Source Time Functions for a M4.5 Aftershock Within the 2019 Ridgecrest, California Earthquake Sequence

Session: Observations From the 2019 Ridgecrest Earthquake Sequence [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

The M6.4 and M7.1 Ridgecrest earthquakes of July 4 and 5, 2019 were the largest to hit Southern California in 20 years. Within days of the mainshock several temporary accelerograph networks had been deployed to supplement the existing permanent network in the area, resulting in a rich dataset that enables the modelling of moderate aftershocks using the recordings of smaller aftershocks as empirical Green’s functions (EGF). We calculate the source time functions (STF) of a M4.5 aftershock at 14 accelerometer recording sites within 30 km. These STFs reveal the moment-rate on the fault as seen by a particular station and include the effects of directivity and any other rupture time characteristics. They do not assume a point source. We compute STFs in the time domain by an iterative least-squares algorithm that uses positivity constraints to deconvolve an EGF from the waveform of the larger earthquake. An F-test is used to ensure that each added sub-event reduces the variance. The resulting deconvolved time-series is interpreted as the time release of slip or stress events during the larger earthquake. For the EGF, we use a M2.7 aftershock that occurred less than 0.5 km from the M4.5 earthquake in epicentral distance and depth. Both events are located within the primary NW-SE trending seismic zone of the Ridgecrest sequence. The resulting STFs at sites toward the NW are generally single, symmetric pulses with widths between 0.5 and 1s, while sites toward the SW and NE have a similar but slightly wider shape, and STFs toward the SE are broader and more complex with multiple pulses. These results indicate that the rupture propagated toward the NW, which is consistent with the published NEIC moment tensor solution and suggests that aftershocks of the M7.1 Ridgecrest earthquake have their own complex structure in space and time. We are able to verify STF results using at least one of three additional EGFs (2≤M≤2.5) at 10 of the 14 sites.

Presenting Author: Jemile E. Erdem

Authors