A gravity inversion method is developed to estimate three-dimensional subsurface structures by assuming a flat layered structure with lateral density variation. The method is applied to observed data in the western part of Central Japan, the Hokuriku district. Results show that this method gives well consistent density structures with geological and explosion seismological data in this area.
Average horizontal crustal strain rates during interseismic period in the Japanese Islands, except the Hokkaido district, are deduced from precise geodetic survey, which has been conducted since the Meiji era. First, distance change rates between the first order triangulation stations are calculated using the weighted least squares method. In regions which suffered from disastrous earthquakes or volcanic eruptions, data before the events are neglected in the above calculation. Second, strain rates in arbitrary triangular regions are estimated. The estimated strain rates ate less than 6×10-7/yr, which is greater than those derived from seismic moment release rate or displacements of active faults. The Japanese Islands may be divided into several provinces on the basis of disstribution of strain rates. In the Tohoku district, a N-S to NE-SW extension is prevailing, and there is a zone of small strain rates which runs parallel to the Oga-Ojika tectonic line proposed by MOGI (1985). This zone may be a boundary between crustal blocks. In the northern Kanto district, strain rates are very small, while a NW-SE compression is predominant in the southern Kanto district. These facts indicate that compressional forces due to the subduction of the Philippine Sea plate may not be transmitted to the northern Kanto district. There is a zone of large compression which runs from Niigata to Gifu through Matsushiro. These high strain rates show a good correlation with seismic activity in the crust. Although compressional strain rates are prevailing in both the Chubu and the Kinki-Chugoku districts, principal axes abruptly change their directions from NW-SE to E-W at around lake Biwa. The westernmost Chugoku and the northern Kyushu districts are characterized by small extension, which suggests that compressional stress predominant in the Chubu and the Kinki districts may fade out. In the Shikoku district, NW-SE compressional strain rates are prevailing. However, temporal variations in baseline lengths show that the elastic rebound associated with the 1946 Nankaido earthquake did not occur in the western part of shikoku. It is considered that the coupling between the subducting Philippone Sea and the overriding Eurasia plates may vary significantly from east to west. Large N-S trending extensions appear in the central Kyushu district, which is consistent with the spreading of the Beppu-Shimabara graben.
Seismic quiescence which has recently appeared in the west coast region of Suruga Bay is investigated. The quiescence is clearly recognized for earthquakes with M≥2.5 since the beginning of 1988. Southern boundary of the quiescence region corresponds to the Irozaki-Shizuoka tectonic zone and its extension to the north-west. However, the whole focal region of the expected Tokai earthquake and its surrounding areas show quiescence since the middle of 1988, when earthquakes with M≥3 are considered. The seismic quiescence seems to have started first in the northern part and extended to the south. The feature that the Irozaki-Shizuoka tectonic zone forms boundary of the quiescence region is very meaningful when it is combined with the fact that a gap exists in the hypocentral depth of earthquakes at the tectonic zone. It suggests that the Philippine Sea slab is split at the Irozaki-Shizuoka tectonic zone and the stress states in the two segments of both sides are more or less independent. It is noted that seismic activity in the regions where the quiescence appeared had been high in the long period of recent several decades.
Three problems in the field of engineering seismology; spatial variation of seismic motions, two-dimensional modelling in seismic simulation, and local generation of surface waves, are investigated through spectral ratios, correlation coefficients, propagation directions, and phase velocities which are estimated from array data of four moderate-sized earthquakes. As for the first problem, spectral ratios manifest that spatial variation at frequencies above 2 Hz is primarily influenced by the inhomogenity near ground surface. Correlation coefficients show they decrease with distance and frequency and, in particular, seismic motions separated over 70 m are uncorrelated at frequencies above 6 Hz and those over 150 m are uncorrelated at 3 Hz. As for the second problem, large variation of propagation direction means that seismic simulation based on two-dimensonal modelling does not reproduce recorded seismograms satisfactorily in amplitude and duration. Finally, as for the third problem, phase velocities show that just one of four events contains locally generated surface waves in large quantities, which suggests local generation of surface waves occurs on some limited conditions.
Group velocities of PL waves recorded at Osakayama and Amagase Observatories of Kyoto University for earthquakes occurring in the east and northeast regions of Japan show a considerable regional difference with respect to propagation paths. The group velocities for propagation paths through the northern region (Tohoku and northern Chubu Districts) are much lower than those for propagation paths through the southern region of Honshu Island (Kanto and Southern Chubu Districts). Comparison between the observed and theoretical group velocities shows that the low group velocities for the propagation paths through the northern region should be attributed to a significant decrease in the velocity in the lower-crust. The theoretical group velocities were calculated by using OLIVER and MAJOR'S (1960) method, since our examination showed that this method was more appropriate than that of SU and DORMAN (1965). It is shown that a surface layer with low velocity, such as a sedimentary layer, could create a condition favorable for generation of large-amplitude PL waves. It is also shown that, at least in the range of period between 15 and 30 sec., group velocities of fundamental PL waves are more sensitive to shear velocity than to compressional velocity. There remains a problem, however, that it is difficult to explain dispersions both of Rayleigh waves and PL waves by one and the same structural model.
