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The Roles of Coseismic Slip and Afterslip in Driving On-fault Aftershock Distributions: An Analysis of Behaviourally-varied Continental Case Studies

Description: 

Spatio-temporal complexity is a common characteristic of aftershock sequences and may require explanation beyond the role of coseismic Coulomb static stress change. Aseismic afterslip has long-been proposed as a driver of aftershock sequences, generally based on observations of spatial co-migration and a shared decay. However, observational evidence is limited to case studies, and efforts to test systematically have not indicated that afterslip is the outright driver of aftershocks. To investigate the role that afterslip may play in specifically driving the spatial distributions of near/on-fault aftershocks, we investigate seven behaviourally varied Mw6.0-7.6 continental earthquakes (in terms of relative afterslip moment and relative aftershock productivity), using high resolution coseismic and afterslip models and regional seismic catalogs. We project slip models and aftershock data onto a best-fitting plane, and statistically test whether coseismic slip, afterslip or total cumulative slip (and each of their respective gradients) best explains near/on-fault aftershock density. In six out of seven cases, we show that total cumulative slip correlates with near/on-fault aftershock density moderately-strongly, and fairly stably through time (up to ~1000 days). However, to explain why we see aftershocks occurring in regions which have: 1) apparently already ruptured (having gone coseismic slip), and 2) theoretically velocity-strengthening regions (having undergone afterslip), we propose that abundant fine scale rheological heterogeneity and/or significant amounts of conditional stability/instability must be present in fault zones. We discuss the role that uncertainty may have on our results, but confidently conclude that near/on-fault aftershock density is well-predicted by total cumulative slip, for analyses at these (2x2 km) resolutions. Pragmatically, this may be used to update our understanding of likely aftershock distributions in real-time.

Session: New Methods and Models for More Informative Earthquake Forecasting

Type: Oral

Date: 4/19/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Robert M. Churchill

Student Presenter: No

Invited Presentation: 

Authors