Earthquake damage is usually related to ground conditions, which is determined by the history of the area. Old maps can be important in revealing the ground history. Land register maps (1/600 scale) from the early Meiji age can provide important information on the ground condition of Japan in the 1880's.
Successful use of the maps is demonstrated with an application to the disaster at Matsumi-machi, Araya, Akita, which suffered from severe damage due to liquefaction of sandy ground in the Nihonkai-chubu Earthquake, May 26, 1983. The damage was almost completely limited to a narrow area (about 30m×100m) of nearly flat ground recently made by leveling the long bank of a rifle range constructed by the Japanese army in 1897. The ground condition before constructing the bank could not be known from any other available maps and information. Land register maps revealed that the damaged area was land reclaimed from a pond. Comments on the maps extend usefulness of the maps which differ from the usual topographic maps.

View full abstract

A stochastic model was used to quantify the heterogeneity of stress distribution in rocks subjected to uniaxial compression stress. In the model, the micro-fracturings are supposed to occur when local stress exceeds the average strength of the rock. The stress heterogeneity is measured by correlation distance in the autocorrelation function of stress distribution. The smaller the correlation distance, the larger the stress heterogeneity. The calculated results based on the model show that the m-value of an ensemble of acoustic emissions (i) increases with decrease of the correlation distance and (ii) decreases as the difference between applied stress and average strength of rock becomes large. In comparison with the experimental results, it is found that the correlation distance decreases as the axial stress approaches the fracture stress. This implies that the rock sample in dilatant state becomes heterogeneous with increase in axial stress.

View full abstract

The shape of aftershock area is investigated and numerically calculated by using energy releasing rate g at the tip of the elliptical crack. When g is independent of the eccentric angle φ of ellipse, that is ∂g/∂φ=0, energy state of the elastic medium is thought to be most stable. If aftershock phenomena are considered to be quassi-static process, it is easy to calculate g.
Using these assumptions, it is concluded that the crack acted by shear stress any direction approximately becomes the elliptical crack of which the ratio between long radius and short one is about 4:3. However it gradually approaches the circular crack as the tension perpendicular to the crack face increases. These results apply to comparatively small earthquakes whose boundary conditions are considered to be infinite, isotropic and homogeneous.

View full abstract

Graphic data are transferred to the video-frame memory by a TV camera, and curves are automatically traced to create digital curve data that consist of successive grid points of the video-frame memory with high grey levels corresponding to dark points. The curve data are first fit to a line every small segment, and then approximated by a smooth connection of circular arcs. Contours on a map are processed in this way, and the altitude at an arbitrary point is evaluated by an interpolation. Namely a curve that passes this point and intersects perpendicularly to neighbouring contours is introduced, and the contour values are interpolated, based on the distances to the contours along the curve.

View full abstract

A new system for the temporary observation of earthquakes have been developed. Analogue memories by CMOS-RAM and usual audio cassette stereophonic recorder are used for the system. They are well developed and cheap. Seismic wave signals and time code signals are holded in the memories by the trigger signal and recorded into a cassette as the audio signals converted by high frequency sampling clock. Multichannel observation is possible by the channel serial output.
Playback of signals is done by the simple memory system converting signals from audio frequency to the suitable frequency. Audio signal analyzers are also available for the direct playback signals of the tape recorder.
In the case of two-channels. recorder which wee call OM-2 type, , using WM-D6C recorder with Dolby-B noise reduction system, 46-db dynamic range can be gotten for each channel. We are observing microearthquakes at more than 20 temporary stations by the recorders with a high-gain and a low-gain channel for one component vertical senser to get records of microearthquakes and unsaturated records of largee shocks. About 900 eventss are stored in a 60 minutes cassette tape in this case.

