Crustal Strain, Crustal Stress and Microearthquake Activity in the Northeastern Japan Arc

Abstract

Ishii, H., Sato, T., Tachibana, K., Hashimoto, K., Murakami, E., Mishina, M., Miura, S., Sato, K. and Takagi, A., 1983. Crustal strain, crustal stress and microearthquake activity in the northeastern Japan arc. In: P. Vyskočil, A.M. Wassef and R. Green (Editors), Recent Crustal Movements, 1982. Tectonophysics , 97: 217–230. Crustal movement and seismic activity in the northeastern Japan arc are produced by a reaction of the Pacific Plate sinking under the continental plate. We have investigated the time variation of crustal strain obtained from analysis of extensometer installed at eleven crustal-movement observatories in the northeastern Japan arc. We then investigated, using the finite element method, the spatial distribution and time variation of the crustal strain and stress caused by plate subduction. We finally compared the results with microearthquake activity observed by the seismic network of Tohoku University. The spatial distribution and time variation of microearthquake seismic energy were elucidated in order to know the seismic circumstances around the eleven observatories. A contour map of the seismic energy distribution was obtained, showing a lineation of the seismic activity. The time variation of various strain components from 1967 to 1981 were revealed. With the accumulation of additional data, it is possible to recognize the propagation of the maximum shear strain in the direction S50°E at 40 km/year from the Pacific Ocean side to the Sea of Japan. A similar time variation of areal-dilatation was found at the five observatories located at a distance of more than 100 km. The directions of the compression axes derived from the composite focal mechanism solution of microearthquakes coincide well with those of principal strain obtained from data gathered using extensometers, indicating E-W compression in most of the observatories and N-S compression around MYK. The spatial distribution and the accumulation of crustal stress caused by plate subduction were investigated using the finite element method, and a comparison with a hypocenter distribution of microearthquakes was performed. The distribution of the crustal stress is closely related to the crustal structure, but the amplitude of the observed strain is affected by local structures around the stations.