Keichi Kasahara

Migration of Crustal Deformation

Abstract Observations on the migration rates of crustal deformation, as recently discovered in several tectonic areas, such as the south Kanto and central Tohoku districts, Japan and the West Cordillera Mts., Peru, has opened up a new opportunity for the study of crustal dynamics. Briefly, these examples from coastal areas are characterized by migration landwards with a velocity of about 10–100 km/yr. This agrees well with the velocity of mIgration of seismicity as previously known. Dispersion and dissipation of the deformation waveform are also noted as characteristics. Simple extrapolation of the migration path back toward the ocean may locate a possible origin of the event. In the case of the south Kanto district, for example, the deformation front seems to have originated in the early 1950s from the vicinity of the junction of the Japan and Izu—Mariana trenches. The deformation front in the central Tohoku district, on the other hand, is thought to have originated in the northern part of the Japan Trench in the late 1960s. One may suppose that either a repeated irregular aseismic plate motion generates the deformation events, or that it results from a periodic seismic slip at a plate boundary. In the latter case, the 1953 Boso-oki and the 1968 Tokachi-oki earthquakes might be suspected of generating the deformation fronts in the south Kanto and central Tohoku districts respectively. As Scholz speculated, the migration of a deformation front might trigger earthquakes, if it hits areas of high seismic potential. Studies of migration events can contribute significantly to earthquake prediction studies.

Aseismic Faulting Following the 1973 Nemuro-Oki Earthquake, Hokkaido, Japan (a possibility)

SummaryThe possibility of delayed strain release by aseismic faulting following a main shock is studied in relation to the recent earthquake off Nemuro peninsula, Hokkaido, Japan 17 June, 1973. The main purpose of this paper is to obtain a comprehensive view of the land movement in eastern Hokkaido with respect to the seismic cycle there This region, particularly the Nemuro peninsula area, has been subject to extensive land subsidence in the past several decades. At Hanasaki, for example, it amounted to −60 cm in the past 70 years. The long-term accumulation of subsidence in the same area, as read from remaining marks of the ancient sea level, is only −2 m in the past 5000–6000 years. There-fore, the rate of the recent land movement is twenty times or more as high as the long-tern one, which is known from the geomorphological evidence. Such disagreement between the two kinds of rates leads us to expect a future uplift phase which will compensate the previous subsidence.Delayed strain release by aseismic faulting is suggested in order to account for the former Nemurooki earthquake (1894) as inferred from the tidal record at Hanasaki. The model is basically a compound fault system with brittle-ductile transition at a certain depth in the plate boundary. If this were the case, the tide-gauge at Hanasaki would record a significant land uplift following the 1973 earthquake for 40 cm or more in the forthcoming several years. The slip mode in the ductile part is a matter of concern from the viewpoint of earthquake prediction, as a secondary shock may occur if the fault should slip quickly. The recent sea-level change at Hanasaki seems to suggest slow propagation of fracture in the down-dip direction. A brief discussion is added on the rheological properties of the plate boundary to estimate the viscosity coefficient for future reference.

A strike-slip fault in a laterally inhomogeneous medium

The paper introduces a model of a two-dimensional strike-slip fault in a laterally inhomogeneous medium, i.e., in an elastic half-space which is divided by two vertical interfaces into three adjoining parts of different rigidity. On the basis of the elasticity theory of dislocation, formulas are derived for the displacement, strain and stress fields. They are applied to the case of a medium with a low-rigidity zone of the width H at the centre of which a vertical fault is introduced. Numerical tests, in which parameters refer to the data from the San Andreas fault, demonstrate the possibility of significant distortion of the displacement and strain fields attributed to the low-rigidity zone around the fault. In comparison with the standard (homogeneous medium) model, diminution of the surface displacement with distance, and consequently the surface strain, tend to predominate in the vicinity of the fault (low-rigidity zone). If this type of pattern is interpreted by the standard model, then the vertical extension of the fault D may be underestimated. The reduction coefficient of D in this sense can reach up to 50%, depending on the width of the zone and its rigidity contrast.

Aseismic faulting following the 1973 Nemuro-oki earthquake, Hokkaido, Japan (a retrospective study)

Tidal records at Hanasaki, southeastern Hokkaido, are analysed to clarify the post-seismic crustal vertical movement in the 1973 Nemuro-oki, Japan, earthquake. The result shows that the post-seismic uplift, as speculated byKasahara (1975), terminated in late 1975 and reverted to the previous rate of rapid subsidence of about 1 cm/yr. Ceasing of the post-seismic uplift, which occurred much earlier than previously speculated, could be explained by supplementary mechanisms, such as frictional resistance against creeping at the fault surface. This unexpected mode of movement, however, raises a new question about the accumulation of tectonic movements in this district. Since the subsidence rate extrapolated from geomorphological data for the past several thousand years is almost one order of magnitude lower than the recent rate as mentioned above, we have to provide some alternative explanation for the discrepancy between the two rates. In other words, simple repetition of co-, post- and inter-seismic movements in seismic cycles due to plate subduction, must be modified in this district. Several possible modifications to the subduction process are suggested, and a brief discussion of vertical movements expected in each case is given.

Mechanical interaction between neighboring active faults—an application to the Atera fault, central Japan

Abstract Static stress changes across the Atera fault, central Japan, due to the neighboring large earthquakes are examined. Several large earthquakes in the past 100 yrs which might have dominantly affected the fault are selected in order to calculate coseismic changes in the fracture stress. The result suggests that the stress increased at both the northern and southern ends of the fault, which amounted to as much as 6 bar in total. Among these earthquakes, the Nagano-seibu earthquake ( M = 6.8) of 14 September, 1984, seems to have most seriously affected the Atera fault in such a sense as to increase the fracture stress.

South Kanto Monitoring Chain on Crustal Activities and Its Processing System of Multi-Dimensional Data

The group of university scientists in crustal movement studies has constructed a series of Monitoring Chains on Crustal Activities (MOCCA) in Japan. The Chain is a concentrated station system for monitoring of crustal activities along a belt, which crosses areas of seismotectonic importance in such a way as to involve the existing observatories and principal routes of geodetic surveys. Each Chain is about 150 km long with 8–10 multi-purpose stations at average separation of 15–20 km. The close spacing of stations will allow our cross-correlational discrimination of the seismic precursory signals which might originate from the interior of the earth’s crust.