Leah Langer, Théa Ragon, Anthony Sladen, Jeroen Tromp

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 the Earths characteristics such as topography, fault geometry and elastic properties. These characteristics are difficult to include in simulations and our knowledge of them is incomplete, leading many to believe that there is minimal advantage to be gained by accounting for these effects. However, 3D structure can be easily included with the newly-developed software package SPECFEM-X, and topography and bathymetry can be measured directly and are well known all over the Earths surface. Accounting for topography thus seems to be an efficient strategy for incorporating accurate and extensive information into the inverse problem. Here, we explore the impact of topography on static slip estimates, with a particular focus on how topography may impact slip resolution on the fault. We also wonder if the influence of topography can be accounted for by simply using a receiver elevation correction. To this end, we analyze the 2015 Mw7.5 Gorkha, Nepal, and the 2010 Mw8.8 Maule, Chile, earthquakes within a Bayesian framework.
The regions affected by these events represent two different types of topography. Chile, which contains both the Peru-Chile trench and the Andes mountains, has a greater 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 two main factors: poor observational constraints and high elevation gradients. In particular, we find that accounting for topography improves slip resolution where topographic gradients are large and the data is less informative: at depths for the Gorkha event, and near the trench for the Maule event. When topography has a significant impact on slip resolution, which is probably the case of many subduction events, the receiver elevation correction is not sufficient.