With an aid of geophysical and geological information, we discussed relationships between characteristic patterns of the shaded relief maps of gravity anomalies and subsurface structures in and around the Fossa Magna region, central Japan. Lineament-like distributions of gravity anomalies are one of the characteristic patterns in the shaded relief gravity anomaly maps in this area. Such lineaments of gravity anomalies are presumably caused by fault like structures which are covered by thick sediments or volcanic products. A very large (about 80km) gravity depression is also clearly appeared over the Kanto-Plain. This depression represents a basin structure in this area. The shaded relief maps are also suggesting a number of pull-apart structures of basement rocks. We also examined relationships between distribution of shallow earthquakes and lineament-like patterns of gravity anomaly. Although we could not find apparent correspondence between the linearment-like patterns and smaller earthquakes, better correspondence for larger, magnitude grater than 6.0, ones. These results presumably indicate that the shaded relief maps provides new tool for exploring hidden faults and other subsurface geologic structures.
The author proposes a formulation of the discrete wavenumber—boundary integral equation method for three dimensional problem, using Green's function for homogeneous half-space calculated by the Sommerfeld integral, based on the consideration about the orthogonality of the horizontal wave function. This orthogonality gives a relation between Green's function for each point source and the total wave field. A wavenumber component of the total wave field is composed only of the wavenumber components radiated by each point source, which correspond to the same wavenumber. For the total wave field, it can be said easily that it is sufficient to evaluate the integral over wavenumber up to that a little superior to the wavenumber corresponding to the fundamental mode of the Rayleigh wave. Then, it is found that the integral over wavenumber to obtain Green's function for each point source can be truncated at the same wavenumber. This may not be exact, mathematically, but it is reasonable in term of the wave theory. The calculation for some simple problems with axi-symmetrical surface irregularity, gives the stable and reasonable wave forms. It is expected that this strategy can be applied not only to axi-symmetrical surface irregularity but also to arbitrary shape of irregularity of surface and of interface.
Osaka Bay is surrounded by several mountains and considered as tectonically sinking block by fault movements in Quarternary period. Reflection sparker survey was applied in 1966 to study shallow geological structure in Osaka Bay. Since then, many reflection seismic investigations were conducted in Osaka Bay to study foundation problems in civil engineering and geotechnical purposes. These surveys have been limited in depth less than a few hundred meters. This papaer is the first to study deep basin structure of Osaka Bay area by airgun reflection seismic method. Because of the heavy marine traffic in the bay, the survey lines were limited as small as three lines to keep the safety on the sea. The first line is 40km in length with NE-SW direction along long axis of Osaka Bay. The second line is 28km in length with EW direction at the central part of the bay. The third line, parallel along the first line and 5km east of the first line, was 10km in length. The third line is planned to include the deepest boring (400m) point in Osaka Bay and extend to cross the second line. The length of the streamer cable was shortened to 500m and the number of the hydrophones was limited 20 groups with interval of 25m. Two airguns with volume 300inch3 were fired at every 25m along the line under 7m in depth from sea surface. The results obtained are as follows: 1) The base rock of Osaka Bay was found from 1, 200m to 3, 000m in depth, 2) The general trend of basin structure is monotonous dipping to NW direction, 3) Many clear reflection events from surface to base rock in the recorded section suggest the alternation of sand and clay formations, which are considered mainly young sediments based upon the estimated velocities from 2, 000 to 2, 500m/s, 4) An active fault is found in the west end of the EW line, the west block of base rock thrust over the east block with dip movement of 1, 000m
Although the Median Tectonic Line (MTL) fault system in Shikoku, southwest Japan, is one of the most active faults during the late Quaternary in Japan, the present-day seismicity along the MTL is not so remarkable. However microseismic activities have recently been reported in some regions along the MTL in Shikoku. It has been pointed out that these activities are those at segment boundaries of the MTL fault system. We carried out a temporal microseismic observation in one of these regions, the region around borders between Ehime, Kagawa and Tokushima prefectures, in order to determine the earthquake hypocenters precisely so that we can discuss the hypocenter distribution in context of the tectonics of the MTL. The seismometer network consisted of six observation points and had its areal extent of about 10km by 10km. Surface trace of the Ikeda fault goes through the network with its strike of N75°E. About 10km to the west of the network, there is a segment boundary of the MTL fault system which divides the Ikeda fault and the Ishiduchi fault. Hypocenters of 82 earthquakes are located during November 1990-August 1991. The seismic activity is high beneath the network, and no hypocenter is located near the segment boundary of the Ikeda and Ishiduchi faults. Most events located in the southern side of the MTL are in a depth range of 5-8km, and the distribution is almost flat or slightly inclines toward south from the MTL. The distribution pattern of the hypocenters in the northern side of the MTL is widely different from that in the southern side. The hypocenter depth increases with being away northward from the surface trace of the MTL. Although the number of events located is too few to conclude that the distribution is planar, the dip angle of about 50° is measured from a cross-sectional view on a vertical plane perpendicular to the strike of the MTL. The gap in the hypocenter distribution just beneath the MTL dipping almost vertically to the north, which has been reported by KIMURA and OKANO (1992) as an evidence that the MTL is near vertical, is not found in the present study.
