摘要:
云南川西地区是多震的地方(图1),从公元前116年到1974年6月,据记载共发生M≥4.75级的地震481次,其中6≤M<7的99次,7≤M<8的14次,M≥8的3次。震源深度一般小于30公里,均为浅源地震。本区强震多发生在深大断裂带上,而且都有一定的地质标志可循,与近代地壳运动、板块构造在成因上有着密切关系。
Abstract:
This paper deals with the correlation between the great fundamental fault zones and intense earthquakes. These great fundamental fault zones might be the contact zones or their influence zones between old plates. Though the region under study is situated in the interior of the Eurasian plate, its fault zones are quite similar to plate contact zones in formation and development, and quite liable to produce intense earthquakes.The direct collision between the two continental crusts of Indian plate and Eurasian plate results in a differential movement, during which the local tensional stress produced brings about in the great fundamental fault zones fault-depression, which forms a demarcation line between two fault blocks. Thus a fault basin is formed and the epicenters of strong earthquakes are mostly found in the fault basin zones. This of course doesn’t mean that such fault depression will produce earthquakes. It is just an indication that in the mobile belt of the earth crust, lower tiers and upper ones react differently to identical lateral compression. Therefore fault basins can only be regarded to a certain extent as "indicator structure" for strong earthquakes, not as "seismogenetic structure". Obviously such basin zones differ from continental rift in mechanism.In this area active great faults are mostly almost vertical. As earthquake foci are basically situated in the extension parts of active fault zones, intense seismic zones are often located in such zones bordering on fault blocks. From the extent of displacement of the active fault zones, an approximate calculation can be made of the intensity of an earthquake.The fact that the distribution of epicenters of strong earthquakes in a plane figure is highly identical with that of fault basin zones and active fault zones can be regarded as an important feature of strong earthquake zones formed by direct collision of the two continental crusts.The Mesozoic surficial folds in this area roughly reflect the stress-field produced by the downthrust of the two oceanic crusts (i.e. the Pacific plate and the Tethys plate) into a continental crust (i.e. Eurasian plate). The Cenozoic stress-field is caused by the direct collision of two continental crusts (i.e. Indian plate and Eurasian plate), which affects the whole of the Chinese mainland.