How well do surface offsets represent earthquake slip at depth?
Abstract
Earthquake ruptures originate in the base of the unstable velocity-weakening part of the seismogenic layer and propagate into the velocity-strengthening upper stability region. Resulting deformation is transmitted to Earth's topography by driving slip along faults and block motions. Two factors affect this process: (1) the three-dimensional geometrical complexity and roughness of fault segments that control rupture continuity, and (2) spatiotemporal strength variations of the seismogenic layer that control the mechanical behavior of the fault zone. Earth's topography preserves evidence of past earthquakes as localized strain along fault scarps and fractures or distributed strain via off-fault folding and warping. These structures may be modified postseismically via afterslip and geomorphically degraded years to decades after rupturing. We investigate how well displaced geomorphic markers represent coseismic slip as it is transmitted from the seismogenic layer through to Earth's surface. We use lidar-derived measurements of single- and multi-event offset geomorphic markers to provide surface constraints on surface slip distributions of past earthquakes. Using FIMozFric, a numerical earthquake simulator that incorporates complex geomechanical properties of the seismogenic layer, we test various fault structural configurations and mechanical properties of the 2010 El Mayor-Cucupah earthquake. Initial simulations investigate the effect of simple fault complexities, such as stepovers and bends, on surface offsets using a simple two-layered seismogenic zone geometry. We then explore the effect of varying the mechanical complexity of the seismogenic zone by varying the relative geometries of the velocity-weakening and velocity-strengthening portions, thus simulating the variable mechanical properties of the upper lithosphere through which earthquakes propagate. Our results show that the geometrical complexity of faults controls the distribution of surface slip. More importantly, our simulations demonstrate that the mechanical configuration of the seismogenic layer within which faults are embedded affects how slip is distributed along single fault geometries and partitioned across zones with multiple fault strands. Variability in the depth of the velocity-weakening/velocity-strengthening interface appears to control the magnitude of single-event surface slip. These results shed light on the ability to confidently interpret paleoseismic and topographic records of the magnitude and recurrence of earthquakes, especially in relation to slip in a single earthquake. This is an especially important insight for efforts such as the Uniform California Earthquake Rupture Forecast (UCERF3) that aim to use lidar-derived slip in the most recent event as geologic constraints for expected slip magnitudes of future earthquakes but without accounting for the detailed lithology and structure of a fault zone.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T23C2600H
- Keywords:
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- 8100 TECTONOPHYSICS