Many active faults trending N-S along basin-mountain boundaries are recognized in Northeast Japan, but only a few of them have experienced surface faulting in historical time; most of them seem to have been quiescent in the past several hundred years or more. Thus earthquakes are anticipated to occur from these active faults in the near future. To detect the recurrence intervals of faulting, which can be obtained by the excavation study, is indispensable for the long term prediction of earthquakes. We excavated a trench at Kitasakai, Sakata City, across the Kannonji fault, one of the eastern boundary faults of the Shonai plain, Northeast Japan, in order to reveal its late Holocene activity including a possible faulting event associated with the Shonai earthquake (M=7.0) of 1894 A. D., which caused severe damage along this fault. Our excavation has revealed that (1) the last surface faulting event on the Kannonji fault occurred in a period from 2, 500 years B. P. to 1894 A. D., and that (2) no surface faulting occurred (at least at the trenching site) in association with the Shonai earthquake of 1894. Careful examination of historical records, however, strongly suggests that the earthquake of 1894 was also generated from this fault; it is likely that thick, unconsolidated sediments prevented the rupture from propagating up-dip to the surface. These results indicate that the interval between the last two earthquakes originating from the Kannonji fault is less than 2, 500 years. It could be 1, 000 years, because the event revealed by excavation is possibly correlated to the historically-documented earthquake of 850 A. D..
A seismic refraction experment was carried out around the Bandai volcanic area, the southern part of the Tohoku District, by using a large capacity (9 liter air chamber) marine airgun. Shallow crustal structure (down to about a 4km depth) obtained along the 30km-length profile shows the swelling of the second layer with P-wave velocity more than about 4.0km/s just beneath Bandai volcano in parallel to the surface topography. The swelling of the second layer by about 1km is in good agreement with that of the basement estimated from the Bouguer anomaly. Seismic signals from the surrounding telemetered stations of Tohoku University at epicentral distances from 10km to 230km, are continuously recorded during the airgun experiments. In the stacked records we can detect the clear first P arrivals from the airgun shots at the stations with distances ranging up to 150km, which shows the usefullness of the marine airgun for studying the crustal structure on land. Anomalously late P arrivals or unclear P arrivals are observed for the ray paths which cross active volcanoes. This result and the shallow crustal structure obtained along the refraction profile suggest the existence of the anomalous zone beneath Bandai volcano at depths deeper than about 4km.
Compressional and shear wave velocities, Vp and Vs, in three ultra basic rocks with hydrous minerals were measured simultaneously to 900°C under 1GPa at closed system. Both Vp and Vs in all three rocks decreased rapidly with the dehydration reactions of hydrous minerals. The relation between the velocity drop of Vp, Vdrop and the amount of extricated water, W (kg/m3) at 900°C after the dehydration reactions under 1GPa is represented in the following equation: Vdrop=0.008W(km/s) W depends on the amount of hydrous minerals in the rocks, and therefore, Vdrop can be estimated from the amount of hydrous minerals in the rocks. The low velocity layer intervenes between the descending lithosphere and the overlying asthenosphere beneath Japanese Island Arcs, and that the ratio of Vp drop in the low velocity layer to the velocity in the lower layer of the descending lithosphere is approximately 5%. 5% of Vdrop could be explained by 10-20% in volumetric ratio of antigoritic serpentinite in the descending slab if it is assumed that the hydrous minerals are antigorites.
Based on tide-gauge records, the characteristics of tsunami associated with aftershock and swarm near Japan during 1896 to 1983 are investigated, comparing with tsunami of the main shock. The obtained results are as follows: 1) The main shocks having magnitude more than M 6.8 have been accompanied with the second tsunami. Seventy percent of the main shocks with M7.8 to 8.1 have the maximum aftershocks accompanied with tsunamis. These tsunami magnitudes on the Imamura-Iida scale, m, are 1 to 2 smaller than those of the main shock. 2) Tsunami source of the aftershock is situated near the edge of long axis or neighborhood of the main shock area. The area of the tsunami source is 1/5-1/10 of that of the main tsunami source. 3) Most of the tsunami magnitudes for the swarms are in the range of m -1-0. A part of the source area overlaps each other. Tsunamis which generated by the aftershock of large earthquake hit locally. As we need tsunami warning for the main shock, so we need precaution against tsunamis associate with aftershocks.
We study the preseismic, coseismic, and postseismic vertical surface displacements in the Shikoku, Kinki, and Chugoku districts, which are associated with the 1946 Nankaido earthquake (M=8.2), one of the largest interplate earthquakes which occurred in southwest Japan. These displacements are precisely estimated from the first-order leveling data (1890-1980) by using an epoch reduction method. The preseismic distribution of vertical surface displacement is characterized by the seaward tilts in the Kii peninsula and the eastern part of Shikoku, which can be interpreted in terms of a steady state subduction of the Philippine Sea plate. The coseismic distribution is the same as that caused by the low-angle thrust faulting near the plate boundary between the Philippine Sea and Asian plates. The most notable feature of the postseismic distribution is the uplift localized in the coseismically subsided region. Such an uplifting tendency lasts at least 30 years after the earthquake, and then would turn gradually into the preseismic one as described above.
