A search into the faulting mechanism of the 1891 great Nobi earthquake.

Abstract

The 1891 Nobi earthquake was the greatest (M@/*6/@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.

Examination of source fault model for the Gifu-Ichinomiya fault based on seismic intensity data

“…They obtained a distribution of seismic intensities based on the relationship between the peak horizontal ground velocity and seismic intensity , and compared this with the distribution of the damage ratio of wooden houses (Muramatu, 1983). They found that the distribution of the calculated seismic intensities for their model, which was constructed by referring to the distribution of coseismic displacement and that of micro-earthquakes, corresponds to the distribution of the damage ratio rather than the distribution of calculated seismic intensities from the model of Mikumo and Ando (1976). For the Gifu-Ichinomiya fault, Fukuwa et al (2003) assumed a reverse fault with an inclined fault plane having a length of 35 km and a dip of 70 degrees.…”

“…Nakano et al (2007) applied the inversion method to the coseismic vertical displacements along two leveling routes across the Umehara fault and the buried fault in order to establish a fault model of the Gifu-Ichinomiya fault. They obtained a fault model having an inclined fault plane with a dip of 60 degrees located 5 km east of the location put forward by Mikumo and Ando (1976). As described above, various interpretations have been proposed for both the existence, and the geometry, of the Gifu-Ichinomiya fault.…”

“…We also apply two fault models (Model D and Model E) that refer to previous fault models (Mikumo and Ando, 1976;Fukuwa et al, 2003). The dip angles of the GifuIchinomiya segment in Models D and E are 90 degrees and 70 degrees, respectively.…”

“…Mikumo and Ando (1976) calculated the theoretical vertical displacements on the basis of several fault plane models and compared them with the observed vertical displacements along two leveling routes across the Umehara fault and the buried fault. By applying the forward modeling method, they found that movements of four faults, including the buried fault, were needed to explain the observation.…”

“…With the aim of constructing a source fault model of the 1891 Nobi earthquake, the rupture of the Gifu-Ichinomiya fault during the earthquake, and its fault geometry, have been discussed (Mikumo and Ando, 1976;Fukuwa et al, 2003). Here, we construct various source fault models that take into account several types of geometries for the Gifu-Ichinomiya fault (Gifu-Ichinomiya segment).…”