Viscoelastic lower crust and mantle relaxation following the 14–16 April 2016 Kumamoto, Japan, earthquake sequence

The 2016 Kumamoto, Japan, earthquake sequence, culminating in the Mw=7.0 16 April 2016 main shock, occurred within an active tectonic belt of central Kyushu. GPS data from GEONET reveal transient crustal motions from several millimeters per year up to ∼3 cm/yr during the first 8.5 months following the sequence. The spatial pattern of horizontal postseismic motions is shaped by both shallow afterslip and viscoelastic relaxation of the lower crust and upper mantle. We construct a suite of 2‐D regional viscoelastic structures in order to derive an optimal joint afterslip and viscoelastic relaxation model using forward modeling of the viscoelastic relaxation. We find that afterslip dominates the postseismic relaxation in the near field (within 30 km of the main shock epicenter), while viscoelastic relaxation dominates at greater distance. The viscoelastic modeling strongly favors a very weak lower crust below a ∼65 km wide zone coinciding with the Beppu‐Shimabara graben and the locus of central Kyushu volcanism. Inferred uppermost mantle viscosity is relatively low beneath southern Kyushu, consistent with independent inferences of a hydrated mantle wedge within the Nankai trough fore ‐arc. Plain Language Summary GPS measurements of motions of Earth's crust in the months following the April 2016 Kumamoto, Japan, earthquake sequence reveal the mechanisms of fault motions and flow in Earth's interior that continue long after the earthquakes. Additional movements of the causative faults continued for about half a year after the earthquakes, while flow of high‐temperature rock in the crust underlying an active volcanic belt in Kyushu has produced motions of the crust even far from the earthquake rupture zone. Key Points Crustal motions following the 2016 Kumamoto, Japan, earthquake sequence were rapid for several months following the sequence Afterslip and viscoelastic flow of the ductile lower crust and upper mantle control the near‐field and far‐field deformation, respectively Inferred lower crust and mantle viscosity structure is consistent with the effects of temperature and water content on rock rheology