Acoustic emissions (AE), which occur after turning out a gas stove, were observed. The purpose is to consider whether we can derive, from such an experiment as the present one, any information on a possible model for the occurrence of aftershocks of earthquakes. The results obatined are as follows. (1) The frequency of occurrence of AE keeps nearly constant for about 100 seconds after turning out the gas stove. (2) It however turns to decay according to power law after large AE's occur most frequently. (3) It decreases exponentially with increase of magnitude of AE. (4) The phenomena (1)-(3) are well reproduced in repeated experiments. Since characteristics of the activity of AE, summarized above, bear resemblance to that of aftershocks of earthquakes, it is anticipated that the temporal change of stress or strain at the focal region of AE is possibly similar to that of aftershocks of earthquake. Therefore, further investigation of temperature, stress and strain at focal region of AE will reveal some intrinsic mechanism that account for the aftershock phenomena.
Recent K-Ar age data of fault gouge from some active faults in central Japan clarified that shattering and gouge formation induced by the fault movement occurred in the time of Late Cretaceous-Early Paleogene, synchronous with the ages of granite intrusion around the faults. Close spatial association of the faults and granitic intrusion suggests some causal relation between them. Optical observations of granitic rocks around the Atotsugawa and Atera faults demonstrate that grain boundary migration and brittle-ductile deformation are commonly found in the granitic rocks. Based upon the close association of granitic rocks with the faults, a tectonic model of granite intrusion is proposed. In this model, granite intrusion was syntectonically induced at gaps formed by block rotations due to strike-slip faulting. We suggest that fault activities provide a favorable tectonic setting for granite intrusion in central Japan.
Regional variation of Rayleigh wave group velocities was investigated in the Philippine Sea area. The Group velocities were determined in a period range from 30 to 80 seconds by applying the Moving Window Analysis to long-period vertical-component seismograms recorded at six WWSSN stations, which are located around the Philippine Sea. We divided the Philippine Sea area into seven regions, based on tectonic feature, age, and topography of the ocean floor. The group velocity in each region was determined as a function of period by the least-squares method. The region covering the Amami Plateau, the Daito Ridge, and the Oki-Daito Ridge, where the ocean floor is the oldest in the studied area, shows low group velocities at short periods. This is interpreted as that this region has thick oceanic crust of about 20 km. The West Philippine Basin region has the highest group velocity, and thickness of the lithosphere is estimated to be about 60 km with normal oceanic crust. The group velocity in the Shikoku-Parece-Vela Basin region is somewhat lower than that in the West Philippine Basin region, and thickness of the lithosphere is about 40 km. The difference in thickness may be due to difference in plate's ages. The lowest group velocity is found in the Mariana Trough region and the region covering the Okinawa Trough, Ryukyu Arc, and South-West Japan. This may be attributable to existence of the extremely lower velocity layer in the upper mantle.
Since the beginning of the anomalous crustal vertical movement in the Izu peninsula, Honshu, Japan, so many repeated precise levellings have been carried out by the Geographical Survey Institute. A special technique is required to adjust the results of these levellings, because they had been carried out for different epochs along each levelling route and also they had been affected by so rapid vertical crustal movement. We apply least square adjustment with velocity model in which corrections to the assumed height and linear rates of vertical deformation at any bench marks are assumed to be unknowns. We apply free net adjustment to this case. We can select several special bench marks in which height or velocity is given from any suitable results of tidal analysis then usual or free adjustment is applied according to the type of network adjustment. The results show that the peninsula was inclined to the south-west as a whole during 1973-1986. Not only the north-east part of peninsula was upheaved as many investigators had pointed but also there occurred remarkable subsidence in the south-west of the peninsula. The detected rate of inclination is (2-8)×10-6 rad/a and several times higher than the one during past several ten years. Thus inclination was proved to be accelerated. The detected deformation is concentrated in the area where Nakamura (1980, 1984) pointed out the bending of the Philippine Sea Plate. The peninsula shows bending upward in the shallower crust along almost ENE-WSW direction before subduction. The detected crustal deformation covering the whole area of the peninsula is the direct evidence of the plate. The bending is asymmetric as detected deformation shows. It is remarkable in the eastern part of the plate and not so in the west. When the plate bending is accelerated, extension becomes remarkable at the earths surface and contraction becomes remarkable at the deeper part of the subcrustal layer. This will be a cause of various kind of the contemporary crustal activities such as earthquakes in or near the Izu peninsula or volcanic eruption of the Oshima in 1974 and during 1986-1987.
An automatically measuring system with GM-survey meter and micro-computer was developed for γ-ray observation. The system shows sufficient stability during two years observation of γ-rays in the tunnel for crustal movement observation. Based on comparison of radon measurement using flow-type ionization chamber and γ-ray spectroscopy using Nal (T1) scintillation counter, it was especially clarified that the system was available for measurement of radon concentration in tunnel air, since radon concentration is 300-40, 000 times in tunnel air higher than in outdoor air. In addition, it has also become clear that a simultaneous measurement using several instruments is desirable to make a precise survey.
