A method for computing focal mechanisms by using polarities and amplitudes of the P wave first motions was designed. In the first step, some suitable solutions were picked up by using only polarities. A method based on the ‘Fourier method’ designed by AOKI (1986) for fast computation was adopted. In the second step, the most suitable solution was selected by using amplitudes. In order to avoid the path effect on the attenuation of the P wave amplitude, an earthquake cluster occurring in a very small region was analyzed and the amplitude correction for each station was made.
This method was applied to the analysis of the aftershock sequence following the earthquake with M4.9 which occurred in Kyoto-Osaka border region on May 28, 1987. As a result of this analysis, some characteristic features of the distribution of focal mechanism types were found. Most of focal mechanisms of aftershocks showed the same type as that of the main shock, that is, east-westward P axis and thrust type focal mechanism. But those of the aftershocks which located at the edge of the aftershock area or slightly distant from the fault plane of the main shock almost showed strike-slip type or intermediate type focal mechanism. Such focal mechanisms may be related to the stress field disturbed by the dislocation of the main shock.
It is conculuded that this method is very effective to calculate focal mechanisms for a group of earthquakes whose polarity data are scarce with sufficient accuracy.

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The observation of the concentration of ²²²Rn in air in tunnels have been carried out using flow-type ionization chambers in Tokai area. Observations started in April, 1977 at Toyohashi (TY), June, 1980 at Kikugawa (KI) and August, 1986 at Toyone (TN), Inabu (IB) and Asahi (AS). The ²²²Rn concentration observed at Toyohashi and Kikugawa shows pulse-like increase after heavy rainfall and is also affected by the atmospheric pressure change. It shows seasonal variation: it increases in summer and decreases in winter.
Measurements of ²²²Rn concentration in groundwater flowing in the tunnel at Toyohashi had also been carried out using the method of a toluene extraction-liquid scintillation counting for 13 months from May, 1986 to May, 1987 in order to compare it with the ²²²Rn concentration in air in the same tunnel. The ²²²Rn concentrations in groundwater also showed seasonal variation, though its amplitude was about a half that in air.
We assumed that the ²²²Rn rich soil gas supplied the tunnel with ²²²Rn. We estimated the pulse-like disturbances of ²²²Rn concentration in air observed after rainfall from local strain variation caused by rainfalls. In this calculation, it was supposed that the amount of the ²²²Rn rich soil gas squeezed out from the basement rock, increases in proportion to the contraction rate of the basement rock. Relatively good agreement was obtained between observed and calculated values. We infer that the seasonal variations of ²²²Rn concentration in air and in groundwater in the tunnel are due to the seasonal variation of the ²²²Rn concentration in the soil gas in the basement rock.

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The height of a sea wall is usually designed based on water levels of large tsunamis experienced in the past. Behaviors of tsunamis exceeding the design height are examined by means of numerical experiments taking the sea walls at Matsuzaki, Shizuoka prefecture and at Taro, Iwate prefecture. In the case of Matsuzaki, the inundated area reduces to 70 percents of the original one by the presence of the wall although the invasion of tsunami through the river mouth, where no water gate is constructed, is noticeable. In the case of Taro, the sea wall height is designed at 10 meters that equals the height of the 1933 Sanriku tsunami. According to our experiment, however, if a tsunami has the same height as that of the 1933 tsunami, the water level close to the wall front becomes higher than the height of the sea wall. In this case, the water level at some places in the town becomes so high as to give rise to human and material damage in spite of the construction of the sea wall. It is recommended, therefore, that local inhabitants take refuge onto hilltop, unless a small height is predicted by the tsunami warning.

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Non-uniform slip on a heterogeneous fault is investigated to describe the detail of earthquake rupture processes. The fault is parameterized by average stress drop and fault size to model the deterministic part of the faulting process. The size of small scale fault heterogeneities and the variance of local stress drop are introduced to describe the stochastic part of the faulting. Nonuniform slip (or non-uniform stress drop) tends to increase the high frequency contents of S-waves, and thus controlls the excitation of strong ground motion. Short-waves from the heterogeneous fault are generated as an energy-additive form by the stochastic fracturing of random fault heterogeneities. This brings us a seismic directivity effect particular to the incoherent short-waves, which is different from the seismic Doppler effect on the long-waves. This short-period seismic directivity predicts the azimuthal variation of short-wave amplitudes as
(1-v cos θ/β)^-1/2
for a unilateral faulting and
{1-(v cos θ/β)2}^-1/2
for a bilateral faulting, where v is an average rupture velocity, θ is the station azimuth from the fault-propagating direction, and β is S-wave velocity. Maximum and root-mean square accelerations are derived theoretically in terms of the variance of local stress drops, number density of fault heterogeneities, and fault size considering the short-period seismic directivity.