View full abstract

In automatic processing of seismic waves, it is important not only to make efforts to read the arrival times of P and S phases with better accuracy for each observation station but to check the validity of results of reading in order to reject the misread items automatically. With this point of view, a practical method for reading and checking the seismic data in automatic processing system was designed, making a comparison with data from the manual processing system of the Regional Center for Earthquake Prediction, Kyoto University.
An outline of the method is summarized as follows:
(1) Basic principle for reading the phase times.
Reading the arrival times is equivalent to dividing a section into two parts in time series. For the criterion of dividing, AIC (Akaike Information Criterion; defined as AIC=-2{maximum log. likelihood—number of free parameters of model}) was adopted.
(2) Reading P times
a) Use of low cut filter for elimination of microseisms. b) Use of normal distribution model and also autoregressive model for further improvements.
(3) Checking P times
a) Quantification of the degree of certainty of reading values. b) Check by means of apparent velocity between observation stations. c) Check by means of hypocenter calculation.
(4) Reading S times
a) Finding out a time window for S times when a hypocenter cannot be determined. b) Applying a bivariate normal distribution model to horizontal components if available.
(5) Checking S times
a) Use of P time-(S-P) time graph (the Wadachi diagram).
(6) Hypocenter calculation
a) Use of robust estimation.
This method was tested for the seismic wave data observed by the network at the Regional Center, from 5th to 8th and from 17th to 19th December, 1984. As regards the differences between the respective P times read out manually and automatically, the relative frequency within ±0.05sec. was amounted to about 85 percent for earthquakes in the neighborhood of the network. Next, we checked the validity of S phase determination, fitting a straight line to the relation between P time—origin time and S-P time. The gradient of the line from automatic determination was estimated as 0.69 and their standard deviation is 0.32sec., while, 0.67 and 0.21sec. for manual reading. These differences, however, were so slight as to be almost negligible. By using an ordinary minicomputer, it took only several seconds to process a channel, so that, an earthquake was treated exhaustively during a few minutes.
According to the results of these tests, the method for processing the seismic wave data may be proved to be useful for the practical purpose.

View full abstract

By using an array of pop-up type ocean bottom seismometers (OBS), we investigated seismicity in and around Suruga Bay, the source region of the hypothetical Tokai earthquake, for about one month in 1983. We deployed 9 OBSs and successfully recovered all of them. Jointly analyzing arrival time data of the OBSs and land stations of NRCDP, we located about 50 earthquakes in and around the OBS array. The resulting hypocenter distribution shows the following characteristic features in seismicity, (1) high microseismicity in the middle to southern part of Suruga Bay, (2) quiescence in microseismicity in the north-eastern part of the Nankai Trough, (3) several earthquakes occurring along the Zenisu Ridge. For the events occurring beneath Suruga Bay, we estimated station corrections of travel times and determined their hypocenters precisely. The difference in station correction between the OBS stations and the land stations is very large, 1 to 2s for P wave and 2 to 3s for S wave, showing the importance of station corrections for precise hypocenter determination, particularly in the case of assembling data from OBSs and land stations. The depths of re-determined hypocenters by using the station corrections range from 5 to 20km for events beneath the western side of Suruga Bay. This depth range is shallower than that determined routinely by NRCDP by 5 to 10km, suggesting that the earthquakes beneath the Suruga Trough occur at the upper-most part of the subducting Philippine Sea plate.

View full abstract

Hypocenter determination for aftershocks of the 1983 Japan Sea Earthquake (M7.7) is made by using the seismic networks of Tohoku University and of Hirosaki University. The obtained aftershock area is 160km long in north-south direction with a width of 40km, distributing itself along the eastern margin of the Japan Basin. Most of the aftershocks are located within the area bordered by the 2000m and 3000m isobaths, northern and southern ends being surrounded by the Sado Ridge and the Oshima Plateau, respectively. Precise hypocenter distribution deliniates an eastward dipping fault plane with a shallow dip angle. Almost all the aftershocks are located in the crust, which is consistent with the fact that the P_MP phase is clearly observed from most of the aftershocks.
A remarkable later phase is observed at many stations 4-7 sec after the P arrival. This later phase is interpreted as the reflected wave both at the sea surface and at the Moho discontinuity (_pwP_MP). Focal depth distribution estimated from arrival time differences between P_MP and _pwP_MP phases also shows the eastward dipping fault plane with a shallow dip angle.
Foreshock activity started 12 days before the occurrence of the main shock within a concentrated area in the vicinity of the main shock hypocenter. All the foreshocks are classified into two groups: one with high peak-frequency and the other with low peak-frequency, each having very similar wave forms. Hypocenters of low peak-frequency events are located at shallower depths than those of the main shock and high peak-frequency events.

