The Earth isn't Flat: The Large Influence of Topography on Geodetic Fault Slip Imaging

Thomas Thompson, Harvard University

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Images of earthquake slip serve as the foundation for understanding the distribution of coseismic energy release, theories about rupture propagation, and fault mechanics. Geodetic slip images are inferred using elastic models that link surface observations to fault activity at depth. To date, almost all elastic models used for these common calculations include the assumption that the Earth is flat, even for earthquakes in regions of extreme topographic relief like the 2008 MW=7.9 Wenchuan earthquake at the Longmenshan range-front. Here we develop three-dimensional boundary element models that explicitly include topographic relief and find that the classical flat-Earth assumption may induce errors greater than 100 percent in imaged coseismic slip. This topographic effect is important even in regions with small topographic gradients (1-5 percent, 1992 MW=7.3 Landers) and can lead to the inaccurate inference of a shallow slip deficit. These results illustrate that understanding earthquake behaviors will require more accurate models not only in terms of physical processes but also in terms of geologic fidelity.

Abstract Author(s): T. Ben Thompson, Brendan J. Meade