Impact of 3D structure on static Green’s functions for slip inversion

Leah Langer, USGS

Wednesday, February 10, 2021 at 10:30 AM

Location
Online-only seminar via Microsoft Teams

When an earthquake occurs, slip models of the event may be produced by inverting geodetic data for slip on a finite fault. This process generally requires coseismic Green's functions, which must be calculated in advance. The vast majority of such studies use Green's functions that ignore any 3D structure that is present in the region. The planet’s largest earthquakes, magnitude 8-9 subduction zone ruptures, occur underneath regions with the largest topographic gradients ranging from deep ocean trenches up to onshore mountain ranges. These regions also include wide variations in material properties. Inland earthquakes can also occur in regions with significant heterogeneity, such as mountain ranges or sedimentary basins, which have the potential to affect the deformation field.

Here, we investigate the effects of 3D structure on forward models of coseismic deformation and on earthquake static slip inversions. Using a newly-developed software package, SPECFEM-X, we show that the presence of topography alters the shape of predicted surface deformation patterns for the 2010 Maule and 2015 Gorkha earthquakes. We then compute coseismic Green's functions for these earthquakes in domains with and without topography, and perform Bayesian inversions using geodetic data. In both cases, we find that the use of Green's functions with topography yields a different distribution of slip. We then turn our attention to 3D elastic structure, and use SPECFEM-X to examine the impact of sedimentary basins on forward models of coseismic deformation. Our analysis of the 1984 Morgan Hill earthquake, which occurred near the Evergreen Basin, and of a hypothetical earthquake on the Sanchiao Fault near the Taipei Basin show that sedimentary basins generally affect the magnitude rather than the shape of surface deformation patterns. These findings suggest that the effects of 3D elastic structure may be qualitatively different from those of topography, and that 3D structure should be accounted for when estimating static slip in regions with significant heterogeneity.

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