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We developed a real-time seismic data processing system to detect and locate local earthquakes occurring in and around Hokkaido. The seismic network operated by the Research Center for Earthquake Prediction (RCEP), Hokkaido University consists of thirty four stations equipped with short period seismometers, including six stations of Tohoku University and three of Hirosaki University.
Our 32-bit computer-based processing system is composed of two parts, which are connected with a control data file and a seismic wave data file. In the first part, digital data of seismic waves telemetered to RCEP are compiled at every 1.387 sec and events are detected based on comparison of the Walsh spectrum of real-time input data with that of noise data at the system setup time. In the second part, arrival times of P waves are estimated by using the AIC (the Akaike's Information Criterion) method for AR (Autoregressive) model and the location and magnitude of the event are calculated. The hypocentral parameters are printed out within 2.5 to 6min after the first arrivals of P waves at the nearest three stations for the events. These results are automatically sent to the Earthquake Prediction Data Center, the University of Tokyo by telephone line.
4049 events were located in 1987 by using P and S phases on eye reading in the RCEP routine and 1993 events among the 4049 were located by the real-time system. The systematic differences between the epicenters located by the rouine and by the real-time system are less than 3km for the events occurring around the center of the network and more than several 10km for events occurring off the southern Kurile Islands. The large discrepancy for the second group of events may be explained in terms of the lateral heterogeneity of the upper mantle structure around Hokkaido.

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Great earthquakes had migrated from south to north along the trenches of the Japan Arc until 1611. But the direction of migration reversed, flowing in a north-south direction beginning in the year 1677. The influence of the change in the migrating direction reached as far as Northeast China and included the Korean peninsula. Then, seismic activity changed suddenly in these two regions and a long aseismic period began. It continued for about 150-250 years. A seismic activity started again in the Japan Arc with the 1843 earthquake in the Hokkaido region. The seismic activity in the Korean peninsula began again with an earthquake in 1906, and the last epoch of seismic activity in Northeast China began in 1970.
The velocity of the migration of activity from the Japan Arc to Northeast China is estimated to be about 20-30km/y.
Great earthquakes along the trenches have repeatedly migrated from north to south. Recently, seismic activity has again turned northward, where two great earthquakes occurred in 1952 and 1968. If this order of migration were to hold, the next earthquake would occur in the Kanto region around the year 2000.

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It is important for earthquake resistant design of critical structures like nuclear power plant to evaluate characteristics of near-field ground motion. The intensity of the near-field ground motion during the 1984 Western Nagano Prefecture earthquake (M6.8) is estimated from the observed upthrow of boulders. To simulate the upthrow, the Distinct Element Method is employed, because it can take into account not only interaction but also separation of soil and boulder. According to ambient vibration test of the boulders and seismic prospecting of surface ground in the field, the parameters used in the simulation are determined. As a result of the simulation, the intensity which cause large displacement of boulder due to the upthrow is approximately estimated to be 2g in terms of peak horizontal acceleration, 150 to 200cm/s in terms of peak horizontal velocity and 1g in terms of peak vertical acceleration. These values are consistent with those of the previously observed strong-motion records in near-field.

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Mutual relations of the temporal variation of seismicity in the long period in five areas in West China and its neighboring regions have been analyzed. Time series of events in Hindukush, Tibet Plateau, Xinjiang, the northern part and the southern part of the North-South seismic belt (NSB) of China were compared with each other.
Seismicity of the northern part of NSB was high in the period from 1560 to 1740. Then a quiet period followed until 1920. From 1920 an active period began again and has continued upto present. On the other hand, seismicity of the southern part has been reversed in long period variation against the northern part.
The seismicity in Tibet Plateau and Xingjiang regions was high around 1910, 1930's, 1950 and 1970's. The active and quiet periods can be correlated to each other in these areas. Synchronous short period variations of seismicity in these areas implicate that the earthquake generating stresses in a wide area of West China are formed under common tectonic conditions. These active and quiet periods also correpond to the variation of large earthquake activity along a long boundary between the Indian and Eurasian plates. They also suggest that there is the transmission of tectonic forces into the wide area of West China by the collision between the Indian and Eurasian plates along Himalayan Front.
The long period variation of historical seismicity in the northern part of NSB is consistent with that in the North China and Korea regions. It suggests that the long peroid variation of seismicity in the northern part of NSB is caused by the tectonic force from the Pacific plate through Korean Peninsula and North China.

