The Convoluted Earthquake Cycle of a Creeping Fault
Abstract:The San Juan Bautista (SJB) segment of the San Andreas fault is an area of complex faulting; it experiences steady creep and produces slow slip events (SSEs) and earthquakes. While the historic record indicates as many as seven M>6 events in the 19th century, the largest earthquake in the last 100 years was the 1998 M5.1 San Juan Bautista earthquake. This earthquake occurred within an area of heterogeneously distributed creep and subsequently creepmeters and strainmeters detected a triggered SSE equivalent to a M5 that evolved over a week. Here, we present the results of a multi-pronged investigation into slip throughout the earthquake cycle on the San Juan Bautista segment. This includes a re-examination of the aftershock sequence of the 1998 M5.1 earthquake, which more clearly defines the fault area ruptured in the event. A new set of characteristically repeating earthquakes (CREs) provides insight into the decay of the triggered SSE and the distribution of interseismic creep, which compares favorably with a model from geodetic data. The analysis indicates that at seismogenic depths, the time period of enhanced creep was as much as six years, rather than one week. CREs also reveal pulses of aseismic slip in the Northern portion of the Creeping Section of the San Andreas fault near SJB. We further use 20 years of InSAR data over the San Juan Bautista region to investigate the variability of surface creep within this time span; using the Small Baseline Subset approach to generate an ordered time series from a set of interferometric pairs. While distinguishing annual variations due to groundwater changes from tectonic motions is difficult, we use principle component analysis to isolate creep variations due to the 1998 earthquake and SSE.
These diverse data sets provide valuable information on the interactions between earthquakes and aseismic slip that occurs in various depth ranges in the fault zone. Results from this work provide insight into the role of aseismic fault slip transients in earthquake occurrence and clustering.