Low Seismic Activity Research Articles

東北地方の地殻応力及び地殻変動の特徴 (I)

Seismic activity and crustal movement in the northeastern Japan arc seem to arise from the mutual action between the continental plate and the Pacific Ocean plate. It is, therefore, very interesting and important to investigate numerically the spacial distribution and the time variation of the crustal stress and the crustal movement caused by the subducting plate.We apply a two-dimensional finite element method to estimate the stress field and the displacement field by use of the crust and upper mantle structure precisely determined by explosion seismology, and compare the results with the observed facts. The lower crust and the upper mantle are assumed to be viscoelastic materials, considering such facts as much lower seismic activity in these layers than in the upper crust beneath the land and the existence of migrating shear strain.The numerical computation is carried out for two types of boundary condition. One is that a normal stress of 100bars is given to the Pacific Ocean side, and the other, a shear stress of 100bars is given along the boundary of two plates.It is found that the stress produced in the crust is almost E-W compressional regardless of boundary condition in good agreement with the results of earthquake mechanisms and crustal movement observations. It is striking that the stress in the upper crust becomes much larger with time than that in the lower crust and upper mantle, corresponding to such facts as the high seismicity of the former and the lower seismicity of the latter. It is clarified that for the shear stress boundary condition a remarkable subsidence and a slight uplift are characteristic at the coast of the Pacific Ocean and at the coast of the Japan Sea, respectively. It is also made clear that horizontal movements are towards west having larger displacement in the Pacific Ocean side than in the Japan Sea side. These computed results are in harmony with observations. It is revealed that the distribution of crustal stress is closely related to the changes of crustal structure, leading to the recognition that the precise determination of the crustal structure is of very importance.

北海道の微小地震活動

The seismicity in and near Hokkaido, Japan during the period from 1976 to 1980 has been studied by means of the telemeter observation system having nine high-gain seismograph stations.The highest activity of shallow microearthquakes (0 to 60km depth) is in Pacific coast from the coast of Urakawa to east off the Shimokita peninsula. In northern part of this area great earthquake with M7 or greater has not occurred for 50 years. In southern part of this area the largest aftershock (M7.5) of the 1968 Tokachi-Oki earthquake (M7.9) occurred. Seismicities in the aftershock areas of the 1968 Tokachi-Oki earthquake and the 1973 Nemuro-Oki earthquake (M7.4) are inactive, while the number of microearthquakes in the aftershock area of the 1952 Tokachi-Oki earthquake (M8.1) has an increasing tendency since 1979. Teshikaga-Akan area of eastern Hokkaido, in which nine shallow earthquakes with magnitudes 5 or 6 occurred during the period from 1938 to 1967 shows low seismic activity during the period of observation.The accurate location of microearthquakes in deep seismic zone shows two dipping zones 25-30km apart beneath the area from the southwest to the middle of Hokkaido. Beneath southern Hokkaido (the northern side of the Northeast Japan arc), the seismicity of the upper zone is more active than that of the lower zone, while beneath central Hokkaido (the western side of the Kurile arc), the seismicity of the lower zone is more active than that of the upper zone.

Ocean bottom seismograph measurements in the Central Aleutians

Bathymetrie profiles of most oceanic trench regions show a well-defined outer rise seawards of the trench with numerous normal faults between the trench axis and the crest of the outer rise. Several investigators (see ref. 1 for review) have shown that the gross bathymetrie features can be modelled quite well if they result from a simple mechanical bending of the subducting lithospheric plate. These results have encouraged searches for seismic evidence of the bending stresses predicted by the models. However, the available teleseismic studies2 do not determine precisely the relative depths of events on the outer rise, or the precise locations of events relative to the trench. In 1979 we successfully deployed nine ocean-bottom seismometers3 (OBS) which operated for 6 weeks on the outer trench slope in the Central Aleutians between the trench axis and the crest of the outer rise (Fig. 1). The locations, depths and focal mechanisms of the earthquakes located by this network all strongly support the bending hypothesis. In particular, we observed a band of seismicity on the outer trench slope extending ∼60 km seaward of the trench axis (Fig. 1). The shallow events (Fig. 2) exhibit normal faulting (Fig. 3) and suggest tension in the upper part of the plate, while the deeper events (Fig. 2) exhibit thrusting mechanisms and suggest compression (Fig. 3). We suggest here that low magnitude seismic activity may be a common feature associated with the well-known normal faulting on the outer trench slope.

Aseismic bearing pads: A new sliding pad technique used in the construction of nuclear power stations for protection against earthquake damage

Construction of a nuclear power station requires a thorough analysis of seismic conditions of the proposed site since most regions of the world suffer significant levels of seismic activity. In order to construct power stations which can withstand seismic disturbances, Electirité de France and Spie Batignolles have developed a sliding pad foundation system which permits a standard method of construction used in zones of low seismic activity to be used in high activity areas.

