Search for repeaters in the central part of the Chilean subduction zone

Repeating earthquakes, corresponding to the rupture of the same asperity over time at more or less regular intervals, can be used to estimate the slip rate on a subduction plate interface. The purpose of this work is to build a catalog of repeaters for the central part of the Chilean subduction zone, extending from 24°S to 33°S latitude and centered on the Copiapo seismic gap. As a basis for our study, we used the seismicity catalog from the Centro Sismológico Nacional (CSN).

The similarity between waveforms gives a good criterion to assign earthquakes to a similar asperity. To measure it, we calculated for each pair of events the coherency, correlation and associated time lag between the vertical components of their P waves, on a 5 s window starting 1 s before the P arrival. We tested different frequency bands (1-4 Hz, 3-12 Hz, 5-20 Hz, ...) and kept each time the one with the best coherency value. We selected all pairs of earthquakes with coherency higher than 0.95, at three or more stations. To ensure a stable measurement, we imposed that the time lags from cross-correlation and coherency differ by less than 0.01 s. To verify that the earthquakes of a repeaters family take place at the same location, we relocated the events using a double-difference method and created clusters based on both coherency and location similarity. As coherency values are calculated on a 5 s window, we relocate the centroids of the events, i.e. the center of mass of the rupture. To estimate the surface rupture, we calculate the rupture radius based on the seismic moment and stress-drop values (Eshelby 1957), estimating the stress-drop and seismic moment with SourceSpec (Satriano 2023).          

As preliminary results, we found 347 families, mostly located between 30-60 km deep, and between 29-33.5°S. Almost no repeaters were found before 2015 due to the lack of available stations. Obtained families contain a few events, with 11 earthquakes in the biggest one. We compared the obtained repeaters with the coupling along the plate interface.  Most repeaters are located at the transition between strong and low coupling zones in the Illapel area, making a circle shape around the deep part of the Illapel coseismic slip. Furthermore, we investigated the evolution of the number of repeaters with time in different areas and found potential aseismic slip marked by repeaters' activity consistent with previous observations, such as before the 2017 Valparaiso sequence (Ruiz et al., 2017) or after the Atacama sequence (Klein et al., 2021).

In order to obtain more complete repeaters families, we created a new machine learning based earthquake catalog for the study area with SeisBench (Woollam et al., 2022), using data from permanent and temporary networks in Northern and Central Chile. We are currently applying our analysis to this new catalog.