大陆活化克拉通与裂谷地堑地区的地震构造模型

SEISMOTECTONIC MODELS FOR THE CONTINENTAL AREAS ASSOCIATED WITH REACTIVATED CRATONS AND RIFTING GRABENS

摘要:

经常观察到这样的现象:活化的克拉通在其演化的不同阶段经历过裂谷作用,在这些地区有分割的地堑构造,有些地堑已变成现代的活跃地震带,如中国山西临汾地堑,中国河北邢台地堑和美国的新马德里地堑。大量资料表明,这些地区的地震成因与许多地质、地球物理条件有关,而不仅仅是活动断层。以临汾地堑为例,地震发生过程涉及到下列因素:(1)地堑周围强度较大的变质岩,这些岩体在区域应力场作用下产生应力积累;(2)地堑内5—15km深度发震层内强度较低的介质,地震发生时发震层介质发生破裂,释放能量,辐射出地震波;(3)发震层之下的异常低阻层或低速层,它对震源过程具有延性的调节作用;(4)地堑两侧的断裂带,它具有魔棱岩化特征,对孕震过程具有控制作用,这些因素结合构成发震构造模型,它能解释大陆活化克拉通与裂谷地堑地区发生地震的原因。应用数值模拟对所提出的模型进行了检验。

Abstract:

It is often observed that there are separated graben structures in the reactivated cratons where rifting occurred durig different poriods of their evolution. Some of these graberls behave as present-day active seismic zones,such as the Linfen graben, Shanxi,China; the Xingtai graben, Hebei, China; and the New Madrid graben, the United states. Increasing data have demonstrated that the causes of earthquakes in these areas are in-volved with many geological and geophysical conditions besides the active fauting.As a typical case,studies on the Luifen graben show that the seismogenic process is associated with the foliowing factors:(1)The metamorphic rocks with greater strength around the graben,in which stress is built up under the action of the regional tectonic stress field.(2)The medium of the seismogenic layer with lower strength at the depth range of 5 15 km within the graben,which ruptures during the seismic source process to release energy and radiate seismic waves. (3)The abnomal layer with low velocity or low resistance beneath the seismogenic layer,which has the ductle adjusting function to the seismic source process.(4)The fault zones along the both sides of the graben ,which are character-ized by mylonitization and have controlling effects on the process of earthquake brewing.The combination of these elements offers seismogenic models which can explain the causes of some seismic zones in the continental ar-eas associated with reactivated cratons and refting grabens.Numerical modeling has been used to test the pro-posed models.