Strain and stress changes in the Tokai region of central Japan expected from a 3D subduction model

Abstract

Using a 3D simulation model with a rate- and state-dependent friction law, Kuroki et al. (2002) discussed a process of a hypothetical great earthquake in the Tokai region, where the Philippine Sea plate subducts beneath the Eurasian plate. One of the main concerns was characteristic changes in volumetric strain and displacement on the ground surface which are caused by the evolution of the coupling between the two plates, i.e. evolution of a strongly coupled region between the plates which results in a preslip of the earthquake. In the present paper we discuss other observable phenomena which might help us to identify the stage of the coupling. The preslip of the earthquake could be more effectively detected by using full information about the change of strain rather than volumetric strain alone; the change in rotation angle of principal strain axes should amount to several tens of degrees while the order of the change in volumetric strain is 10 −8 to 10 −7 for 1 day before the earthquake. The spatial pattern of the displacement field on the ground surface provides us with information on the intermediate-term precursory changes in the plate coupling. Information given by micro-earthquakes is less direct. The seismicity should change considerably when a highly shear-stressed ring on the plate interface passes nearby, and ups and downs of seismicity rate will be estimated by Coulomb failure stress. On the other hand, focal mechanisms are rather insensitive to the progress of plate subduction. The changes may be not significant even at the time of the preslip. The interplate coupling yields a stress field that should produce reverse fault type mechanisms, but the stress field is modulated by a curved shape of the plate interface. Superposition of a regional tectonic stress to this field explains observed spatial distribution of focal mechanisms in the Tokai region which involve large strike-slip components.