A Secondary Zone of Uplift Measured After Megathrust Earthquakes: Caused by Early Downdip Afterslip?. Issue 7 (31st March 2023) - Ragon, Théa
Abstract: A secondary zone of surface uplift (SZU), located from 200 to 400 km 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. Published coseismic finite‐fault models for these events do not reproduce the measured SZU. One interpretation is that this SZU is universal, driven by volume deformation around the slab interface (van Dinther et al., 2019, https://doi.org/10.1007/s00024-019-02250-z ). In contrast, we demonstrate the SZU may instead result from slip on the slab interface, and suggest it might be caused by rapid afterslip. We can reproduce 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. Plain Language Summary: Large earthquakes in subduction zones induce displacement of the ground surface, which usually include large amplitude uplift offshore, transitioning to a region of less pronounced subsidence further inland. After the largest instrumented earthquakes, such as the 2011 Mw 9.1 Tohoku (Japan), the 1960 Mw 9.5 Valdivia (Chile), and 1964 Mw 9.2 Alaska earthquakes, a secondary zone of uplift (SZU) is detectable even further inland. The origin of this SZU remains enigmatic, but one interpretation is that it derives from deformation of the volume around the subducting fault (van Dinther et al., 2019, https://doi.org/10.1007/s00024-019-02250-z ). In this study, we investigate alternate interpretations of its origin. A simple slip model with realistic variations in crustal elastic properties allows one to reproduce the SZU. We then focus on the 2010 Mw 8.8 Maule (Chile) event, for which some measures of the SZU peaked at a few centimeters. Unlike previous studies, we can reproduce the SZU with on‐fault displacement, located significantly deeper than the region of estimated coseismic slip. This deep slip potentially occurred in the hours to days after the earthquake. Key Points: A secondary zone of uplift (SZU) has been measured several hundred kilometers from the trench after large subduction earthquakes The SZU is not reproduced by coseismic finite‐fault models that neglect 3D elastic heterogeneities in lithospheric structure The SZU is reproduced using plausible models of 3D elastic heterogeneities, with slip downdip of the main coseismic patch