The three-dimensional Q structure down to 150 km beneath the Hidaka Mountains, Hokkaido, Japan was determined using an inverse method. Apparent Q values were determined along ray paths from 456 earthquakes to the eight stations operated by Research Center for Earthquake Prediction of Hokkaido University. The hypocenters of these earthquakes were located beneath the Hidaka Mountains, with magnitude of 3.4-4.0. This region was divided into 600 blocks and then absolute Q values of the individual blocks were determined from these apparent Q values by using the inverse technique with least-squares method and constraints. In order to determine the Q values, the pulse broadening method (PB method) was used. We did not use spectral methods which need a time window with, at least, a few seconds from P arrivals, because seismograms observed at the eight stations have many phases of unknown origins which appear just after P arrivals. On the other hand, the PB method requires only one pulse of the first P arrivals; the Q values obtained by the PB method are hardly affected by scattering and reflections. The Q values determined by the PB method, however, involve considerable errors because of the narrow time window. We introduced some constraints on the Q values in the inversion process. The result shows that an extremely low-Q (Q=50) region is located beneath the area from the western part to the central part of the Hidaka Mountains and it extends to the depth of 100 km, and that relatively high-Q material exists beneath the summit of the Hidaka Mountains. Comparisons of the obtained three-dimensional Q structure with a three-dimensional Vp structure and with a seismic activity map, show the low-Q zones correspond to low-Vp and high seismicity zones. It is concluded that the low-Q zone is not caused by high temperature but composed of highly fractured rocks because no volcanic activity is found and rather low heat flow was observed in this region.
Erosion is a key to understanding mountain building processes at both orogenic and postorogenic stages. The height of a mountain range, if no uplift, decreases exponentially with time by erosion with an erosion time constant τ which is influenced significantly by climate and erosivity of rocks. τ is also a function of the wave length of topography; shorter wave-length topography decays more rapidly. Observations of sediment yields have indicated that the erosion time constants for large mountain ranges such as the Appalachians and the Alps are of the order of 2.5-6, 5 Myr. However, there exist very old (> 200 Ma) mountain ranges, such as the Appalachians and the Urals, which still preserve prominent topographic relief and have evidence for dramatically-large unroofing up to ten kilometers or more. This indicates that the process of erosion-isostatic uplift plays an essential role in the geomorphic development of large-scale mountain ranges at a postorogenic stage. On the other hand, it is expected that a smaller-scale mountain range supported by lithospheric strength will loose its topographic prominence very rapidly (in several Myr or less) after the orogenesis terminates. Also at an orogenic stage, erosion plays an important role in the development of large-scale mountain ranges. Observed data of erosion and uplift rates were examined for selected mountain ranges formed by continent-continent or continent-island arc collision; several points of interest have come to light: (1) The Southern Alps of New Zealand is an active orogene, characterized by extremely high rates of uplift (>10 mm/yr) and erosion. The rate of loading by crustal shortening is equal to the rate of unloading by erosion, indicating that crustal shortening is accommodated by uplift, not by crustal thickening, and all the uplifted mass is eroded simultaneously. (2) The Central Range of Taiwan is another active orogene, and has very high rates of uplift and erosion (>5 mm/yr). Both rates are approximately in balance, and therefore the Central Range is nearly in an equilibrium state just like the Southern Alps. However, the foreland basin west of the Central Range is still subsiding, suggesting that a small fraction of crustal shortening is accommodated by crustal thickening. (3) The high rates of erosion in the Central Range and the Southern Alps, which are due primarily to the high erosivity of rocks (slate, phyllite, and schist), contrast strongly with the low rates of erosion in the Appalachians and the European Alps which had a deep crustal root at the end of orogenesis. It is suggested that, if the Southern Alps and the Central Range had a long erosion-time constant, the crustal shortening would have been accommodated mainly by crustal thickening until they would have attained a deep crustal root and higher elevation.
The southern Kanto district, central Japan, is characterized with its quite complex tectonic environment. Both of the Pacific and the Philippine Sea plates are subducting beneath this area and interfering each other. Adjoining this area, there exists the Izu Peninsula to the west as a collision zone between the Pacific and the Eurasian plates, while there exists trench-trench-trench triple junction of the Pacific, Philippine Sea and the Eurasian plates to the east. Several models of the plate configuration in this area have been proposed based onthe hypocentral distribution of microearthquakes, the focal mechanism solutions, the three-dimensional velocity structure, the fault model of major earthquakes, the geomorphological and geological evidences, and so on. Comparing these models, considerable varieties are seen in the iso-depth contours of the Philippine Sea plate, whereas those of the Pacific plate are more or less similar. These varieties came from the differences in the data source, the strike of the cross sections which were mainly used for delineation of the contour lines, and referenced data on which each researcher put different stresses. Based on these tectonic models, some explanations are given to the paired earthquake phenomenon and synchronized seismic activation among the adjacent regions. Also, several candidates for forthcoming earthquakes of M7 class are proposed as a consequence of these tectonic backgrounds.