Relations among several earthquake magnitude determination formulas were investigated by using data obtained by high resolution digital accelerometers. Digital accelerometer, JMA-87 type electromagnetic strong motion seismograph (ESM) which was recently deployed at 74 stations over the Japanese Islands by the Japan Meteorological Agency (JMA), is one of the wide-frequency band and large-dynamic range seismograph. Digital records enabled us to simulate any type of seismograms from the same data and to measure several maximum amplitude values for corresponding magnitude determination formulas. To evaluate consistency among the magnitude formulas, magnitude values from the same station were compared with each other in the present study. One of the advantageous points of the method is its independency from the influence of site condition and inhomogeneous attenuation along ray path on the observed amplitude.
As a first step, magnitudes which are derived from the Tsuboi's formula (M_T) and the JMA-EMT formula (M₆₇ or M₇₆), both of them are employed as standard JMA magnitudes, are compared with each other. M_T is calculated from the maximum amplitude of horizontal ground displacements recorded by medium period seismograph (JMA-59 type and JMA-S type, T=5s) and M₆₇ and M₇₆ are from the maximum amplitude of vertical ground velocity recorded by short period seismograph. JMA's magnitude is given as a arithmetic mean of both formulas. Although JMA-EMT formula was designed as a substitute of Tsuboi's formula for smaller earthquakes which can be recorded only by sensitive short period instruments, as several studies have already pointed out, it is consistent with Tsuboi's formula only in a narrow magnitude range. Compared to the Tsuboi's formula, while the JMA-EMT formula provides smaller magnitude values for larger earthquakes, it gives larger values for earthquakes which magnitudes are less than 3 and discrepancy increases as the size of earthquake becomes smaller. Introduction of a coefficient to the formula which adjusts the relation between magnitude value and logarithm of maximum amplitude value seems to be indispensable to correct the discrepancy.
Beside the coefficient, constant in the EMT formula depends largely on conditions of installation sites. Otherwise, borehole installation in a hard basement gives smaller magnitude value and surface installation on an alluvial soil basement gives large magnitude value. Classification of site effect is important to determine appropriate constant for EMT formula.
As a second step of this study, it is indicated that amplitude data within 50km epicentral distance provide smaller magnitude values for both Tsuboi's formula and EMT formula. The EMT formula also gives smaller magnitude value for data beyond 200km epicentral distance. There by, the study shows importance of appropriate selection of epicentral range for applying the formula.
Finally relation between JMA's magnitude and Richter's local magnitude (M_L), which is the oldest magnitude scale and of the international scale, was investigated by using simulated Anderson-Wood seismograms. Results show that M_L from simulated seismograms is generally larger than M_T for smaller earthquakes. However, difference becomes smaller as earthquake magnitude becomes larger.

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The heterogeneity and anisotropy in the lower mantle such as seismic P wave velocities and geoid anomalies point to a fundamental Pacific/African bipolarity. The ridge/subduction zone systems have some relationship with the hemispheric hotspots grouped through the mantle convection and so may have nearly antipodal axes under the influence of convection flow. A global seismicity map suggests that most of earthquake epicenters are distributed on a part of great circles over the Earth's sphere. We propose a method for detecting great circles from the global epicenter distribution by using the Hough transformation. As a result, we extract two dominant great circles which intersect perpendiculaly each other at the equator as an axis of symmetry. One of these two great circles is passing through the northern part of the circum-Pacific seismic zone and the Africa-Antarctic plate boundary, while the other is passing through the Eurasian seismic belt. The reason of the orthogonality and the symmetry of these two great circles to the equator is discussed qualitatively in comparison with the long-wavelength geoidal anomalies and the Earth's rotation.

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The ability of the automatic data processing in seismic observation was examined by comparing the results of the hypocenter and focal mechanism determinations with those by manual processing. The mean rate of the hypocenter determination is estimated to be 53% for the data observed within a circle with a 200km radius, which is regarded as the network observation area covering the Kanto and the Tokai districts. The accuracy is certainly good within that area. In the case of the automatic processing for focal mechanism determination, the rate reaches 41% and is almost 100% for earthquakes of M3 or greater. It has been noticed that the lack of reliability becomes a conspicuous problem in the results obtained by the automatic processing for the earthquakes occurring outside the network area.

