Magnetotelluric (MT) measurements on three transects have been obtained in the central Tohoku district of the northeastern Japan, where the Pacific plate subducts beneath the Eurasian plate. In this report observations and analysis of the MT data on the northernmost line (known as Line A) of 33 sites are described. Broadband magnetotelluric sounding data (0.00005-1, 000s period) were collected at the points along the line A of about 140km length running approximately east-west. Measurements at two sites on each transect about 70km apart were conducted simultaneously for the sake of remote-reference data analysis.
The impedance- and tipper- strikes show roughly north-south structural direction in spite of variable distributions both in space and frequency suggesting a considerably complex feature of the resistivity structure in the region. We assumed a two-dimensional structure running roughly north-south. Both strike-parallel and strike-perpendicular components of the MT apparent resistivity and phase data of the broadband measurement were subjected to a modified Rapid Relaxation Inversion to obtain an approximate two-dimensional smooth resistivity model for the northernmost transect, which was compared with the one-dimensional model through the coordinate invariant apparent resistivity. The two-dimensional model indicates that there are six conductive anomalies of several ohm·m with different degree of plausibility, for instance, at Japan Sea side of Tohoku, Central Basin Range and Kitakami River region. Those conductive regions are suggested to be Quaternary sediment layers. The resistivity structure is discussed with reference to the seismic velocity distribution, Q-value of the seismic wave attenuation, seismicity, and temperature distribution.

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Strong ground motion in Tokyo metropolis during the Sep. 1, 1923 Kanto earthquake was important factor for determining the seismic design code in Japan. However, it has not been clarified in detail, because of no strong motion record near the source region of the Kanto earthquake. The author collected the descriptions and made a data set of 548 personal experiences in and around the southern Kanto district to investigate the strong ground motion from the Kanto earthquake. Many descriptions in Tokyo metropolis indicated that three severely strong shakings arrived at this area during the Kanto earthquake. The first shaking was, of course, caused by the main shock (M=7.9) at 11:58AM (JST). The second and the third shakings were caused by the aftershocks occurring 3 minutes and 4.5 minutes after the main shock, respectively. These aftershocks were identified on a seismogram obtained at Gifu observatory and magnitudes of them were determined to be 7.2 and 7.3, respectively from this seismogram [TAKEMURA (1994)]. The results of the analyses of the personal experiences also indicated that the duration time of the first shaking due to the main shock was 30 to 40sec. The strength of the second shaking was as strong as the first shaking, of which seismic intensity was estimated to be VI in JMA scale, while duration time of the second shaking was shorter than the first shaking. On the other hand, the third shaking was weaker than the first and the second shakings, of which seismic intensity was estimated to be V. The sequence of strong shakings within 5 minutes after the occurrence of the main shock of the 1923 Kanto earthquake was elucidated in Tokyo metropolis in the present study.

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Tectonics and geological environments in and around Ashigara valley are very complex, especially surrounding rock outcrop sites are not on the same geology. Therefore, it is our concern to find the best reference site and to determine the seismic response at a site in the valley. We had an opportunity to use the ground motion data sets from the very far (>700km) and large (>M7) events to compare the rock sites motion as well as the response of sediments site. Advantages of using far and large events are that the source and path effects will be common with a sufficient approximation and that the ground motions of wide frequency band content are expected. Selecting one rock site, seismic responses of the sediment sites and rock sites were determined in terms of frequency as well as time domain amplitudes applying band-pass filtering techniques. Results are as follows; (1) Spatial distribution of seismic responses as a function of frequency in the valley are displayed. The spatial distribution of amplitude in low frequency range under 0.2Hz is relatively simple and similar to Bouguer anomaly map. The characteristics of spatial distribution in high frequency are more complex. (2) Deviations of spectral amplitude at rock sites were determined by taking the spectral ratio of the site to the average spectra of 6 rock sites. Only lower frequency motion than 0.07Hz are the same and the deviations of spectral ratio are a factor of 2 in the frequency range of 0.1-10Hz. (3) The level of spectral ratios of very far earthquakes are more stable than that of near earthquakes in 0.1-10Hz.

