Effect of topography on slip inversions using geodetic data: application to the 2015 Gorkha, Nepal earthquake

Abstract:

A common simplifying assumption in inversions of geodetic data for the earthquake source is that the geodetic observations were made on a flat surface at zero elevation. In many cases, this approximation is reasonable, however in the case of large thrust faults, where the hangingwall of the fault is often accompanied by significant relief, including steep slopes and high elevations, it is much less secure.

We can anticipate two potential biases in source models that do not account for topography. First, geodetic observations made at high elevations are correspondingly further away from the faults that generated them, compared with a flat surface at sea level, and will thus require greater amounts of fault slip to provide the same surface displacements; models that do not account for topography may, therefore, underestimate seismic moment. Second, in situations where the topographic profile contains slopes on a length scale similar to that of the fault, the different elevations of the actual observation points may cause a bias in dip in a model that does not take topography into account.

We test these hypotheses against data from the 2015 Gorkha, Nepal earthquake. Using interferometric surface displacements imaged by the ALOS-2 satellite, we first invert for the geometry of the source fault, and its corresponding slip distribution, assuming a flat elastic half space. Our best fitting model has a dip of 15 degrees, steeper than seismic estimates (7-10 degrees), tentatively supporting our hypothesis of a possible bias. We will present the preliminary results of our modeling of the same data using finite element models that include a realistic representation of the topography, using both our preferred dip from our half space model, and the shallower preferred dip from seismology.