Spatio-temporal foreshock evolution of the 2019 M 6.4 and M 7.1 Ridgecrest, California earthquakes

The 2019 M 6.4 and M 7.1 Ridgecrest, California earthquake sequence provides an ideal opportunity to study the seismicity evolution and interaction among multiple complex fault structures. Here, we apply the matched-filter detection method to obtain a relatively complete (magnitude of completeness approximate to 0.9) and precisely relocated earthquake catalog. The results show a short-duration (similar to 31 minutes) foreshock sequence with 28 events, before the M 6.4 earthquake. The foreshock sequence started with a M 4.0 event and was aligned along the NW-SE direction. This implies that the M 6.4 rupture initiated on a NW trending fault segment, before rupturing the primary SW trending fault. Repeating earthquakes before and after the M 7.1 event are separated in space and bound the areas of large coseismic slip in the M 6.4 and M 7.1 events. This might reflect local slow slip acceleration near the edges of coseismic rupture asperities. The NW-striking fault zones illuminated by seismicity are separated into several subregions with distinct pre-M7.1 seismicity rate evolutions. The M 7.1 event nucleated in a region of local seismicity concentration which intensified similar to 3 hr before the M 7.1 mainshock. The M 7.1 nucleation zone is characterized by a significantly low b value of events that occurred since the M 6.4 event, which might indicate local failure conditions approaching a critical state. (C) 2020 Elsevier B.V. All rights reserved.