The southwestern region of Hyogo Prefecture, Japan is one of the quiescent regions of seismicity in Japan. The earthquake with the maximum intensity IV and magnitude M of 5.6 occurred in the region on May 30, 1984, breaking the silence for twenty years. Under the seismic circumstances as mentioned above, it will go far toward the planning for prevention of seismic disasters to investigate behavior of the regional inhabitants for earthquakes. Especially, actions of the persons situated as a leader when disasters occurred, will affect magnitude of disaster and stationing themselves promptly for rescue position. The present paper is the report on the information obtained by the questionnaire method about the their behavior at the earthquake. The result of the questionnaire are as follows: (i) The staff of city hall, town offices and the like must have enough knowledge to care first the safety of the community and the inhabitants. But, they who answered the questionnaire have very insufficient awareness for disaster prevention and rescue. (ii) Staff of firehouse, school and company have relatively high awareness for disaster prevention and rescue. (iii) There are a small number of questionnaire recovered from hospital, banking organ, supermarket and hotel. Accordingly, we cannot help withholding the conclusions for them.
A computer-based system for automatic seismo-geochemical observations has been developed. At Hoshina hot spring, Nagano city, as a monitoring station, a personal computer regulates observation instruments such as gas chromatograph and water flow-meter, and it records the data on its floppy disks. In this study, the data transmission with a simplified telemetry system was improved. Through a public communication line and MNP modems, the data are sent to a personal computer in our laboratory. The modem enables us to transfer the data reliably without troubles caused by noise and phase delay, because it contains the function for correcting the error during correspondence. The simple BASIC program can be easily revised for expansion of measurement instruments. In view of cost, size and simplicity, this processing system is applicable for continuous on-site measurements for seismo-geochemistry.
Izu-Oshima volcano erupted on Nov. 15, 1986, 12 years since the last eruption in 1974. It is an important subject to determine the volcanic structure for a future eruption prediction. The shallow seismic profiles in the western and the central part of the island have been studied, but the profile in the eastern part of the island has not yet been determined. In March 1987, a seismic experiment was carried out to determine the structure of the island and its periphery. In this paper, we derive and discuss the shallow seismic profile along the east coast of Izu-Oshima island with the refraction data generated by a large volume air-gun in the sea around the island. The air-gun was blasted along a line circling round the island. We presents the results obtained by analyzing the two series of blasting carried out off Okata-harbor, the north of the island, and off Habu-harbor, the south of it. The results derived from other blasts will be shown in following papers. The derived seismic profile shows that the thickness of a surface layer in the northern area is thinner than that in the southern area and a slightly concave-like structure is locating in the middle of the profile.
We present a brief review on recent studies of upper mantle structure with surface wave inversions, and waveform and travel time inversions of body waves, focussing on the regional variation of low velocity zone (LVZ) structure. Distinct features summarized are 1) increase of plate thickness with age, 2) increase of S wave velocity in LVZ with age, 3) less distinct LVZ in the ocean older than 130 Ma, and 4) no LVZ in the continental upper mantle. These seismological observations can be interpreted in terms of the thermal/mechanical structures of the upper mantle once the factors that affect elastic and non-elastic preperties of rocks are known. Although current materials science studies in this field are very limited, they suggest the solid-state mechanisms of inelasticity in the upper mantle, wich are consistent with the seismological observations. Thus, partial melting is limited to young ocean if it does exist in the oceanic upper mantle. In these mechanisms, the elastic and non-elastic properties vary strongly with temperature and presumably with a magnitude of regional tectonic stresses due to plate motion. In this vein, age-dependent variation of LVZ structure under the same plate can be attributed to temperature decrease with age in LVZ. The difference of LVZ structure between plates with different absolute plate motion velocities may reflect the difference in tectonic differential stresses in LVZ generated by the plate motion.
The earth mostly consists of polycrystals. Plastic flow in the polycrystalline earth's materials creates lattice preferred orientation (LPO). LPO can be formed by lattice rotation in crystals, which is induced to compensate the difference in rigid body rotation between the inside and surrounding of each crystal. LPO may be modified or reinforced by dynamic recrystallization, which occurs at large strain. Little is known about the LPO development by dynamic recrystallization. Seismic anisotropy can be caused by LPO because some minerals constituting the earth are elastically anisotropic. We can infer the flow behavior in the earth from seismically observed anisotropies. Seismic anisotropy is found in most parts of the earth, notably in the upper mantle. Recently, anisotropy in the inner core has been documented, and the possible role of anisotropy on the anomalously high seismic wave velocities in subducting slabs has been suggested. Seismic anisotropy could be caused by microcracks in the crust under tectonically active regions, where hydrothermal fluid may exist along microcracks resulting in the increase of pore fluid pressure at depth, and thus the opening of microcracks. Anisotropies in the transition zone and lower mantle have not been known yet, but are very likely. To clarify the anisotropies in these regions is an important future subject for understanding the feature of mantle convection.