A seismic reflection profile implies that foothills and terrace surfaces in the eastern margin of the Suzuka Range are underlain by an east-facing monocline. Based on the stratigraphy of the late Pliocene-middle Pleistocene Tokai Group and a drill core of 500m deep, we correlated distinct seismic reflectors to the boundaries between the Komeno Formation and the Oizumi Formation of the Tokai Group. Analysis of growth strata demonstrates that monoclinal uplift of the Tokai Group and Paleozoic rocks began since the beginning of the deposition of the lowermost part of the Tokai Group (ca. 3.0Ma). Total vertical growth and shortening of the bottom of the Tokai Group during the deposition of the growth strata are estimated to more than 1, 800m and 1, 000m, respectively. Using an age of 2.9±0.2Ma for the Ichinohara volcanic ash, which overlies near the bottom of the Tokai Group, based on fission track method, uplift rate since late Pliocene is >0.58-0.68mm/yr, and horizontal shortening rate is >0.30-0.37mm/yr. The resulting long-term structural growth rates are comparable to those of the Fumotomura fault during late Quaternary based on previous geomorphic studies, which suggests that the Fumotomura fault is responsible for the growth of the monocline. If so, the fact that terraces are deformed more narrowly than the Tokai Group implies that the monocline and the warping of terrace surfaces is fault-propagation fold.
Many earthquakes occur after a destructive shallow earthquake or before a volcanic eruption. It is very important to determine accurate hypocenters as early as possible at a time of such a huge seismic activity, since seismicity data are essential for the understanding of the crustal activity. We developed an automatic data processing system of seismic waves which has a swarm mode processing and can locate accurate hypocenters even for a huge seismicity. The main difference between the swarm and ordinary modes is that the former assumes hypocenters for all events to be in a small area. Event detection is made by estimating approximate origin times calculated from each picked arrival time of P and S waves for several low noise stations close to the swarm area and each theoretical travel time from the swarm area to these stations. We assume events having similar origin times to be seismic events. The system picks P and S wave arrival times not only for these stations but also for all stations by setting time windows at their arrivals which are calculated from the approximate origin time. The application of swarm mode to waveform data at a huge seismicity after the 1996 Onikobe earthquake sequence showed that the new system is very effective and can determine more precise hypocenters rather than manual pickings.
The Noto-Hanto-Oki earthquake (MJMA=6.6) occurred NE off Noto Peninsula on February 7, 1993. According to JMA (Japan Meteorological Agency), the focal depth of this event was 25km, which was deeper than most inland shallow events in Japan. There were distinct differences among the focal mechanisms obtained from the data of the P-wave initial motions, the long-period seismic waves, the short-period strong ground motions, and the tsunami waves. However, most of them indicated that the earthquake fault of this event was of the reverse fault type. We evaluated the source depth and the focal mechanism of the event through parametric simulation studies in the period range from 4 to 20 seconds of long period strong ground motions observed at 4 local stations. These stations were located from 46 to 133km from the epicenter. First, the subsurface structure from the source to each station was estimated as a multi-layered model by an inversion analysis of the group velocities from the observed Rayleigh waves. The structure obtained for each station was consistent with the distribution of gravity anomaly. It was concluded from the best fit of the simulation and the location of aftershock region that the fault plane of the event dipped 45° toward N140°E with a slip angle of 70°. We found that the fault plane simultaneously dipped 45° toward N330°E with a slip angle of 70°. However, this case was inconsistent with the deformation of the sea bottom by active faults. The depth of the equivalent point source, whose location corresponds to the center of the fault plane, was 10km in the best case of the simulation. The depth was significantly shallower than the focal depth reported by JMA and was consistent with other inland shallow earthquakes in Japan. All the results were obtained from the simulation using the point source model. However, it was also found that the conclusion did not change for the results from the model of finite moving source of this event.
The tectonic stress, which was found from focal mechanisms of earthquakes in the eastern and the central area of Tottori prefecture, were investigated. Focal mechanisms in this area were the strike slip type of WNW-ESE compression, which is nearly agreed with the mechanism of the 1943 Tottori earthquake. Tectonic stress in this area has conserved the same pattern as the Tottori earthquake, 1943. But the seismic activity near the Yoshioka-Shikano faultsystem shows lower than the other regions. Though their focal mechanisms show a large irregularity, the averaged P-axis indicates NW-SE compression compatible with the strike of the Yoshioka-Shikano faultsystem. This may be attributed to the situation that this area agrees approximately with the bright spots of the 1943 Tottori earthquake.
