Aftershock activity of the 1987 Chiba-ken Toho-oki earthquake (M6.7) is investigated using JMA data. It is found that aftershocks during two weeks just after the main shock occurred mostly in a region to the east side of the fault plane which is nearly north-south direction with a steep dip to the east. However, aftershock activity in the area to the west side of the fault plane became high since the beginning of January 1988. The contrast between spatial distribution of aftershocks in December 1987 and that after January 1988 is conspicuous. The later activity was concentrated to a rather small area and the largest aftershock occurred on 16 January in the active region. The mechanism of the largest aftershock was reverse type in contrast to the mechanism of the main shock which was strike slip type. Further, pattern of temporal decrease of aftershock activity deviated notably from the Omori's formula when the later activity was started. All these characteristics suggest that most earthquakes which occurred in the region to the west of the fault plane of the main shock after January 1988 are not the so-called aftershocks in a narrow sense, but that they represent an appearance of a new fracture, which occurrence might be caused by the stress concentration due to the fault motion of the main shock. The phenomenon that aftershock activity in the either one side against a fault plane is higher than that in the other side is frequently observed, even for fault motions of strike slip type. It is interesting to note that seismicity before the main shock was also asymmetrical, i. e. it was active in the region to the west of the fault plane of the 1987 earthquake. The seismicity in the recent one year also seems to be active in the west region. These features may show that the western block to the fault plane has taken a positive part in the accumulation process of stress in the focal region of the 1987 Chiba-ken Toho-oki earthquake.
We analyzed P-wave coda from a deterministic point of view. A temporary observation of local earthquakes using a small aperture seismic array at Atagawa in the Izu Peninsula was carried out in order to determine the structure of scatterers near the array. The array system consisted of 10 seismic stations and the dimension was 125m×100m. The small aperture array observation was found to be useful to identify and locate scatterers near the array. Directions and magnitudes of slowness vectors of incoming waves across the array were determined from P-wave coda of 5 local earthquakes by using the semblance analysis. The analysis revealed that the early P-wave coda waves for 2s from an onset of the P-wave arrived from the same direction and had the same apparent velocity independent of a source direction. This result indicated that the coda waves come from the local Scatterer which was located about 4km north of the array and was 3km in depth. We interpreted that the scatterer corresponded to inhomogeneity of velocity distribution due to geothermal activity or faulting in the crust. Since the resolution of the scatterer's location is, however, 3km or more, an alternative origin of the coda wave is topographical irregularities such as a valley.
A scaling relation of source spectra is discussed, which is a basis of semi-empirical methods for synthesizing the strong ground motion of a large earthquake from the observed records of small earthquakes. Inhomogeneous displacements on a fault plane may be represented as the distribution of a displacement of a small sub-event multiplied by random variables with a standard deviation of SD. The source spectrum of a large event in very high frequency range is determined by the two key parameters of the source spectrum of a small event and the standard deviation SD, while in the low frequency range it is determined by the seismic moment ratio of the large to the small event. We also obtained the result that the power spectral density of the synthesized waves for the large event is proportional to SD2ΔσE4/3Δσ2/3 in the high frequency range, where ΔσE is the stress drop of the small event and Δσ is the average stress drop in the fault plane of the large event. Another characteristic of strong ground motion is determined by an effect of fault heterogeneity, in which the seismic directivity effect is not shown for the amplitude of the Fourier spectrum of the synthesized waves in the high frequency range. This is consistent with the results obtained by using other stochastic fault models. It is found that the parameter SDΔσE2/3Δσ1/3 corresponds to the local stress drop and/or the rms stress drop of the stochastic fault models. This indicates that the stress drop ΔσE of the small event is an important parameter which determines the heterogeneity of faulting quantitatively, when the strong ground motion due to large earthquake is synthesized by the semi-empirical method.
Based on tide-gauge records observed at the Hokkaido and Sakhalin coasts facing the Okhotsk Sea, the characteristics of seven Kurile tsunamis (1918-1978) passing through the straits are investigated. The ratio of wave-heights at the Okhotsk Sea coast to Hanasaki located at the Pacific side was about one-half for tsunamis generating off the Urup Island, and the ratio has a tendency of decrease when the tsunami source moves to western direction toward Hokkaido. It suggests the effective tsunami energy passes into the Iturup Straits than other straits or channels. The travel time at Hanasaki becomes long when the tsunami source moves to the east direction, but that of the Hokkaido-Sakhalin region in the Okhotsk Sea is usually unchanged: For example, travel times of the Kurile tsunamis were mostly about 1.5 hours at Abashiri and 3 hours at Wakkanai. We found through the refraction diagrams that the unchanged travel times were caused by sea-bottom topography in the Okhotsk Sea. The wave rays emitted from the Iturup Straits concentrate in Sakhalin, while those from the Kunashiri Channel concentrate in Hokkaido. For future tsunamis coming-into the Okhotsk Sea, it is indispensable to take into considerations of the effect of tidal currents.