Tsunami earthquakes and subduction processes near deep‐sea trenches

A tsunami earthquake is defined as a shock which generates extensive tsunamis but relatively weak seismic waves. A comparative study is made for the two recent tsunami earthquakes, and a subduction mechanism near a deep‐sea trench is discussed. These two earthquakes occurred at extremely shallow depths far off the coasts of the Kurile Islands and of eastern Hokkaido on October 20, 1963, and on June 10, 1975, respectively. Both can be regarded as an aftershock of the preceding larger events. Their tsunami heights and seismic wave amplitudes are compared with those of the preceding events. The results show that the time constants involved in the tsunami earthquakes are relatively long but not long enough to explain the observed disproportionality between the tsunamis and the seismic waves. The process times are estimated to be less than 100 s. The spatio‐temporal characteristics of the two events suggest that they represent a seaward and upward extension of the rupture associated with a great earthquake which did not break the free surface at the coseismic stage. The amplitude and phase spectra of long‐period surface waves and the long‐period P waveforms indicate that this extension of the rupture did not take place entirely along the lithospheric interface emerging as a trench axis. It rather branched upward from the interface in a complex way through the wedge portion at the leading edge of the continental lithosphere. This wedge portion consists in large part of thick deformable sediments. A large vertical deformation and hence extensive tsunamis result from such a branching process. A shallowest source depth, steepening of rupture surfaces, and a deformable nature of the source region all enhance generation of tsunamis. The wedge portion ruptured by a tsunami earthquake is usually characterized by a very low seismic activity which is presumably due to ductility of the sediments. We suggest that this portion fractures in a brittle way to generate a tsunami earthquake when it is loaded suddenly by the occurrence of a great earthquake and that otherwise it yields slowly. Upward branching of the rupture from the lithospheric interface produces permanent deformation of the free surface which is relative uplift landward and relative subsidence trenchward of the zone of surface break. This surface break zone geomorphologically corresponds to the lower continental slope between the deep‐sea terrace and the trench. Such a mode of permanent deformation seems to be consistent with a rising feature of the outer ridge of the deep‐sea terrace and a depressional feature of the trench. This consistency implies a causal relationship between great earthquake activities and geomorphological features near the trench.

Environmental and structural implications of Rhode Island seismic events, 1965–1976

Rhode Island, like most of New England, is in a mainly quiet seismic region, and thus, few epicenter locations are available to determine seismically active fault traces. This lack of information has hindered environmental impact siting studies for nuclear power plants because such studies must locate any “capable” (active) faults. Reinvestigation and evaluation of four Rhode Island earthquakes that occurred during the past 12 yr (1965, 1967, 1974, and 1976) indicate that the earthquakes were related to regional lineaments, which are probably caused by faults. In this area of low seismic activity, only isoseismal mapping–rather than simple epicenter locations–can indicate these lineaments. Two of these seismically located lineaments, paralleling each other and trending east, are at the head and mouth of Narragansett Bay, and a third lineament trends northeast through Narragansett Bay itself and along the Taunton River into Massachusetts. Recent re-exploration of coal resources in the Pennsylvanian Narragansett basin in this area has suggested potential new faults. No field relationship between this seismicity and the few known, mapped faults exists, but extensive Pleistocene glacial deposits in the area obscure many geologic structures. In addition, the faults indicated by these regional lineaments may be deeply buried basement features that have been reactivated at various times, at different degrees of activity, up to the present.

Seismotectonic maps of southwest Asia region comprising Eastern Turkey, Caucasus, Persian Plateau, Afghanistan and Hindukush

abstractThree seismic activity maps, the A-value map, the b-value map and the returnperiod map for earthquakes with magnitude 6 and above, have been prepared for the southwest Asia region using the Kaila and Narain (1971) method, and the same are compared with regional tectonics. For the preparation of these maps, a modified relation A = 6.36b - 1.00 has been used instead of the earlier relation where A and b are constants in the cumulative regression curve represented by log N = A − bM. The A-value seismicity map reveals that the Southwest Asia region consists of a number of seismic high zones such as the Caucasus-Abul Samsar high, the Zagros high, the Shahrud-Doruneh high and the Hindukush-West Pakistan high. The Caucasus-Abul Samsar seismic high shows two superimposed trends, one NW-SE which is consistent with the Caucasus tectonic trend and the other NE-SW which is parallel to the Abul Samsar fracture zone. The Zagros seismic high runs in the NW-SE direction almost parallel to the Zagros thrust zone with diversions to the northeast at the two ends. High seismic activity is revealed in the Zagros foothills area rather than the thrust-zone. The Shahrud-Doruneh high shows a NW-SE trend parallel to Kopet Dagh, and, toward the west, it bends down aligning itself almost parallel to the Elburz mountains, thus indicating the possibility of a connection between this high and the Zagros high. The Hindukush-West Pakistan high runs in the NNE-SSW direction consistent with the tectonic trends in this area, indicating the highest seismic activity near the Yasman fracture zone. The b-value seismicity map also reveals the same seismic features as brought out by the A-value map. The b-values obtained by this new method over various regions of southwest Asia agree fairly well with those reported by other workers obtained from earthquake regression curves. The return-period map further brings out the zones of high and low seismic activity which are quite consistent with the A-and the b-value maps, and the regional tectonics.

Unusually low seismic activity in the focal region of the great Kanto earthquake of 1923

The epicenters of earthquakes which occurred in the vicinity of Tokyo during the period 1926–1967 have been plotted by an IBM 360/40 computer at the Earthquake Prediction Observation Center. As a result, a seismically quiescent area is found to the south of Tokyo. The area coincides with the shallower part of the focal plane of the great Kanto earthquake of 1923 as obtained from the analyses of seismological and geodetic data. In historical times, many large earthquakes having a magnitude of 7.0 or greater took place in this area. If it is assumed that the strain energy has been constantly accumulated in this area, an energy potentially equivalent to that of a large earthquake of which the magnitude exceeds 7.7 seems to be stored in the crust there.