脈動、地震探査、重力異常から推定される中国雲南省麗江盆地の地下構造

Abstract

1996年麗江地震により生じた麗江盆地の異常震度分布の原因を究明するために, 脈動観測, 屈折法地震探査および重力測定を実施して, 盆地の地下構造, 特に基盤岩構造を調べた。その結果, 脈動や地震探査から求まる速度構造と重力異常から求まる密度構造とでは, 基盤岩深度に違いはあるものの, 類似の盆地基盤構造が得られた。盆地西縁の雪山断層に伴って基盤岩は700m程度急峻に沈降している。盆地南部を斜めに横切る麗江一剣川断層に沿っては基盤岩に顕著な起伏はない。断層に関係する基盤岩構造が盆地内の震度異常の一因であることが推測される。

Array observation of microseisms (long-period microtremors), seismic-refraction exploration and gravity survey were conducted to investigate the subsurface structure of Lijiang basin, Yunnan, China.Special attention was given to the relation between the configuration of bedrock and the anomalous distribution of damage to wooden houses and RC buildings in the basin caused by the M7.O Lijiang Earthquake of February 3, 1996. Microseisms were analyzed with horizontal-to-vertical spectral ratio (H/V) method and spatial auto-correlation method. The results were reported by Morikawa et al. (1998) and Akamatsu et al. (1998). The seismic exploration was performed along E-W profile crossing the central part of the basin and NW-SE profile crossing the southern part. Observed P-wave travel times were interpreted using 2D seismic ray-tracing method developed by Cerveny et al. (1977) to determine the velocity structures along the profiles. The gravity measurements were carried out at 130 sites inside the basin, mainly in its central and southern parts. Terrain and Bouguer corrections were conducted within a distance range of 60km. Three dimensional two-layer model of density structure was obtained using an inverse method developed by Komazawa (1984).The obtained structural models still remain preliminary because of poor constraints on velocity distribution in the bedrock and the lack of gravity data on surrounding mountains; there is a difference in the estimated depth to bedrock in the basin between the velocity and density structural models. However, both models suggest that, the bedrock subsides steeply along the west edge of basin. This is probably related to vertical displacements of bedrock across the Xueshan fault which runs near the west edge of basin in the N-S dirction and is considered to be a seismogenic fault of the 1996 Lijiang Earthquake. It is important to note that, the most severe earthquake damage in the basin was found not on the west edge but in the areas situated about 1.5km apart from the edge. This situation reminds us of the similar damage distribution in Kobe-Hanshin area caused by the 1995 Hyogoken-nanbu (Kobe) earthquake. On the other hand, no significant change in the configuration of bedrock was found across the Lijiang-Jianchuan fault which runs through the southern part of basin in the NE-SW direction, thereby implying that the displacements across the fault is mainly of strike-slip type. It is suggested out of this that, the high seismic intensity observed in the Old Town lying on this fault should be attributed to reasons other than configuration of bedrock; for example, to the effect of fractured rocks along the fault on seismic ground motions.