FAULT MODEL OF THE 1891 NOBI EARTHQUAKE FROM HISTORIC TRIANGULATION AND LEVELING

The faulting associated with the 1891 Nobi earthquake, central Honshu, is re-examined on the basis of horizontal displacement vectors determined between the periods 1882/1899-1950/1961. These vectors are modelled in terms of two components: (1) strain accumulation and release along the Nankai Trough and Suruga Trough, and (2) postseismic relaxation following the 1891 earthquake. We assume a rheological model of the central Honshu crust and upper mantle consisting of a 33-km thick elastic lithosphere overlying a 130 km thick viscoelastic sublithosphere. The modelling demonstrates that the observed displacement vectors are dominated by post-seismic displacements following the Nobi earthquake. The Nukumi-Neodani faults are inferred to have accommodated 7-8 m of left lateral strike-slip displacement, in good agreement with those estimates based on the observed surface rupture. In addition, we identify two northeast-southwest trending blind thrust faults, together comprising a 70-km-long region of compressional deformation, each of which accommodated several meters of reverse slip during the Nobi earthquake. These thrust faults consist of a 20-km-long western segment and a 50-km-long eastern segment. The western thrust begins near the southern termination of the strike-slip faulting component, has shallow (31-degrees) dip, is buried by 15 km and has a prominent coseismic signal in the leveling data. Examination of the topography east of Nagoya suggests that the eastern thrust is buried 5-10 km below the surface and represents the subterranean continuation of the exposed thrust fault on the eastern flank of the Kiso Range to the northeast. The model of strain accumulation and release includes the various effects of the 1854 Ansei and 1944 Tonankai earthquakes. as well as the effect of steady state Philippine Sea interplate convergence. A model consisting of only tectonic strain accumulation and release along the Philippine Sea interplate boundary provides a poor fit to the data. The modelling suggests that the 1944 rupture did not extend beyond the Atsumi Peninsula. The observed displacement vectors and an independent set of Suruga Bay uplift data contain a moderate signal due to 1854 postseismic displacements. Considering that this signal has tradeoffs with the slip accumulation rate in Suruga Bay, this enables us to constrain the central Honshu sublithospheric viscosity to lie near 2 x 10(19) Pa.s.