern California: Agreement in Active Regions but Discrepancy Elsewhere. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Community Earth Models (CEM)