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Relative Afterslip Moment Does Not Universally Control Aftershock Productivity, But Do Special Case-studies Reveal More Nuance?

Robert Churchill

Submitted September 11, 2022, SCEC Contribution #12206, 2022 SCEC Annual Meeting Poster #030

Aseismic afterslip is postseismic fault sliding that may significantly redistribute crustal stresses and drive aftershock sequence. However, evidence for the link between afterslip and aftershocks is mostly case-study based, including observations of shared Omori-type decays and early spatio-temporal co-migration. Here, we use data from a compilation of 41 mainshocks to constrain first-order relationships between afterslip moment and the gross characteristics of aftershock sequences, focussing on potentially damaging (Mw≥4.5) aftershocks in the first 18 months. Whilst relative afterslip moment (Mrel, afterslip moment divided by coseismic moment) varies from <1% to >300%, we find that this does not correlate with aftershock number or cumulative moment (absolute or relative to expectation), seismic rate change, b-value, or Omori exponent p of the aftershock sequence. Adding afterslip moment to mainshock moment does not improve the correlation with absolute aftershock number. We therefore reject the hypothesis that afterslip moment alone controls Mw≥4.5 aftershock productivity, implying that other mechanisms are dominant. However, given the existing case-study evidence, we explore whether afterslip may influence other characteristics of aftershocks sequences such as the spatio-temporal distribution or at other magnitudes or timescales. To do this, we investigate case-studies with varied aftershock behaviours using high resolution regional seismic catalogs and detailed afterslip and coseismic slip distribution models. We explore correlations through space and time between the distributions of Mw≥2.5 aftershocks, coseismic slip and evolving afterslip (and their respective slip gradients). For example, we contrast characteristics of the relatively productive aftershock sequence following the 2004 Parkfield earthquake (>100% Mrel) and the relatively quiet sequence following the 2014 Napa earthquake (~30%Mrel). Our findings imply that relative afterslip moment is not a useful constraint on the production of damaging Mw≥4.5 aftershocks, but the spatio-temporal distributions of smaller aftershocks may be linked to afterslip in specific case studies.

Key Words
Aftershock Productivity Afterslip

Citation
Churchill, R. (2022, 09). Relative Afterslip Moment Does Not Universally Control Aftershock Productivity, But Do Special Case-studies Reveal More Nuance?. Poster Presentation at 2022 SCEC Annual Meeting.


Related Projects & Working Groups
Seismology