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We estimated source spectra, an attenuation function, and site amplification factors using the K-NET records for six earthquakes (M_J=4.4-5.9) that occurred in the border of Akita and Miyagi Prefectures in August, 1996. These earthquakes occurred inland at the focal depths from 6 to 11km. The hypocentral distances of the stations used in this study range from 5 to 200km. We analyzed 212 source-station pairs×3 components, that is, radial, transverse, and vertical components. The Q^-1 estimated by assuming the geometrical spreading to be the reciprocal of the hypocentral distance showed strong decay with frequency. It was modeled by Q^-1=0.005f^-1.5 in the frequency range from 0.3 to 1Hz in which Q^-1 had almost the same value among the three components. We also found that the Q^-1 has no clear difference between the eastern and western regions to the volcanic front. The site amplification factor at IWT09 (Daito station) in the Kitakami region whose geology is categorized into pre-Tertiary rock is smallest in the frequency range from 0.1 to 4Hz. Therefore we examined site amplification factors and source spectra in the frequency range from 0.1 to 4Hz using IWT09 as a reference site. The site amplification factors relative to IWT09 vary from site to site very strongly and their standard deviation for all the used 59 sites is the largest, about a factor of 2.5 of the average, for the horizontal components in the frequency range from 0.5 to 2Hz. For the vertical components that is a factor of 2.2 of the average in the similar frequency range. The source spectra for all the six earthquakes have predominant peaks at the frequency about 0.3Hz for all the three components. We found that the remarkable dominance of the horizontal components in the frequency of 0.3Hz at MYG05 (Naruko, the nearest station with an epicentral distance of about 10km) are caused by the source spectra as well as its site amplification. The Brune's stress drops were estimated to be 5 bar, much smaller than the average stress drop of 200 bar estimated for subduction-zone earthquakes in the Tohoku district. Acceleration magnitude M_pga, which is determined to match the observed peak ground acceleration with an empirical attenuation relation, is 0.3 to 0.7 smaller than the JMA magnitude M_J. The relation between the M_pga and the seismic moment M₀ of the Harvard CMT solution, also suggested low static stress drops from 5 to 15 bar. However, the relation between the M_J and M₀ is consistent with the SATO's empirical relation which is equivalent to the static stress drops of 50 bar. We interpreted this discrepancy to be caused by the prominent generation of waves in the frequency range around 0.3Hz at which M_J is determined.

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During the Hyogo-ken Nanbu earthquake of 1995 we have observed conspicuous concentration of damage in an extended area from Suma in the west end of Kobe to the center of Nishinomiya as a form of the so-called damage belt whose width was very small (≅1km) compared to its length (≅20km). We need to find what caused this damage pattern in Kobe. In our previous study we attributed it to “the basin-edge effect”, the amplification effect caused by the constructive interference of the direct S-wave with the basin-induced diffracted/surface waves. However, several researchers have recently reported that seismic motions at the bedrock have a narrow-band distribution due to the directivity effect and that the enhancement of this distribution should be taking place through the interaction of the directivity with the basin structure.
In this paper we analyzed again a simple two-dimensional basin model subject to various types of three-dimensional incident waves to confirm that the basin-edge effect is present irrespective of the input wave field. After proving it then we analyzed a simpler basin to fully understand the wave interference phenomena around the edge. It is found that the interference is actually happening twice, first with the basin-induced diffracted P-wave and second with the basin-induced diffracted S-wave plus Rayleigh wave. Quantitative estimation on the contribution of the edge-induced waves during the Hyogo-ken Nanbu earthquake revealed that it must give at least 50cm/s additional amplitude in terms of the peak ground velocity inside the damage belt.

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The Hokkaido Nansei-oki earthquake (M_JMA=7.8) occurred off the Japan Sea coast of southwest Hokkaido, Japan, on July 12, 1993. We deployed four digital ocean bottom seismographs (DOBSs) at intervals of about 5km aiming at the southernmost part of the aftershock region, where a land seismic network detected the highest seismicity. A purpose of the DOBS network is to obtain a high resolution aftershock distribution which allow us to investigate a local fault system. We acquired high quality data from three OBSs, and the OBS network detected approximately 1000 earthquakes, during observation period from August 31 to September 6, 1993. The magnitudes of observed earthquakes ranged from 1 to 4. The use of DOBSs enabled us to pick later phase arrivals (e. g. S wave and converted wave arrivals) with high accuracy. Most of the observed aftershocks were located in an area of 20×15km, and a depth range of 4-18km. A total of 518 hypocenters were obtained with spatial errors of less than 1km. The aftershock distribution correlates well with the sea floor topography. The region where the water depths are between about 1900m and 2100m seems to be characterized by relatively low seismicity. In the area west of the low seismicity region, the hypocenter distribution shows a westward dipping plane, and the shallowest earthquake occurred at a depth of 4km. This feature in the western area suggests that the hypocenter distribution corresponds to the fault plane of the main shock, and that the fault extends to near the sea floor. In the area east of the low seismicity area, the hypocenter distribution does not show any clear dipping plane, and seems to be clustered. We infer the existence of a complicated fault system from the hypocenter distribution in the eastern region. The obtained aftershock distribution suggests that the source region consists of several blocks and that the fracturing in each block is different.

