A synthesis method is developed for estimating deterministically strong motions during the mainshock, using the records of small events such as foreshocks and aftershocks which occurred within the area of the mainshock fault. This synthesis formulation is based on the kinematic source model of Haskell type and the similarity law of earthquakes. The parameters for this synthesis are determined to be consistent with the scaling relations between the moments and the fault parameters such as fault length, width and dislocation rise time. If the ratio of the mainshock moment Mo to the small event one Moe is assumed to be N', then the mainshock fault can be divided into N x N elements, each dimension of which is consistent with that of the small event and N events at each element may be superposed with a specific time delay to correct the difference in the rise time between the mainshock and the small event and to keep a constant slip velocity between them. By means of this method, the mainshock velocity motions are synthesized using the small event records obtained by velocity-typestrong-motion-seismographs for 1980 Izu-Hanto-Toho-Oki Earthquake (M=6.7). The resultant synthesized motions show a good agreement with the observed ones in the frequency range lower than 1 Hz. Further, the synthesis formulation is improved to be applicable to the higher frequency motions, especially acceleration motions. This revised synthesis for the higher frequency motions is effective when we use the records from the small event having the fault length Le-=V,-•T(Vr: rupture velocity and T: rise time of mainshock). The synthesized accelerograms by this revised method are in good agreement with the observed ones in the frequency range up to 5 Hz.

https://repository.kulib.kyoto-u.ac.jp/dspace/bitstream/2433/124917/1/b33p2n298p04.pdf

Semi-Empirical Estimation of Strong Ground Motions During Large Earthquakes

The results of twenty-eight seismic refraction profiles recorded in the various physiographic provinces of the Philippine Sea as part of the United States and Japan Science Cooperation Program are presented in four schematic structure sections. The basins of the Philippine Sea have fairly normal oceanic crust that includes, between the sea floor and layer 2, a layer of about 3.5-km/sec velocity controlling the characteristic rough topography. Crustal thickening beneath the Nansei Shoto, Oki-Daito, Kyushu-Palau, and the Honshu-Mariana ridges is associated mainly with an increase in thickness of the 3.5-km/sec layer and a thick underlying section of material with a velocity between 5.5 and 6.0 km/sec. Beneath the Nansei Shoto trench and the Honshu-Mariana trench, there is a tendency for layer 2 to increase and layer 3 to decrease in thickness as the trench is approached from the adjacent oceanic sector.

Crustal structure of the Philippine Sea

Maximum-likelihood estimation of hypocenter with origin time eliminated using nonlinear inversion technique

Seismic velocity structure of the crust beneath the Japan Islands

In the 1940’s, several destructive earthquakes occurred in western Japan. Seismograms in this period were usually recorded on smoked paper and the quality was poor compared to modern digital data. But the recent development of image processing technology enabled us to reconstruct feasible waveform data, whereby we investigated source rupture processes of two devastating earthquakes: the Tonankai earthquake (M7.9) of December 7, 1944, and the Mikawa earthquake (M6.8) of January 13, 1945. The results for the Tonankai earthquake show that the source roughly consists of a single asperity with a length scale of 100 km, having no segment structure with a smaller length-scale. Such a feature seems to be reflected to the sea bottom topography above the source region. The main source parameters are as follows: the seismic moment = 1.0 × 1021 Nm (Mw=7.9); the fault area = 140 km × 80 km; (strike, dip, rake) = (225°, 15°, 79°); the maximum and averaged dislocations = 4.4 m and 3.0 m, respectively. The analysis of the seismograms for the Mikawa earthquake shows that the source is a reverse fault with a slight left-lateral component. The pressure axis is directed to ENE-WSW, which is a little rotated from the EW compression axis prevailing in western Japan. This fault can be regarded as the southern extension of the Nobi earthquake fault system. The main source parameters are as follows: the seismic moment = 1.0 × 1019 Nm (Mw=6.6); the fault area = 20 km × 15 km; (strike, dip, rake) = (135°, 30°, 65°); the maximum and averaged dislocations are 2.1 m and 1.1m, respectively. The slip distribution mainly consists of two asperities: the one near the hypocenter and the other 10–15 km northwest from it. The heavily damaged area is well correlated with the northwestern asperity.