View full abstract

Focal machanisms are determined for about 300 shocks, which occurred in the Kanto-Tokai District during the period 1979 through 1983. First-motion data are taken from Japan Meteorological Agency, National Research Center for Disaster Prevention, Earthquake Research Institute (7 stations) and Abuyama Seismological Observatory.
In the Northern Kanto District, low-angle thrust fault type solution is predominant for earthquakes shallower than about 70km. Most of these belong to the upper seismic plane. Earthquake in the lower seismic plane are characterized by down dip tension and those in the upper seismic plane by down dip compression but with various azimuths.
In the Southern Kanto District, earthquakes of reverse type with E-W compression and of normal type with E-W extension are considered to be related to the subduction of the Pacific plate, and those of reverse type with N-S compression and of strike slip type with N-S compression are supposed to be related to the subduction of the Philippins Sea plate. In and around the Izu Peninsula, tension axes are systematically distributed so as to make coaxial circles whose center is at a point in the north of Suruga Bay.
In the Tokai district, the upper crustal earthquakes are predominantly of strike slip type with E-W to SE-NW compression, whereas the subcrustal earthquakes are of strike slip or of normal faulting with E-W extension.

View full abstract

The epicenters of large earthquakes and active faults are located on the boundaries between mountainous area and plains or between uplift and subsidence areas. These boundaries are found on the inclined topography. The relation between the crustal movement such as active faults and large earthquakes and the gradients of the topography is investigated. The gradient of slope is estimated on a space of each 12km×12km.
In the Chugoku district, Southwest Japan the gradient peculier to the crustal movement such as seismic activity is about 3/100, in the Chubu district about 7/100. These gradients of topography indicate the wavelengths of topography related to the crustal movement. The wavelength in the Chugoku district is longer than that in the Chubu district.
For extracting these wavelengths of the topography, a bound-pass filter which was designed by SEYA (1963) for gravitational prospecting, is applied to the topography.
The filter with the central wavelength of 40km and band width of 20-90km, extracts the long wavelength topography. Large earthquakes and active faults exist along the boundaries between positive: upheaval and negative: subsidence areas in the Chugoku district. Another topography extracted by the filter with the central wavelength of 20km and band width of 10-40km explains the distribution of epicenters of large earthquakes and active faults. This topography is also agreement with the pattern of distribution of large earthquakes in the Chugoku district.
Microearthquakes are also correlated to the topography. The gradient of 3/100 is a key value for the location of vigorous activity of microearthquakes. Epicenters of microearthquakes observed by the Hokuriku Microearthquake Observatory are related to the extracted topography by Seya's filter with the central wavelength of 16km or less.

View full abstract

A seismic observation using an extensometer in combination with pendulum seismometers was carried out at the Osakayama Crustal Deformation Observatory, situated in the sowthwestern part of Shiga Prefecture. The extensometer used for the observation is equiped with a differential-transformer transducer, the output signals of which were passed through an electronic device of automatic zero adjustment in order to minimize the effect of the earth-tide and secular strain signals that contribute ‘noise’ to observation of small seismic signals.
A detailed analysis of some good quality records obtained from recent earthquakes successfully demonstrated an agreement of theory and observation as to the directional response characteristics of the strain- and the pendulum seismometers, though the accuracy of the amplitude ratios of ground velocity to strain estimated from the record of the two instruments was not sufficient to provide the data for determination of phase velocity of surface waves from observation at the single station.
Comparison was made between the strain and the velocity records obtained from two Chilean and 1983 Japan Sea earthquakes and it was shown that seismic waves propagating along the minor or the major arc path of the great circle, such as SS- and SSS-coupled PL waves and some G waves, can be easily discriminated by taking into account the difference of directinal responses of the two instruments to seismic waves. Two examples of ‘strain step’ recorded from large regional earthquakes were characterized by a gradual change in strain rather than step-like one.

View full abstract

The author investigated seismic activity, during the past 100 years, for the earthquakes (5.0≤M) in the southern part of Central Honshu, where the Philippine Sea plate is descending with a low dip angle. The migration of earthquakes towards northwest occurred twice. One, from 1905 to 1930, and the other, from 1935 to the present. The velocity of migration was about 2.5km/year for both cases. It seems that the earthquakes in the block have been occurring periodically rather than at random. The period of the activity is about 6 to 10 years. These results give us useful information for the prediction of earthquake occurrences in long term. It was also found that the area of Lake Hamana is most active in the investigated region.

View full abstract