A Model for Low‐Frequency Earthquake Slip

Abstract

AbstractUsing high‐resolution relative low‐frequency earthquake (LFE) locations, we calculate the patch areas (Ap) of LFE families. During episodic tremor and slip (ETS) events, we define AT as the area that slips during LFEs and ST as the total amount of summed LFE slip. Using observed and calculated values for AP, AT, and ST, 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 (SETS) in distinct subpatches (i.e., AT ≪ AP). In the connected patch model, AT = AP, but ST ≪ SETS. LFEs cluster into 45 LFE families. Spatial gaps (∼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 ∼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 ( and and and , respectively). During ETS, Mc decreases with downdip distance but NT does not change. Over the entire ETS cycle, Mc decreases with downdip distance, but NT 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.