https://link.springer.com/content/pdf/10.1186%2FBF03351745.pdf

Source rupture processes of the 1944 Tonankai earthquake and the 1945 Mikawa earthquake derived from low-gain seismograms

Studies on the crustal structure in the profile across central Japan close to the longitude 137°E were started in 1966 with 20 explosions made off the Atumi Peninsula. In 1967, twenty-one reverse shots off the Noto Peninsula and a supplementary three-ton explosion at the Ebisu Mine were observed at stations on land specially arranged for the derivation of crustal structure. Most of the stations were equipped with magnetic-tape-recording systems which provide us a filter technique to detect signals of low S/N ratio. The quality of record was extensively increased through filtering, and the following characteristic features were revealed:A. Pn waves were extensively observed in the explosions off the Atumi Peninsula but not in those in the Sea of Japan.B. Waves belonging to a granitic layer were clearly observed in the explosions off the Noto Peninsula but were absent or almost absent in the explosions off the Atumi Peninsula except for shots near the peninsula. These marked differences suggest an asymmetric crustal structure in the profile.A time-term method is satisfactory for the derivation of sediments under the Sea of Japan, but not for the derivation of the Moho discontinuity. A new method utilizing long wave components, was developed and used in the present study resulting in many possible structures. The crustal structures thus obtained are, in general, continental under the mountain area, oceanic under the Pacific, and intermediate under the Sea of Japan. The most probable one out of many is shown Fig. 17. Gravity data were also used in the selection of the best model. The thickness of the basaltic layer remains unknown, but it should be very thin in the southern half of the profile. It forms a striking contrast with the structure in the profile along the longitude 139°E which is located within a short distance of only 220km.A velocity gradient for the upper mantle was also obtained from the explosions off the Atumi Peninsula.

Crustal structure in the profile across central japan as derived from explosion seismic observations

The third Kurayosi explosion was fired by the Research Group for Explosion Seismology on November 21, 1970 in order to detect the seismic waves reflected from deep crustal boundaries. Very clear reflected waves from the Conrad discontinuity were recorded by the instrument for seismic prospecting (E.T.L. recorder). There was also weak indication of the reflection from the Mohorovicic discontinuity in the records obtained at closely spaced temporary observation points equipped with FM data recorders at the shot distance of about 100km. On the basis of these results, a revision was made for the previous model. The crustal thickness obtained is about 35km and the Pn velocity, about 7.8km/sec.

The third kurayosi explosion and the crustal structure in the western part of japan

Crustal Structure in the Western Part of Japan Derived from the Observation of the First and Second Kurayosi and the Hanabusa Explosions. : Part 2. Crustal Structure in the Western Part of Japan

The seismic moment of the 1891 Nobi, Japan, earthquake has been evaluated from the historical seismogram recorded at the Central Meteorological Observatory in Tokyo. For this purpose, synthetic seismograms from point and finite source models with various fault parameters have been calculated by a discrete wave-number method, incorporating the instrumental response of the Gray-Milne-Ewing seismograph, and then compared with the original records. Our estimate of the seismic moment (Mo) is 1.8×1020 N m corresponding to a moment magnitude (Mw) 7.5. This is significantly smaller than the previous estimates from the distribution of damage, but is consistent with that inferred from geological field survey (Matsuda, 1974) of the surface faults.

/pdf/seismic-moment-of-the-1891-nobi-japan-earthquake-estimated-4bzfwd5jnq.pdf

Seismic moment of the 1891 Nobi, Japan, earthquake estimated from historical seismograms

61. Crusted Structure in the Western Part of Japan Derived from the Observation of the First and Second Kurayosi and the Hanabusa Explosions (Continued) : Part 2. Crustal Structure in the Western Part of Japan (Continued)

Ikuei Muramatu

Papers

Crustal structure in the profile across central japan as derived from explosion seismic observations

The third kurayosi explosion and the crustal structure in the western part of japan

Crustal Structure in the Western Part of Japan Derived from the Observation of the First and Second Kurayosi and the Hanabusa Explosions. : Part 2. Crustal Structure in the Western Part of Japan

Seismic moment of the 1891 Nobi, Japan, earthquake estimated from historical seismograms

61. Crusted Structure in the Western Part of Japan Derived from the Observation of the First and Second Kurayosi and the Hanabusa Explosions (Continued) : Part 2. Crustal Structure in the Western Part of Japan (Continued)