A SEARCH INTO THE FAULTING MECHANISM OF THE 1891 GREAT NORI EARTHQUAKE

抄録

The 1891 Nobi earthquake was the greatest (M≅8) inland earthquake in Japan, which was accompanied by surface fault breaks extending over 80 km, with unusually large displacements with a maximum displacement of 8 m, and also caused pronounced vertical and horizontal tectonic movements, extremely strong ground motions and heavy seismic damage. All these geophysical and geological data are used together to recover the general features of the faulting mechanism from a modern seismological viewpoint based on dislocation theories. The fault rupture seems to have initiated at a depth around the northwestern end of the fault, and propagated southeastwards almost unilaterally at velocities of about 2.5 km/sec, forming successive fault slips with a sliding velocity of probably over 100 cm/sec, down to at least 15 km depth along three major preexisting Quaternary faults. The rupture propagation was temporarily blocked probably by the curvature of the fault traces or by small lateral heterogeneities at the southern end of the Neodani fault, and then branched off into two directions including along a deep latent fault extending southward. All seismic faulting appears to have been completed within 40 sec. A uniform geology in basement rocks covering a wide region is considered to have allowed the formation of this long fault. The released stresses are estimated to be 40-150 bars, which appear appreciably different from one place to another. Unusually large displacements and stresses in the southern part of the Neodani fault may be attributed to the nature of the fault plane there. A part of the stresses appears to have been released by a small amount of aseismic slip that continued for longer than 20 years. Possible sources of the stresses that caused this great earthquake are also discussed.