Assumptions on elastic structure in finite-fault models: the case of the secondary zone of uplift measured after megathrust earthquakes

A secondary zone of surface uplift (SZU), located from 200 to 400 kilometers landward of the trench, has been measured after several megathrust earthquakes. The SZU reached a few centimeters hours to days after the 2011 Mw 9.1 Tohoku (Japan) and 2010 Mw 8.8 Maule (Chile) earthquakes. One interpretation is that this SZU is universal, driven by volume deformation around the slab interface (van Dinther et al. 2019). Indeed, published coseismic finite-fault models for these events do not reproduce the measured SZU.

Here, we build on the case of the SZU to understand if, and how, our prior assumptions on the forward model can prevent us (or allow us) to make the most out of our dataset. In particular, we investigate under which assumptions the SZU can, or cannot, be predicted with fault slip. We show the SZU cannot be reproduced with coseismic finite-fault models that neglect 3D elastic heterogeneities in lithospheric structure. In contrast, we can recover the SZU with fault slip if elastic heterogeneities associated with the subducting slab are accounted for, as opposed to assuming homogeneous or layered elastic lithospheric structures. The SZU may therefore result from slip on the slab interface, downdip of the main coseismic patch. We suggest SZU might be caused by rapid afterslip, but a deformation of the volume around the fault cannot be ruled out.

Reference: van Dinther, Y., Preiswerk, L. E., & Gerya, T. V. (2019). A Secondary Zone of Uplift Due to Megathrust Earthquakes. Pure and Applied Geophysics, 176(9), 4043–4068. https://doi.org/10.1007/s00024-019-02250-z