An evaluation of Coulomb stress triggering during the 1997 Umbria-Marche, 2009 L’Aquila, Ridgecrest 2019 earthquake sequences

Abstract:

Static stress change by measure of Coulomb Failure Stress (∆CFS) from large earthquakes is often invoked to explain subsequent earthquakes, including aftershocks and more complex sequences that do not follow Omori’s Law. Other triggering mechanisms have also been proposed, such as transient dynamic stresses associated with the passing of seismic waves. Many past assessments of ∆CFS as a triggering mechanism compared ∆CFS resolved on optimally oriented planes with the spatial distribution of aftershocks. However, the fraction of events promoted by ∆CFS has been reported to diminish when rupture plane orientations and slip directions are accounted for in evaluating ∆CFS. We calculate the static stress change on large magnitude events and on hundreds to thousands of aftershock fault planes for the 1997 Umbria Marche and 2009 L’Aquila, Italy earthquake sequences, as well as the 2019 Ridgecrest, California earthquake sequence. Despite the difference in tectonic setting, sense of slip (normal versus strike-slip), lithology, and potentially other factors, only 60 to 70% of aftershocks appear to be promoted by static stress changes for each earthquake sequence. The unexplained >30% must either have failed by other mechanisms or represent deficiencies in our data and models. Our preliminary results indicate the adopted receiver plane parameters, including slight changes in the location, strike, dip, and/or rake, can impact the magnitude and sign of ∆CFS and thus the proportions of events that appear to have been triggered or not. The inclusion and details of small events preceding larger events can also have an effect on the magnitude and sign of ∆CFS. In this presentation, we will evaluate the uncertainty ranges of earthquake source and receiver fault parameters and their impacts on ∆CFS calculations, with implications for our ability to test the ∆CFS triggering hypothesis.