A Model for Low-Frequency Earthquake Slip

Using high-resolution relative low-frequency earthquake (LFE) locations, we calculate the patch areas (A(p)) of LFE families. During episodic tremor and slip (ETS) events, we define A(T) as the area that slips during LFEs and S-T as the total amount of summed LFE slip. Using observed and calculated values for A(P), A(T), and S-T, we evaluate two end-member models for LFE slip within an LFE family patch. In the ductile matrix model, LFEs produce 100% of the observed ETS slip (S-ETS) in distinct subpatches (i.e., A(T) << A(P)). In the connected patch model, A(T) = A(P), but S-T << S-ETS. LFEs cluster into 45 LFE families. Spatial gaps (similar to 10 to 20 km) between LFE family clusters and smaller gaps within LFE family clusters serve as evidence that LFE slip is heterogeneous on multiple spatial scales. We find that LFE slip only accounts for similar to 0.2% of the slip within the slow slip zone. There are depth-dependent trends in the characteristic (mean) moment and in the number of LFEs during both ETS events (only) and the entire ETS cycle (M-c(ets) and N-T(ets) and M-c(all) and N-T(all) , respectively). During ETS, M-c decreases with downdip distance but N-T does not change. Over the entire ETS cycle, M-c decreases with downdip distance, but N-T increases. These observations indicate that deeper LFE slip occurs through a larger number (800-1,200) of small LFEs, while updip LFE slip occurs primarily during ETS events through a smaller number (200-600) of larger LFEs. This could indicate that the plate interface is stronger and has a higher stress threshold updip.