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The source region of the 1946 Nankai earthquake is examined based on the recent studies of microearthquakes, focal mechanisms and the crustal structure in Shikoku, southwest Japan as well as the aftershock distribution just after the main shock. Although it is generally known that an aftershock distribution is nearly equal to the fault zone and the source area of tsunami, such an agreement has not been recognized in the case of the 1946 Nankai earthquake. In the present study, we point out the misinterpretation about the aftershock area and the focal plane in the previous works, and give a new interpretation indicating good agreement among the aftershock distribution, source area of tsunami, crustal movements, disaster area and fault region. In the previous works, the defect of the observation network in the west of Shikoku and the east of Kyushu was not taken into consideration on the estimation of the aftershock area. Furthermore, the focal plane was not properly estimated owing to misunderstanding of the crustal movements and the source area of tsunami. Since the focal plane exists in the focal layer in the uppermost mantle, we consider that the aftershocks occurred only in this layer and the crustal earthquakes were induced by stress release due to the occurrence of the main shock. Our microearthquake observations show that this layer has a thickness of about 5km from the west of Shikoku to the western part of the Kii peninsula and from the Median Tectonic Line in the north to the Nankai Trough in the south.

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Recently, servo-seismometers (negative feedback seismometers) are used for various fields in seismology and earthquake engineering. First, this report explains five fundamental negative feedback seismometers with a displacement or a velocity transducer. Next, composite negative feedback seismometers, such as STS, VSE and CMG seismometers, are explained. Also, new negative feedback seismometers that have two feedback paths, are discussed. Finally, seismometer noise, the representations of instrument noise and seismometer characteristics, and, some problems in borehole observation by using negative feedback seismometers are discussed.

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Two types of earthquakes have occurred in and around Japan, which have resulted in severe damages. They are inter-plate earthquakes in subduction zones around the Japan Islands, and intra-plate earthquakes in the upper crust beneath the mainland of Japan. The faults associated with intra-plate earthquakes usually exist within a depth of 20km. Six intra-plate earthquakes causing a toll of more than 1, 000 lives have occurred since the end of the 19th century. The faults responsible for these events were estimated from the trains of surface fault breaks and the measured crustal deformations. Three intra-plate earthquakes with magnitude M larger than 7, which killed more than 200 people, have also occurred in the same period. Their causative faults have also been estimated. Many reports of damage surveys of the above nine intra-plate earthquakes were examined to investigate conditions necessary for generating strong ground motion with seismic intensity I=VII (very disastrous) in the Japan Meteorological Agency (JMA) scale, and to elucidate characteristics of strong ground motion with I=VII. According to the original definition of the JMA scale, I=VII (very disastrous) corresponds to “Collapse of more than 30% of wooden houses”. The data on damage to wooden houses for the nine events show that the area of I=VII extends up to 5km on both sides of the fault in the case of faults through mountainous region. On the other hand, in the case of earthquake faults lying underneath basins, where sediments have accumulated from the Late Pleistocene to Holocene, the area of I=VII extends over a wider area and occasionally fills the whole basin. The ground condition is seen to be an important factor in generating strong ground motion with I=VII. A similar result is obtained from an examination of the 1923 Kanto earthquake near source region with the thrusting fault at a depth shallower than 20km. The Kanto earthquake is one of the typical disastrous inter-plate earthquakes in Japan. The data on the directions of simple bodies that have overturned and collapsed wooden houses in the proximity of earthquake faults indicate that systematically larger ground motions occur in a direction normal to the strike than parallel to it irrespective of the type of faulting. The effective periods for these simple bodies and wooden houses are estimated to be in the range of 0.3sec to 1.5sec. These results indicate that the dominant strong ground motion in the direction normal to the strike plays an important role in generating the severe damages in regions hit by intensity I=VII.

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