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Regional distributions of maximum earthquake motion for Algeria and Ethiopia are examined and on the basis of these distributions, the seismic hazard zoning maps, which are available to establish the seismic regional coefficient, are proposed for both areas.
Algeria was damaged by EI Asnam earthquake (M=7.7) of Oct. 10, 1980, due to which the casualties were 5, 000 killed and more than 9, 000 wounded. Algeria had also suffered a great deal of damages from the earthquake (M=6.7) of Sep. 9, 1954. Therefore, Algeria has made a lot of efforts for mitigations of earthquake disasters. The regional distributions of maximum earthquake motion in Algeria show fairly high values at the vicinity of Bi-in-Eker around 24.0°N and 5.0°E and north of 33.0°N. Ethiopa is located on the north-eastern part of African plate, which is in contact with Arabian plate and the East African Rift Valley runs through the central part of its territory. Addis Abeba, the capital of Ethiopia has experienced a few earthquake damages. In consequence, Ethiopia has been ardent and interested in earthquake disasters and its prevention and has made a trial of seismic hazard evaluation. Regional distributions of maximum earthquake motion for Ethiopia are made newly also in this research. Seismic hazards of Ethiopia are high in the region of East African Rift Valley, Afar depression and the northern part of its domain. The regional distribution of maxmum earthquake motions or seismic hazard zoning maps are expected to give useful information for Algeria and Ethiopia to make counterplans for earthquake disasters.

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A microearthquake network covering Hokkaido, Japan, has been operated by the Research Center for Earthquake Prediction of Hokkaido University. The Center is equipped with a data processing system suitable for handling and storing large amounts of waveform data. Digital waveform data over 40GB have been stored since 1984 on optical disks with 1GB memory per cartridge. Manipulating the waveform data becomes as easy as using the bulletin data such as phase readings, hypocentral parameters and so on.
A total of 1476 short period waveform data from earthquakes of M≥3.0 occurring in and around Hokkaido in 1985 are analyzed automatically without visual inspection. Rectilinearities, azimuths, and incidence angles of P wavelets are computed on the basis of the principal component analysis. Results are surprisingly different from station to station. However, spatial distributions of both rectilinearities and deviations of azimuths from the directions of epicenters show systematic patterns related to lateral heterogeneity in the crustral structure. Another analysis of earthquakes occurring off Urakawa from 1984 to 1988 indicates that the rectilinearity has become high at the beginning of 1987. The change may be related to a large earthquake of M7.0 which occurred under the Hidaka mountains on January 14, 1987.

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We devised a low-cost portable seismic array system suitable for mobile field observations. The system consists of 8-Hz geophones, 4-channel preamplifiers, 16-channel analog bus-lines, a 24-channel main amplifier with analog filters and a 12 bit digital data recorder using personal computer. The usage of the personal computer as a data recorder makes a real-time data processing in fields very easy. The system has been found to be very effective and convenient for high-quality data acquisition through the following field surveys: the reflection profiling at Tokai City, the refraction experiment combined with air-gun source in Izu-Oshima Island, and the observation of volcanic tremor in the Izu-Oshima caldera. The system is useful in many kinds of field surveys in addition to those mentioned above, such as an observation of microearthquakes in noisy urban area, a reflection profiling of earthquake source region using aftershocks, and seismic tomography of an active volcano.

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The melting of the great ice sheets in Late Pleistocene and Early Holocene time have produced ongoing spatial and temporal variations in sea-level as the Earth responds to the redistribution of the surface loads. Factors contributing to these sea-level changes include the geometry of the ice loads through time and the rheology of the Earth. By examining different time-intervals of the observed changes in sea-level for the past 20, 000 years from widely distributed geographical areas, it becomes possible to constrain both the melt models and the Earth's rheology. The sea-level changes along the Atlantic coast of North America, close to the margins of the former Laurentide ice sheet, have been widely used to determine the rheological parameters but these sites are also sensitive to the description of the nearby ice loads which are generally inadequately known for the purpose of estimating the mantle viscosity. Observations from sites far from the former ice magins, such as from Australia and the South Pacific, are important for constraining the gross melting history of the Late Pleistocene ice sheets. The Antarctic ice sheets provide a significant contribution to the sea-level rise at a rate that was approximately synchronous, or possibly later than, with the melting of the Laurentide ice sheet. Minor melting of the Antarctic ice sheet continued throughout the Late Holocene. Particularly important observations are the differential observations of Late Holocene sea-level change recorded at sites in the same region as such differences are sensitive to rheology and insensitive to the detailed melting history. A good resolution of the depth dependence of the viscosity parameters and of lithospheric thickness is not, however, achieved and the resultant parameters are effective or equivalent parameters only. The response of the crust to the addition of meltwater in Holocene time depends strongly on the geometry of the oceans into which this meltwater is added and by examining sea-levels from islands of different sizes or from coastlines of different geometry it becomes possible to exploit this dependence to examine whether lateral variations occur in mantle viscosity. Differential sea-levels along the Australian continental margins constrain the effective upper mantle viscosity to be about (1-2) 10²⁰Pa s and the lower mantle viscosity, below the 670km discontinuity, is about two orders of magnitude greater. The estimated ice and rheological models explain many of the Holocene sea-level observations throughout the Australian region, and there is little evidence for tectonic motion. These models also explain the average sea-level curve for Japan and serves as basis for examining tectonic motion. The Pacific Island observations suggest that the upper mantle viscosity beneath the oceans may be less than beneath the continental crust. However, more data from tectonically quiet regions is required for the period 18, 000 years ago to the present in order to examine lateral variations in mantle viscosity.

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