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Group A, Poster #071, Tectonic Geodesy

A secondary zone of uplift measured after megathrust earthquakes: caused by early downdip afterslip?

Théa Ragon, & Mark Simons
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #071, SCEC Contribution #12155 VIEW PDF
A secondary zone of surface uplift, located a few hundred kilometers from the trench, has been measured after several megathrust earthquakes. The secondary zone of uplift reaches a few centimeters in data acquired hours to days after the 2011 Mw 9.1 Tohoku (Japan) earthquake, and more than 30 cm a month after the 1960 Mw 9.5 Valdivia (Chile) and 1964 Mw 9.2 Alaska earthquakes. Less than a day after the 2010 Mw 8.8 Maule (Chile) event, the secondary zone of uplift peaked at 12 cm. Published coseismic finite-fault models for the Tohoku or Maule events do not reproduce the measured secondary zone of uplift.
One interpretation is that this secondary zone of uplift is universal, and driven by volume deformation around the slab interface (van Dinther et al. 2019). In contrast, with synthetic tests and an investigation of the Maule event, we demonstrate the secondary zone of uplift may instead result from slip on the slab interface. Further, we suggest that slip occurs as rapid postseismic afterslip. We show the secondary zone of uplift can only be reproduced by fault slip if elastic heterogeneities associated with the subducting slab are taken into account, as opposed to assuming homogeneous or layered lithospheric structures.