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A stick-slip experiment is performed to investigate generation mechanism for high-frequency ground motion at immediate proximity to a fault. A granite sample with a 40cm long pre-cut fault is compressed by a bi-axial loading apparatus to generate unstable slips on the fault. Ground motion at frequencies from 200kHz to 1MHz is measured with a wide-frequency band AE sensor. The observed records reveal some characteristics of high-frequency ground motion as follows: (1) The amplitude of ground motion strongly depends on the rupture velocity. (2) Near-fault strong-motion duration is much shorter than slip duration and well correlated with the local breakdown time. (3) Near-fault strong motion contains much higher frequencies than those expected from the breakdown zone size. These results indicate that near-fault high-frequency strong motion is generated by irregular rupture during the local breakdown process. The result (2) suggests that the breakdown zone size for an earthquake can be estimated from near-fault high-frequency strong motion records.

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The southern part of Niigata prefecture is located on thick Tertiary sedimentary rock layers. On Dec. 7, 1990, a shallow earthquake (M=5.4, H=15km) occurred in this area and strong motion records were obtained at stations of a seismic array about 24km north of the epicenter. In this study, characteristics of the recorded motions in period range of 1 to 10 seconds are investigated using multiple band-pass filtering technique. The results are as follows:
(1) The displacement seismograms at ground level and at depth of about 300m are very similar. Correlation coefficients of band-pass filtered seismograms show that the ground motions with periods which are longer than natural period of overlaying layers are very similar.
(2) The long periods ground motion with periods about 3 to 8 seconds consists of surface waves. Both Rayleigh wave and Love wave are recognized and they are propagating mainly from the epicenter. So, the correlation coefficient of radial and transverse components show different frequency distribution.
(3) Because of short epicentral distance, the surface waves arrived soon after body waves' arrival. So, the principal parts of seismograms have longer duration than as usual and the principal parts of radial and transverse components have different periods.

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We report hypocenter distribution derived from 24-day OBS array observation on the Reykjanes ridge, SW off Iceland. Our research area was between 62.5°N and 64°N, 150km along the ridge and 40km width. In this area, 18 OBSs were deployed with interval of 10-20km. The object of this experiment was to investigate detailed microseismicity in the area. In the observation period (July 2-26, 1990), about 800 earthquakes were recorded. We selected 444 earthquakes for hypocenter location, and 297 hypocenters were determined. In order to discuss the hypocenter distribution there, we selected 179 earthquakes whose errors are less than 5km. The epicenters are distributed along the ridge axis, with 5km width. Hypocenters seem to be distributed vertically. The focal depths are less than 12km below ocean bottom. Seismic activity is concentrated in space and time; two active swarms were observed (July 4 and 14).

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For the study of macroscopic characteristics of earthquake, it is necessary to investigate both historical and modern earthquakes from the same point of view. Historical earthquakes present informations on damages only. Modern earthquakes have various kind of data obtained from instrumental observations. Intensity data is common to both old and modern earthquakes. As the first step to study historical earthquakes, we studied the seismic intensity distribution of recent earthquakes, in the Tohoku district, which occurred in the interval from 1926 to 1990. Results are summarized as follows.
(1) The isoseismal contours for many earthquakes off the Pacific coast elongate to north and south, and the seismic intensity decays steeply across the line which is almost parallel to the volcanic front. We call this line “Steeply Decay Line of Seismic Intensity (SDLSI)”. On the contrary, the distribution of seismic intensity for shallow inland earthquakes doesn't show this tendency.
(2) We investigated the seismic intensity attenuation in the Tohoku district and obtained the next formula,
I=A-BX
where, I is the seismic intensity, X the hypocentral distance. Coefficients A and B are expressed as follows:
for earthquakes of G1, G2 and G3 (see Fig. 2)
A=0.198+0.679M, A/B=-1332+299.9M (east side of SDLSI),
A=0.944+0.589M, A/B=-1329+279.5M (west side of SDLSI).
for earthquakes of G4
A=-1.315+0.912M, A/B=-410+134.9M
(3) For the earthquakes along the plate boundary, we defined relatively low- and high-frequency earthquakes according to the value of M_J-M_I, where M_J is JMA magnitude and M_I the one determined by comparing observed intensity attenuation data of each earthquake with a curve calculated from formulas in (2). The epicenteral distribution of low- and high-frequency earthquakes does not show clear characteristics. But, off Fukushima Prefecture region, there seems to be a boundary of the high-and low-frequency earthquake's distribution along the plate boundary of about 40km depth. The high-frequency earthquakes are found in the west side of the boundary.

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