Characterizing Earthquake Source Complexity in the San Jacinto Fault Zone: Implications for Calculation of Source Parameters

It is well known that many larger earthquakes have highly complex sources with significant variation in slip and stress drop over the rupture plane. As the quantity and quality of data increase, similar complexity is being observed in sources of well-recorded smaller earthquakes. We find evidence for complex rupture processes in earthquakes of M 2-5 using both time and frequency domain analysis, and investigate whether source complexity depends on structural complexity.

We systematically analyze the source complexity of earthquakes in the trifurcation area of the San Jacinto Fault zone (SJFZ), a right-lateral strike-slip fault system with high structural and geometric complexity that is the most seismically active in southern California. We combine both frequency and time domain analysis based on the empirical Green's function (EGF) technique to investigate the characteristics of source complexity for earthquakes from ~M 2- M 5 to better understand their underlying earthquake physics and the associated seismic hazard. In the frequency domain, we apply the multiple spectral ratio method, in which various EGF events are used to obtain stacked spectral ratios for target events, to ~600 potential target events with M ≥ 2 between 2005 and 2018 in SJFZ. We quantify the complexity of the stacked spectral ratios by examining their deviation from the simple omega-square source model. Preliminary results demonstrate that ~100 events as small as ~M 2 deviate significantly from this simple source model. In the time domain, we apply an iterative deconvolution method to retrieve relative source time functions (RSTFs), starting with selected target events that show significant spectral complexity. Preliminary results for these moderate earthquakes (~M 4-5) reveal clear directivity effects and the existence of multiple subevents. Ongoing RSTF analysis on smaller target events will help us further examine the connection between the spectral and time-domain behaviors of these earthquakes. Our study highlights the fact that the simple source models in common use are inadequate in estimating source parameters such as stress drop and source dimension and may limit our understanding on earthquake source physics.