Ou Backbone range, northeastern Japan, is bounded both at eastern and western edges by active faults with reverse-fault type, and seismic activity is quite high in this range. The known largest event is the Rikuu Earthquake with M7.2 that occurred along the western edge in 1896. Two small aperture seismic arrays were deployed in 1997 and 1998 in order to image spatial distribution of P wave scatterers in the crust beneath the range. Waveforms of 7 and 13 explosions observed by the respective arrays were analyzed in a frequency band of 6-20Hz, Hypocentral distances between explosions and arrays are too large to apply conventional analyses such as CMP stacking or migration to observed data. We developed a method to estimate P-wave scatterer distribution from array observation data. Observed waveforms were slant stacked along various directions from the arrays, and energy density of waveform was further processed by diffraction curve summation. Spatial distribution of P wave scatterers was imaged by the presently developed procedure. The results show that strong scatterers are distributed around the fault plane of the 1896 Rikuu Earthquake and in the mid- to lower-crust of the range. The distribution of scatters in the upper crust correlates to hypocenter distribution of micro-earthquakes.
This is a revision of Hashimoto's (1990) study on average horizontal crustal strain rates in Japan derived from geodetic data collected during the past 100 years. This new study includes Hokkaido since the completion of the latest nationwide trilateration enables us to apply the methods of the previous study and obtain strain rates there. In this method, side lengths of triangulation networks are calculated using adjusted coordinates for different epochs and then the rate of change of each side length is estimated by regression. Finally the principal strain rate in each triangular region is obtained. In order to avoid the effects of large earthquakes or volcanic eruptions, data before or after these events are simply discarded. Unfortunately, due to frequent occurrence of large events in northern Japan, we can use data from only two epochs for most of Hokkaido, which causes large uncertainties in the estimated strain rates. Hokkaido appears to be divided into four provinces according to the characteristics of the horizontal strain rates, although the uncertainties are too large to be definitive. Eastern Hokkaido is characterized by large WNW-ESE contraction. In northern Hokkaido E-W contraction is prevailing, while extension is dominant in southwestern Hokkaido. In other regions the directions of principal axes of strain rates are not noticeably different from those of the previous study. In the Tohoku region, tensile strain rates in a N-S direction are prevailing. A NNW-SSE or NW-SE trending contraction is dominant along the coast from southern Kanto to Shikoku, though it is smaller in the Tokai area and Kii peninsula than in southern Kanto and Shikoku. In the same Shikoku region, there is a remarkable region of NE-SW extension that is as large as contraction. In central Japan, there is a NE-SW trending region of contraction. An E-W oriented contraction is dominant west or northwest of this region, such as in the Kinki and eastern Chugoku districts, while a NW-SE trending contraction is prevailing in the southeast. Kyushu is under an extensional regime with tensional directions of N-S and NW-SE in central and southern Kyushu, respectively. Magnitude of strain rates obtained in this study ranges from 1 to 3×10-7/yr in most regions except in the Shikoku, southern Kanto and Fukui areas where it reaches 6×10-7/yr. Over all, magnitude of strain rates are as large as or slightly larger than those derived from continuous GPS observations, but are larger than geological or seismological strain rates by a factor of 10.
Nozawa et al. (1995) proposed a source model with two big subevents of the same seismic moment for the 1923 Kanto earthquake (M=7.9), through the simulation of the records by the Imamura-type strong motion seismograph (displacementmeter) at Gifu observatory. This model was named Model I in the present study. The first subevent of Model I is located under the Odawara city, having a fault plane with the strike of N290°E and the rake angle of 162°. This fault has much strike slip component, which is consistent with the focal mechanism solution by KANAMORI (1971). However, the direction of the strike is not compatible with the trench axis of the Sagami trough. The second subevent occurring 12s after the first subevent is located under the Miura Peninsula. The fault of the second subevent, having much dip slip component, well explains the geodetic data. Recently, the seismograms by the Imamura-type strong motion seismographs at Sendai (Mukaiyama) observatory and Yamagata observatory were examined and the instrumental responses of the seismographs were revealed. Crustal structure from source to stations was estimated in the present study so as to explain the observed Love and Rayleigh waves at Sendai (JMA) and Yamagata observatories from the recent events occurring near the focal region of the 1923 Kanto earthquake. However, Model I failed to explain the records of the 1923 Kanto earthquake at Sendai (Mukaiyama) and Yamagata observatories, using the obtained crustal structure. Then, we revised Model I to explain these records, in consideration of the newly determined focal mechanism solution by Lallemant et al.. (1996) and iso-depth contour of the upper boundary of the Philippine Sea plate by Ishida (1992). The first subevent of the revised model (Model R) has a fault plane with the strike of N321°E and the rake angle of 128°, and the twice of seismic moment of the second subevent. The direction of the fault strike of the first subevent is parallel to the trench axis of the Sagami trough, while the fault plane of the second subevent is the same as Model I. Model R succeeded in explaining not only the records at Sendai (Mukaiyama) and Yamagata observatories but also those at Gifu observatory in the period range from 2 to 20s. This shows the fault model, being in agreement with the geometry of subduction zone along the Sagami trough, is better to explain the seismic records observed in Japan.