Active faults and folds were mapped on the Japan Sea shelf off Niigata in the vicinity of the main shock and aftershock area of the 1964 Niigata earthquake based on air gun, water gun and 3.5kHz seismic profiles. Three types of active faults are defined on the basis of the age of youngest deformed sedimentary unit. Type A faults cut the Holcene sediments and are considered to be active. Type B faults offset the erosional or sedimentary surface of the last glacial age, but Holocene movements is not clear. Type C faults cut Neogene and early Quaternary sediments, but their late Quaternary activity is not clear. Two uplift zones composed of active faults and anticlines were recognized and named the Awashima Uplift Zone and Niigata-oki Uplift Zone. The Awashima Uplift Zone extends for about 40km NE and 40km SSW of the Awashima Island. The location of the uplift generally coincides with the zone of aftershocks and the area uplifted during the 1964 Niigata earthquake, indicating that crustal deformation accompanying repeated Niigata type earthquakes formed this uplift. Air gun seismic profiles show that the uplift is underlain by more than 1500m of Neogene and Quaternary sediments and consists mainly of two major east-vergent anticlines which are cut by west-side-up reverse faults on their eastern limbs. By analogy, the seismic fault producing the 1964 earthquake is presumed to have been a high-angle, west-dipping reverse fault. Several active faults can be sporadically traced for only 5 to 10km, whereas anticlines are more continuous. This strongly suggests that considerable amount of slip at the basement is dissipated by the formation of the anticlines in the thick Neogene and Quaternary sediments, and only part of the slip reaches on the sea-bottom as active faults. The slip rate of about 0.7m/ky estimated at one of the most active faults at the sea floor is a minimum figure of the slip rate of the seismic fault in the basement. The Niigata-oki Uplift Zone extends for 60km along the shelf edge. The last glacial erosional surface is exposed throughout this uplift. Active faults identified by offset of the erosional surface are generally continuous for over 20km, mainly because this uplift zone is underlain by thinner Neogene and Quaternary sediment cover. The dimensions of this uplift zone is comparable to that of the Awashima Uplift Zone, suggesting the possibility that movement of faults generating this uplift could produce another earthquake comparable in size to the Niigata earthquake.
The middle section of the Itoigawa-Shizuoka Tectonic Line (Middle ISTL) is an active fault system that extends NW-SE for 50km from Matsumoto to Kobuchizawa, in central Japan. The Middle ISTL is characterized by high average slip-rate reaching 8 to 10mm/yr during the Late Pleistocene and Holocene. This is one of the highest slip-rate reported from active faults on land in Japan. Empirical relation between slip-rate and recurrence time indicates that the Middle ISTL may rupture more than once a thousand year. The previously known recurrence time estimates of 3500 to 5000 years were significantly longer than the expected recurrence time. The last faulting event on the Middle ISTL most likely occurred in 841 A. D. according to historic record and paleoseismological works. Since the elapsed time is about 1152 years, the estimation of recurrence time is critical to evaluate the potential of next earthquake. In order to know the history of recent faulting events, we excavated the Gofukuji fault, the northernmost segment of the Middle ISTL at Namiyanagi, south of Matsumoto. Investigation of 6 trenches, 3 test pits and topography around the trenches brought following results. The ages of three most recent faulting events are 445-1386 A. D., 150-334 A. D., and 839-189 B. C. The recurrence time is between 111 and 1236 years. Assuming the correlation of the last event with the 841 A. D. event, the recurrence time is estimated to be 338 to 1172 years. Average recurrence interval in this case ranges 515 to 840 years. The elapsed time of 607 to 1152 years is probably longer than the recurrence time. The average slip rate of the fault since c. a. 6000 B. C. is 9.4±4.5mm/yr left-lateral. The left-lateral coseismic slip during the last event is estimated as 7.5±1.5m.
Aftershock activities for two days after the 1923 Kanto earthquake is reinvestigated from the data of S-P time, maximum amplitude, and its period, which are newly measured on the records at the Gifu observatory. 34 aftershocks with magnitude M from 5.0 to 7.5 are identified on the records. According to locations of epicenters of aftershocks inferred from the S-P times, most of large events concentrated in the vicinities of western or eastern ends of the rupture area of the main shock. The first aftershock activity started in the western region immediately after the main shock at about 12:00 on the 1st of September and included two M=7 class events, whose magnitudes are newly determined from maximum amplitudes in vertical component of these records. At about 12:40, aftershocks began to occur in the eastern region. The next activity began at 13:00 in the western region again. From 17:00 on the 1st to 11:00 on the 2nd of September, the activity was quiet. The largest aftershock of M=7.5 and its related events occurred in the eastern region after this quietness. Then, the activity returned to the western region again at about 21:00. It is found that the number of M≥6 aftershocks is 16 for two days, which is meaningfully larger than that for the other M=8 class events in and around Japan. The natural period of the vertical component of the seismograph was about 1s, while those of the horizontal components were about 5s. Therefore, the ratio γ of maximum amplitude in vertical component to those in horizontal components is used as a parameter which show a frequency characteristic of seismic waves from each aftershock. All the aftershocks in the eastern region have low-frequency characteristics, which show smaller γ values, comparing with those in the western region. We also found some aftershocks of which wave forms are similar to those of recent events occurring in the same region. These results indicate that the aftershock activities included the same type events as we can find in the recent seismicity and suggest that the locations of epicenters of the aftershocks inferred in the present study are proper.
