An asperity model for fault creep and interseismic deformation in northeastern Japan
We explore the potential geodetic signature of mechanical stress shadows surrounding inferred major seismic asperities along the Japan-Kurile subduction megathrust. Such stress shadows result from a decrease in creep rates late in the interseismic period. We simplify the rupture history along this megathrust as the repeated rupture of several asperities, each with its own fixed recurrence interval. In our models, megathrust creep throughout the interseismic period evolves according to velocity strengthening friction, as opposed to common kinematic back-slip models of locked or partially locked (i.e. coupled) regions of the megathrust. Such backslip models are usually constrained by onshore geodetic data and typically find spatially extensive and smooth estimates of plate coupling, a likely consequence of model regularization necessitated by poor model resolution. Of course, these large coupled regions could also correspond to seismogenic asperities, some of which have not experienced a significant earthquake historically. A subset of existing kinematic models of coupling along the Japan Trench, particularly those that use both horizontal and vertical geodetic data, have inferred a surprisingly deep (∼100 km) locked zone along the megathrust or have called upon complex, poorly constrained megathrust processes, such as subduction erosion, to explain the geodetic observations. Here, we posit two scenarios for distributions of asperities on a realistic 3-D megathrust interface along the Japan-Kurile Trench off NE Japan. These scenarios reflect common assumptions made before and after the 2011 M_w 9 Tohoku-oki earthquake. We find that models that include two shallow M9-class asperities (one corresponding to the 2011 Tohoku-Oki earthquake and one offshore of Hokkaido) and associated stress-shadows can explain geodetic observations of interseismic strain along the eastern halves of Honshu and Hokkaido. Specifically, models including localized fault creep can explain most of the observed long-term vertical subsidence in this region during the past century and thus appealing to processes such as deep locking or subduction erosion may not be required.
Additional Information© 2012 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. Accepted 2012 October 12. Received 2012 October 10; in original form 2012 January 13. We thank Hiroo Kanamori for his insights in estimating the set of characteristic asperities used in our simulations.We thank Yaru Hsu for providing the original strain calculation code which we adapted for Japan. RVSK also thanks Prof. John Suppe for providing computational facilities and financial support to complete the project at the National Taiwan University, Taipei, Taiwan. We also thank Roland Bürgmann and an anonymous reviewer, whose constructive criticism significantly improved the clarity of this manuscript. This is Caltech Seismological Laboratory publication 10070, and Caltech Tectonics Observatory publication 192.
Published - Geophys._J._Int.-2013-Kanda-38-57.pdf
Supplemental Material - JapanAsperities_SupportingMaterial_revised_SUB.pdf