The role of rheological heterogeneities in postseismic deformation

Advances in modelling and access to InSAR and GNSS observations have highlighted the role that rheological heterogeneities play in postseismic deformation. Here we discuss three recent studies (Muto et al. 2019, Sambuddha et al. 2022, and Takada et al. in prep) following the 2011 Tohoku-Oki and 2008 Iwate-Miyagi earthquakes, which reveal both localised and along-strike rheological heterogeneities. We construct a self-consistent physical model of the postseismic deformation for these two events using the Unicycle code (Moore et al. 2019, Barbot, Moore, and Lambert 2017), with which we consider coupled fault slip and viscoelastic flow utilising laboratory-derived constitutive laws to simulate the time series of geodetic observations. All three studies illuminate a crustal low viscosity rheological heterogeneity in the vicinity of Mt Kurikoma / Mt Naruko. This is perhaps to be expected, given the proximity to known active volcanic centres, and is commensurate with observations following the 2016 Kumamoto earthquake (Moore et al. 2017) where we found low-viscosity anomalies beneath Mt Aso and Mt Kuju. However, the heterogeneities the data reveal are not restricted to known volcanic regions, because our results also suggest along-arc heterogeneity in the forearc mantle rheology of north-eastern Japan; specifically we find a narrower cold nose in the Miyagi region and wider for the Fukushima forearc. We also find evidence of interaction between the localized crustal heterogeneity and afterslip in both events, highlighting the importance of addressing mechanical coupling for long-term studies of postseismic relaxation. Variations in rheological properties in the lithosphere are not restricted to viscous and thermal effects, and observations of the Iwate-Miyagi earthquake suggest elastic heterogeneities may also play a role. We therefore conclude by presenting expressions for computing displacements and stress due to localised (faulting) and distributed inelastic deformation in heterogeneous elastic spaces with piece-wise constant homogeneous elastic subregions (Sato & Moore 2022), and their application in the context of the seismic cycle.

 

Muto J, Moore J D P, Barbot S, Iinuma T, Ohta Y, Horiuchi S, Hikaru I, 2019. Coupled afterslip and transient mantle flow after the 2011 Tohoku earthquake. Science Advances

Dhar S, Muto J, Ito Y, Muira S, Moore J D P, Ohta Y, Iinuma T, 2022. Along-Arc Heterogeneous Rheology Inferred from Postseismic deformation of the 2011 Tohoku-oki Earthquake.

Moore J D P, Barbot S, Feng L, Hang Y, Lambert V, Lindsey E, Masuti S, Matsuzawa T, Muto J, Nanjundiah P, Salman R, Sathiakumar S, & Sethi H, 2019. jdpmoore/unicycle: Unicycle. In Coupled afterslip and transient mantle flow after the 2011 Tohoku earthquake, Science Advances 2019. Zenodo. https://doi.org/10.5281/zenodo.5688288

Barbot S, Moore J D P, Lambert V, 2017. Displacements and stress associated with distributed anelastic deformation in a half-space. BSSA

Moore J D P, Yu H, Tang C, Wang T, Barbot S, Peng D, Masuti S, Dauwels J, Hsu Y, Lambert V, Nanjundiah P, Wei S, Lindsey E, Feng L, Shibazaki B, 2017. Imaging the distribution of transient viscosity after the 2016 Mw7.1 Kumamoto earthquake. Science

Sato D, Moore J D P, 2022. Displacements and stress associated with localised and distributed inelastic deformation with piecewise-constant elastic variations.