Complex Migration of Tremor Near Cholame, CA, Resolved by Seismic Array Analysis

Tectonic tremor observed along the Cholame and Parkfield sections of the San Andreas Fault has previously been associated with transient slip beneath the seismogenic zone. To study tremor and associated Low Frequency Earthquakes (LFEs), we deployed three dense near-fault seismic arrays for a period of 3 months in 2018. In early August 2018, the arrays recorded a strong 4-day-long deep transient that nucleated in Cholame and propagated toward the northwest. The initiation area is characterized by tremor that persists throughout the transient, while adjacent fault portions located to the northwest host more intermittent tremor. From the rate and location of tremor and LFEs, we infer a deep slip transient, whose along-strike propagation velocity is ∼8 km/day. Secondary tremor fronts are observed to travel ahead of the main slow slip front at speeds that are (Formula presented.) times faster. These rapid migrations propagate in slow slip events (SSE) slip-parallel direction, and are sometimes observed to back-propagate into previously ruptured segments. High-resolution images of secondary fronts obtained by using near-fault borehole stations indicate the active tremor band takes the shape of a narrow strain pulse that is bounded from above by the lower edge of the seismogenic zone. We estimate the stress drops associated with the secondary slip fronts are of the order of a few kPa. Frequency-dependent analysis suggests the tremor signal is coherent at frequencies as high as 16 Hz, and that the spatial distribution of high- and low-frequency tremor radiators is sometimes complementary.