We have simulated long-period ground motions in three 3D subsurface structural models of southwestern part of the Kanto plain for an M4.1 earthquake in the west of the Kanagawa prefecture, Japan with a finite-difference modeling to compare their performances in strong motion prediction. The three 3D models are constructed from the seismic refraction data (KOKETSU, 1995) and mainly from the geological data (SUZUKI, 1999), and the compiled results of microtremor array measurements (YAMANAKA et al., 1999). The major differences of these models are configurations of the sedimentary layers and the basement topographies, especially, the depth to the top the basement of the model from the microtremor array measurements is shallower than the other models.
First, we estimated source parameters by a fitting of synthetic waveforms with observed records at a rock site. The synthetic motions for the models were significantly different at sites in the basin area. The model from the microtremor array measurements was better in reconstructing later phases than the other two models. Because the model from the microtremor array measurements has the low-velocity sedimentary layers around Yokohama, and contains an up-lifted structure of the basement around the Miura peninsula that would be responsible to generation of the later phases. It was suggested that misfit of synthetic motions with observed one are useful information for constructing of the 3D-earth model for the simulation of ground motion.

抄録全体を表示

Excavated areas such as the ore veins, tunnels and shafts of the Old Ikuno Mine, an abandoned mine in western Japan, can be regarded as comprising a huge reservoir of groundwater extending over several square kilometers to a depth of about 1km. In 1988, tidal fluctuations with an amplitude on the order of 1mm were observed in the water level of the Chuo shaft in this mine. The vertical length of the Chuo shaft is 370m, about two-third of which is flooded. Observations were curtailed until 1997, when this well-tide was again observed. The persistence of this phenomenon over a nine-year period implies that the well-tide in the Chuo shaft is an intrinsic feature of this huge underground reservoir. To understand the well-tide in the Chuo shaft, we compared the tidal fluctuations of its water level with crustal movement measured in the same area. We found that (1) the tidal response of the water level to crustal strain is frequency dependent, and (2) the tidal fluctuation exhibits a phase advance. These are quantitatively explained by a finite-cavity model with a drained condition. The estimated volume of the flooded cavity is on the order of 10⁶m³, which is consistent with the excavated volume of the Old Ikuno Mine.

抄録全体を表示

The Japan Sea is a back-arc basin in the northwestern Pacific and is now one of the most well studied marginal seas in the world in terms of its seismic crustal structure. The Japan Sea seems to possess areas of various stages of back-arc opening process. Understanding of each stage is critical in reconstructing the back-arc opening process. One of the key areas from which we lack information on seismic structure is the geologically interpreted failed rifts. In September 1997, an airgun-ocean bottom seismometer (OBS) survey was carried out in the Kita-Yamato Trough, which splits the Yamato Rise into two parts and geologically shows a failed rift structure. Seven digital recording OBSs were deployed every 20km and a 20-liter airgun was used for controlled source. We determined a two-dimensional velocity structure model by forward 2-D ray tracing method. The crustal thickness is about 12km including 1.5km thick sediments. A thin (ca. 0.8km thick) layer with a P-wave velocity of about 4.2km/s underlies the sedimentary layers. The lower part of the upper crust has a velocity of 6km/s with a thickness of about 2km. The lower crust has a velocity of 6.6-6.9km/s and is about 7.8km thick. There is no seismic evidence for existence of velocity greater than 7.1km/s in the lowermost crust. The Moho was detected by reflected waves only so that the Pn velocity remains unconstrained. The crust of the Kita-Yamato Trough is suggested to be not related to large and rapid volcanic activities, because the crust has no high velocity layer in the lower crust indicative of such activities. Since the Trough is adjacent to the Yamato Rise, which is believed to have a continental crust, the Kita-Yamato Trough should have been formed by rifting/stretching of a continental crust. The crustal structure of the Trough is roughly similar to those found in the southern Yamato Basin and the northern Tsushima Basin, which are stretched continental crusts. The resemblance of the crustal structures may indicate that the origins and/or stages of back-arc rifting are similar in the three areas.

抄録全体を表示

Two major historical earthquakes of the 1703 Genroku Kanto Earthquake (M 8.2) and the 1923 Taisho Kanto Earthquake (M 7.9) occurred along the Sagami Trough, where the Philippine Sea Plate subducts beneath the North American Plate. As a result of the coseismic crustai movements associated with these earthquakes and with the repeated characteristic earthquakes of two types (the Genroku-type Earthquake and the Taisho-type Earthquake), Holocene emerged shoreline topography has developed along the southern coast of the Boso Peninsula. We investigated the emerged shoreline topography and beach sediments in the Iwai Lowland with help of a new sampling method using the long geo-slicer to clarify the timing and recurrence interval of the Taisho-type Earthquake. A series of ten narrow emerged beach ridges, which are named the BR-I to BR-X in descending order, are distributed below the highest emerged shore platform, 15.5m high above sea level in the lowland. At least eleven times of coseismic events, probably the Taisho-type Earthquake, can be recognized from the number of the emerged shoreline topography. Based on the ¹⁴C ages and sedimentary sequences related to the BR-I-IX, the Taisho-type Earthquake occurred at least three times between 6825-6719 cal yrs BP and 5304-5055 cal yrs BP. After the other three non-dated events succeeded, four events are confirmed at 3300-3100 cal yrs BP, before 2306-2118 cal yrs BP, 1292-1264 cal yrs BP and around 900 cal yrs BP. The lowest BR-X was judged to be emerged during the 1923 event by comparing between the topographic map issued in 1886 and the present one. Analyzing intermittent emergent age of beach ridges, the recurrence interval of the Taisho-type Earthquake is within 990 years, and is 380 years at shortest.

抄録全体を表示

Long-term seismicity change in and around Shizuoka City is investigated on the basis of seismic observation records at the Shizuoka Local Meteorological Observatory (SLMO). Distance of small earthquakes from SLMO is evaluated by S-P times. Since the start of observation in Jan. 1941, almost all earthquakes with M 3 or larger are estimated to have been reported. It is found that long-term seismicity on the western coast of Suruga Bay has changed substantially several times: It was active in 1940s and became low noticeably after mid 1960s. We show that the decrease of seismicity after mid 1960s can not be regarded as an artifical one, but is real. Interesting facts are that seismicity near Shizuoka city was very quiet during about one year just before the 1944 Tonankai earthquake and many induced earthquakes occurred in the area around SLMO after the event.

抄録全体を表示