昌马地震应力场及发震机制研究

RESEARCH ON STRESS FIELD AND CAUSATIVE MECHANISM OF THE CHANGMA EARTHQUAKE,GANSU PROVINCE

摘要:

本文利用前人大量的地质、地球物理资料,建立了昌马盆地的立体地质模型,利用三维有限元法研究了1932年昌马7.6级地震前后应力场的变化及其昌马地震成因机制,结果表明:
(1)震源断层走向与利用主压应力推测的断裂走向相差65°,可能是由于地壳浅部旋转的主压应力与深部北东方向的主压应力所产生的力矩使块体发生旋转所致,其枢纽点即为震源所在;同时,该旋转也导致了地震断层和震源断层在力学性质和几何性质上的不同。
(2)昌马地震是在闭锁、贯通和块体旋转联合作用下形成的。
(3)区域应力场与局部应力场不一致、应力松弛单元的出现可能与地震的孕育、发生有关。
(4)断裂活动的不均一性与平面最大剪应力分布不均匀成正相关,且各断裂均以相对左旋走滑兼挤压为特征,但是其走滑量的分布是不均匀的。

Abstract:

Based on the geological and geophysical data,this paper establishes a stereogeologic model of the Changma basin. The change of stress field and theearthquake mechanism be-fore and after the Changma Ms7.6 earthquake in 1932 are studied by 3-D finite element-method. The results show that:
1.The difference between the focal fault strike and that inferred by the principal com-pression stress is 65°,this probably is due to the block rotation which results from the force moment produced by the rotated principal compression stress in shallow crust and the NE directional principal compression stress in depth. The hinge point is the focal re-gion.The block rotation results in the differences between the mechanical and geometrical natures of earthquake fault and focal tault.
2.The Changma earthquake is due to the cooperative interaction of closing joint and block rotation.
3.The regional stress field discords with local stress field and the appearance of stress relaxation unit is probably relative tothe earthquake preparation and occurrence.
4.Nonuniformity of fault activity is positive correlative with the inhomogeneous dis-tribution of plane maximum shearing stress.The fracture zones are characterized by rela-tive levorotation, shove and compression,but the distribution of strike slip is inhomoge-neous.