Detailed spatial changes in the stress field of the 1984 western Nagano earthquake region

摘要

We estimate the stress field after the 1984 western Nagano earthquake from numerous focal mechanisms. To precisely determine the focal mechanisms, we analyze the earthquakes that occurred around the eastern part of the main shock fault, where station coverage is fairly good. Most of the earthquakes that occurred in this part, except the main shock fault, are reverse fault types. In contrast, earthquakes that occurred near the main shock fault have T axes distributed in a belt (strike slip, oblique slip, and reverse fault types occurred equally). Using the stress inversion method to quantitatively estimate the stress field of these earthquakes revealed that the σ_2 axis near the main shock fault is close to the vertical direction, whereas it is close to the horizontal direction in other regions. At the western edge of the study area, we observe that the σ_1 axis rotates toward the NS direction and that the stress ratio (σ_1 -σ_2)/(σ_1-σ_3) is low. We estimate that the magnitude of σ-hmin decreases near the main shock fault. Spatial variation of the stress field around the eastern part of the main shock fault is not generated by static stress changes caused by the main shock. Local stress anomalies might have occurred around the main shock fault before the main shock.