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

Frederick Pollitz; Tomokazu Kobayashi; Hiroshi Yarai; Bunichiro Shibazaki; Takumi Matsumoto

doi:10.1002/2017gl074783

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

Frederick Pollitz·Tomokazu Kobayashi·Hiroshi Yarai·Bunichiro Shibazaki·Takumi Matsumoto

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Abstract

The 2016 Kumamoto, Japan, earthquake sequence, culminating in the M w =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.

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Source-reported bounding extent: 31.5° to 34° latitude; 130° to 132° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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BibTeX RIS

Frederick Pollitz, Tomokazu Kobayashi, Hiroshi Yarai, Bunichiro Shibazaki, Takumi Matsumoto. 2017-09-09. Viscoelastic lower crust and mantle relaxation following the 14–16 April 2016 Kumamoto, Japan, earthquake sequence. https://doi.org/10.1002/2017gl074783

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