Source Time Functions for the Anchorage Earthquake of 30 November 2018

Session: What Can We Infer About the Earthquake Source Through Analyses of Strong Ground Motion?
Type: Oral
Date: 4/29/2020
Time: 04:45 PM
Room: 115
Description: 

We compute source time functions for the Anchorage November 30, 2018, M7.1 earthquake from accelerometer records recorded within the nearby region. These source time functions (STF) illustrate the moment-rate on the fault as seen by a particular station. The STFs include the effect of directivity and any other rupture time characteristics. They do not assume a point source. The STFs are computed in the time domain by an iterative least-squares inversion with positivity constraints to deconvolve an empirical Green’s functions (EGF) within the waveform of a larger composite earthquake. An F-test is used to insure that each added sub-event reduces the variance. The STFs are interpreted as the time release of slip or stress events during the larger earthquake. For an empirical Green’s function we use an M 5.7 aftershock that occurred about 6 minutes after the mainshock and which was frequently recorded in the coda of the mainshock on the same record. The depth of the EGF was 40.7 km compared to 46.7 km of the mainshock. The distance between the main event and the EGF was about 7 km, but within the rupture zone of the mainshock. There were eight accelerometer stations within about 45 km and five within about 30 km of the epicenter. For the closer stations the STFs have a dominant pulse about 4.5s wide that is approximately triangular in shape (compared to about 10s wide for a double pulse from teleseismic analysis). Some of these STFs show a smaller pulse about 3s in duration preceding the main pulse. Multiple pulses are apparent in the records of stations to the northeast of the epicenter. The main 4.5s pulse is apparent on all STFs except one at a station to the east of Anchorage. Interpretation of the shape of the pulse depends on how much smoothing is applied, but with some smoothing, the pulses are surprisingly symmetric. Reformed waveforms (the STF convolved with the EGF) show good agreement with the mainshock waveform.

Presenting Author: Jon B. Fletcher

Authors