Frictional properties and rheological structure at the Ecuadorian subduction zone revealed by the postseismic deformation due to the 2016 Mw 7.8 Pedernales (Ecuador) earthquake

Abstract

Postseismic deformation following subduction earthquakes includes the combined effects of afterslip surrounding the coseismic rupture areas and viscoelastic relaxation in the asthenosphere and provides unique and valuable information for understanding the rheological structure. Because the two postseismic mechanisms are usually spatiotemporally intertwined, we developed an integrated model combining their contributions, based on 5 years of observations following the 2016 Pedernales (Ecuador) earthquake. The results show that the early, near-field postseismic deformation is dominated by afterslip, both updip and downdip of the coseismic rupture, and requires heterogeneous interface frictional properties. Viscoelastic relaxation contributes more to far-field displacements at later time periods. The best-fit integrated model favors a 45-km thick lithosphere overlying a Burgers body viscoelastic asthenosphere with a Maxwell viscosity of 3 × 1019 Pa s (0.9 - 5 × 1019 Pa s at 95% confidence), assuming the Kelvin viscosity equal to 10% of that value. In addition to the postseismic afterslip, the coastal displacements of sites north and south of the rupture clearly require extra slip in the plate motion direction due to slow slip events that may be triggered by the coseismic stress changes (CSC), but are not purely driven by the CSC. Spatially variable afterslip following the Pedernales event, combined with the SSEs during the interseismic period, demonstrate that spatial frictional variability persists throughout the whole earthquake cycle. The interaction of adjacent fault patches with heterogeneous properties may contribute to the clustered large earthquakes in this area.