Impact of topography on earthquake static slip estimates

Abstract

Our understanding of earthquakes is limited by our knowledge, and our description, of the physics of the Earth. When solving for subsurface fault slip, it is common practice to assume minimum complexity for characteristics such as topography, fault geometry and elastic properties. These characteristics are rarely accounted for because our knowledge of them is often partial and they can be difficult to include in simulations. However, topography and bathymetry are known all over the Earth's surface, and recently developed software packages such as SPECFEM-X have simplified the process of including them in calculations. Here, we explore the impact of topography on static slip estimates. We also investigate whether the influence of topography can be accounted for with a zeroth-order correction which accounts for variations in distance between subfaults and the surface of the domain. To this end, we analyze the 2015 Mw 7.5 Gorkha, Nepal, and the 2010 Mw 8.8 Maule, Chile earthquakes within a Bayesian framework. The regions affected by these events represent different types of topography. Chile, which contains both a deep trench and a major orogen, the Andes, has a greater overall elevation range and steeper gradients than Nepal, where the primary topographic feature is the Himalayan mountain range. Additionally, the slip of the continental Nepal event is well-constrained, whereas observations are less informative in a subduction context. We show that topography has a non-negligible impact on inferred slip models. Our results suggest that the effect of topography on slip estimates increases with limited observational constraints and high elevation gradients. In particular, we find that accounting for topography improves slip estimates where topographic gradients are large. When topography has a significant impact on slip, the zeroth-order correction is not sufficient.