Tectonics of the Taiwan region is reviewed. Relative plate motions so far obtained are described and rated. The most reliable solution gives 7.4cm/yr Philippine Sea-Eurasian motion in the N50°W direction in central Taiwan. Plate boundary geometries and modes of plate consumption in the region are discussed on the basis of geophysical, geological and geomorphological data. The major thrust zones at the Western Foothills and the Longitudinal Valley constitute mechanical plate boundaries, but a considerable fraction of the relative plate motion is also consumed by the strike-slip faulting off the southeast coast of Taiwan. The cause for the occurrence of the strike-slip faulting off the southeast coast of Taiwan is discussed based on the various models so far proposed. The bending of the Philippine Sea plate due to the subduction at the Ryukyu Trench and the collision in the Longitudinal Valley are similar to the tectonics of the Izu Peninsula region in Japan, and are the most likely cause for the strike-slip faulting. The reconstruction of the motion and plate boundaries in the vicinity of Taiwan for the past 4-6m. y. is reviewed. The Philippine Sea-Eurasian motion for the past 4 Ma is similar to the present one. The motion prior to 4 Ma may be more northward than present due to the absence of subduction at the Philippine Trench. The Luzon arc started to collide with the continental margin of S. E. China around 4 Ma.
The Research Group for Explosion Seismology of Japan, founded in 1950, have continuously conducted many explosion seismic observations in various regions of the Japanese Islands. Active investigations of the Group have provided important data about the crustal structure beneath the Japanese Islands which is quite useful as the most basic information for various researches of geosciences. After ages of the cradle in 1950's, the Group experienced large scale investigations under the International Upper Mantle Project and the International Geodynamics Project in 1960's and 1970's, and general features of crustal structure beneath the Japanese Islands became clear through the investigations in this period. Since 1979, the Group has conducted the investigations under the Japanese Earthquake Prediction Project in order to accumulate basic data for earthquake prediction researches. The series of these investigations have been conducted through highly dense observations, and the obtained data have revealed quite complex structure of the crust beneath the Japanese Islands. Application of some of data processing techniques in reflection survey to those data also revealed clear images of the subducted Philippine Sea plate beneath the Japanese Islands.
This is a review of recent development of estimating strong ground motion in broad band for input motion for efficient earthquake-resistant design and response-controlled design of structures to reduce earthquake damage. We focus quantitative studies on earthquake sources, discussing the following topics: (1) deterministic source models, considering kinematics and dynamics, (2) stochastic source models, describing the spectral properties of ground motion generated from source process, (3) hybrid source model, combining deterministic approaches with stochastic approaches of source process, (4) empirical Green's function techniques relating the deterministic source models to the scalings of source parameters and source spectra, and (5) some problems for source modeling, self-similarity of source process, limitation of ω2 model, heterogeneity of fault motion and combining stochastic simulation with the empirical Green's function. The empirical Green's function method has the advantage of not only containing propagation-path effects and local site effects but also incorporating the source effects due to growing of small cracks to larger ones. Conclusively, the empirical Green's function method is one of the most effective techniques in simulating strong ground motion for the purposes of engineering designs, although further discussion is required about the limitatins and extensions of empirical Green's function.
The recent philosophies and methods in seismic design of buildings are discussed. In these two or three decades, several kinds of new type buildings have been constructed in Japan, that is, high rise buildings, nuclear power plant structures, base isolated buildings, structures on soft soil ground in so called “Water front” areas, and so on. These social demands of constructions have needed new philosophy and method for seismic design. On the other hand, the recent earthquake events occurred in the world, for instance, the 1968 Tokachi-oki earthquake, the 1979 Imperial Valley earthquake, the 1985 Michoacan earthquake and others, showed us important information. The actual situations of building damage and the observed data by strong motion seismographs have showed clearly that the characteristics of earthquake ground motion are greatly effected due to earthquake source processes, seismic wave propagation from source to site, deep subsurface structure in the concerned district and local site conditions. The lessons learned from these recent earthquakes have promoted the developments in the methods of evaluation of intensity of earthquake ground motion to apply the seismic design of buildings. The many kinds of attenuation formulae have been proposed on the basis of the data set of many strong motion accelerograms in the recent destructive earthquakes, and the methods of ground motion simulations have been studied on the theories of physical state on seismic fault, rupture process of seismic source, and wave propagation. Then, the current trends and ideas of evaluation of seismic load, design spectrum or ground motion time histories for response analyses, and new representation of intensity of earthquake ground motion to buildings by “energy spectrum” are discussed in this paper.