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°) 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 ob-served 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 2×10¹⁹ Pa·s.

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Fault plane solutions for small earthquakes in Japan are determined by using the P wave first motion data given in the Japan University Network Earthquake Catalog and the Seismological Bulletin of the Japan Meteorological Agency. By carefully selecting from the catalog the events that occurred inland in Japan, we can determine reliable fault plane solutions for the earthquakes with a magnitude as small as three. We have determined fault plane solutions for 174 earthquakes which occurred in 1987, compared with 107 fault plane solutions reported by the Seismological Bulletin for the same year but for a much wider area. The fault plane solutions obtained in this study reflect the same pattern of the regional and tectonic stress distribution as previous studies. This result indicates that first motion data reported in the Japan University Network Earthquake Catalog are reliable enough to determine fault plane solutions. The construction of a database for fault plane solutions of microearthquakes in Japan is essential for the detailed study of the stress accumulation process in the Japanese islands.

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Geophysical Institute, The University of Tokyo, Bunkyo-ku, Tokyo 113, Japan Both the compressional-wave velocity and the electrical conductivity were measured on a partially molten material. An H₂O ice and KCl aqueous solution system was used as an analog of partially molten Earth's materials; the melt has a triangular tube shape at the edge and corner regions of solid grains. Both the velocity and the conductivity showed discontinuous change at the onset of partial melting. A melt fraction less than 5% causes the conductivity to increase by 1.5-3 orders of magnitude, while the velocity decreases only a few percent. This result is consistent with the geometry and connectivity of melt in the KCl-H₂O system. The large conductivity increase suggests that there will be a high-conductivity partially molten region where a low velocity anomaly is difficult to detect.

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To calculate the elastic deformation due to a thrust fault in the lithosphere-asthenosphere composite, Nur and Mavko (1974) used the exact solution of Mura (1968) for an edge dislocation parallel to the boundary of two elastic half-spaces in welded contact. They approximated the effect of the free surface by considering Mura's solution for two equal and opposite edge dislocations with the free surface midway between them. By considering a simpler problem of a long dip-slip fault in a uniform half-space, we show that the approximate method used by Nur and Mavko does not yield satisfactory results. For small dip angles and beyond a certain epicentral distance, while the exact solution predicts subsidence, the approximate solution predicts uplift. Moreover, the approximate solution yields non-zero values for the surface tractions at the free surface.

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