Implications of Receiver Plane Uncertainty for the Static Stress Triggering Hypothesis. Issue 5 (29th April 2022)

• Record Type: Journal Article
• Authors: Hanagan, C.; Bennett, R. A.; Chiaraluce, L.; Hughes, A.; Cocco, M.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 5(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-04-29
• Subjects: static stress triggering uncertainty -- aftershocks -- L'Aquila -- Ridgecrest -- Coulomb failure -- Umbria‐Marche; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB023589
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Static stress transfer from major earthquakes is commonly invoked as the primary mechanism for triggering aftershocks, but evaluating this mechanism depends on aftershock rupture plane orientations and hypocenter locations, which are often subject to significant observational uncertainty. We evaluate static stress change for an unusually large data set comprising hundreds to thousands of aftershocks following the 1997 Umbria‐Marche, 2009 L'Aquila (Italy), and 2019 Ridgecrest (California) earthquake sequences. We compare failure stress resolved on aftershock focal mechanism planes and planes that are optimally oriented (OOPs) in the regional and earthquake perturbed stress field. Like previous studies, we find that failure stress resolved on OOPs overpredicts the percentage (>70%) of triggered aftershocks relative to that predicted from observed aftershock rupture planes (∼50%–65%) from focal mechanisms solutions, independent of how nodal plane ambiguity is resolved. Further, observed aftershock nodal planes appear statistically different from OOPs. Observed rupture planes, at least for larger magnitude events ( M > 3), appear to align more closely with pre‐existing tectonic structures. The inferred observational uncertainty associated with nodal plane ambiguity, plane orientation, and, to second order, hypocentral location yields a broad range of aftershocks potentially triggered by static stress changes, ranging from slightly better than random chance to nearly any aftershock promoted, particularly those further than 5 km from the causative fault. Dynamic stresses, afterslip, pore fluids, and other sources of unresolved small‐scale heterogeneity in the post‐mainshock stress field may also contribute appreciably to aftershock occurrence closer to the mainshock. Plain Language Summary: Large earthquakes are followed by a decaying number of smaller earthquakes, called aftershocks, which are hypothesized to occur on subsidiary faults and fractures stressed by the mainshock. The percentage of aftershocks potentially triggered by this mechanism depends on the locations and orientations of the aftershock faults, and is susceptible to observational uncertainties in these parameters. We evaluate the percentage of aftershocks encouraged by stress changes for the 1997 Umbria‐Marche, 2009 L'Aquila (Italy), and 2019 Ridgecrest (California) earthquake sequences using large, published aftershock data sets. While only ∼50%–65% of aftershocks are nominally encouraged by mainshock stress changes, observational uncertainties are large enough that any aftershock could be explained by random chance, or could have been triggered. We also test the common assumption that aftershock faults are oriented to maximize failure in the applied stress field. Contrary to this assumption, we find that aftershocks tend occur on planes oriented more consistently with pre‐existing mapped faults. Accounting for aftershock fault plane uncertainties is critical when evaluating the percentage of potentially triggered aftershocks. Additional sources of uncertainty or other triggering mechanisms may be required to explain aftershock occurrence, especially in the immediate vicinity of the mainshock. Key Points: Aftershock uncertainties increase the range of statically triggered events from little better than the chance to nearly any event promoted Optimally oriented planes for Coulomb failure misrepresent aftershock planes, which may be better represented by considering mapped faults Static stress triggering near the mainshock remains uncertain from receiver plane uncertainties, and not only source uncertainties