Explosion seismic refraction experiments were carried out in 1988 for the investigation of the shallow crustal structure in the volcanic island of Nii-jima, northern part of Izu-Bonin island arc, Japan. One hundred and eight seismograms were obtained at 54 temporary stations aligned on the three profiles arranged like a triangle on the island. The main features of the shallow crustal velocity structure revealed by the travel time analysis are as follows: 1) Surfaces of high velocity layers are raised beneath volcanos in the island of Nii-jima. 2) The structure mostly consists of three layers with P-wave velocities of 1.9km/s, 2.8km/s and 4.5km/s, respectively. 3) A layer with a velocity of 1.5km/s exists over the 1.9km/s layer at two explosion sites in shore. 4) There is no 1.9km/s layer beneath Mt. Acchi volcano in the northern part of the island. 5) The top of the 2.8km/s layer is much shallower beneath Mt. Mineji volcano in the central part of the island and beneath Ajiaiso beach in the northwestern part of the island. 6) Both top of the 2.8km/s layer and that of the 4.5km/s layer are much shallower beneath Mt. Acchi volcano in the northern part of the island. 7) The 1.9km/s layer is thicker beneath the small plain in the central part of the inland, beneath the place between Mt. Mineji and Ajiaiso beach, and beneath Wakago village in the northern part of the island.
It is known that great Sanriku tsunamis sometimes hit the Sendai Plain in the north part of the Honshu Island in Japan. Among those great tsunamis, the field investigation had been rarely carried out about the tsunami of July 13th, 869 (11th year of the Jogan Era) because of the lack of the materials of old documents. We carried out the field investigation of the height of this tsunami in the Sendai Plain on the basis of the archaeological view and the sedimentologic examination. The former method is based on the archaeologist's judgement whether historic relics have the trace buried by the running water or not. We estimated the inundated area from the result of the survey for the 8 points in the Sendai Plain. In the latter method, we dipped the test pit and observed the soil layer of the side wall minutely to find the sand layer being supposed to be carried by the tsunami from the dune. Moreover to analyze the character of the sand layer, we made the physical examination, the chemical analysis and the measurement of the age by 14C method. From the results of these investigations, the height of the Sanriku Jogan 11 earthquake-tsunami (A. D. 869) is estimated as 2.5-3m at the point 3km from the coastline in the Sendai Plain. Finally we checked it with the social and geographic circumstances at that time, and we got the result that the estimated height is not inconsistent with the record of this tsunami.
This paper presents some analytical results on the underground velocity structure of OSAKA basin based upon refraction data by explosions. The main shot point was at the OSAKA NORTH PORT area which was under reclamation construction. From the shot point at OSAKA NORTH PORT, four observational lines were extended to the surrounding mountain area. The reverse shot was given to only one of four lines. The dynamite of about 500kg were exploded at the depth of about 100m from the surface as seismic source for each shot point. To cover the shortage of the no reverse shot, the seismographs at the longest distances for normal shot were set up on the rock site, so as to define the thickness of sediment to be zero. Time-terms at the 28 observation points along four lines in OSAKA plane were determined. Several conclusions obtained in this study are as follows, 1) The base rock is estimated to have a simple triangular shape along the observed line with N-S direction, 2) The maximum depth of the base rock along the observational line of N-S direction is more than about 2.0km, 3) The average velocity of the base rock is obtained as 5.37km/s, taking into account of the P wave velocity of 2.1km/s of the subsurface sedimentary in OSAKA basin by reflection survey.
This paper reviews the studies on seismic activities and tectonics in southwestern Japan. We start with the stress state of southwestern Japan with reference to movements of the Philippine Sea plate, the Eurasian plate and other concerning plates. Then we describe subcrustal seismic activity associated with the subduction of the Philippine Sea plate and shallow earthquakes occurring in the upper crust in relation to the active faults and mechanical properties of the crust. We also give a brief summary of studies on microearthuakes in the concerned districts.