第四纪断层上断裂活动的群集及迁移现象

对许多第四纪断层上形成地表破裂的巨大滑动事件及错动位移状况的具体研究表明,在不同时间段它们的数量和强度是变化的,不均匀的。这种断层活动的不均匀性主要表现在两个方面:一是滑动事件的发生往往集中于一定的地段和一定的时间段,并与相对平静的阶段相交替,即所谓断裂活动的群集性;另一方面的表现是集中发生滑动事件段落(或区域)在空间上的迁移变化。这是中国大陆内部第四纪断层活动所表现出的主要特征之一。这方面的实例可见于许多活断层上。如沿郯庐断层、鲜水河断层以及河西走廊断裂带、阿尔金断裂带等。深入研究第四纪断层活动的这一不均匀特性对正确评估活断层的活动程度和潜在地震危险性具有实际应用意义。

GROUPING AND MIGRATION OF FAULTING ON QUATERNARY FAULTS

Rupturings of Quaternary faults are of obvious nonuniformity which is mainly reflected in the following two aspects: one is so called grouping of faulting which means that slip events are often concentrated on some definite segments of faults and in some definite time intervals, and alternated with intervals of relative quiescence; another is migration of faulting which denotes that the segments or regions producing grouping of slip events usually show special changes. These two important features are verified by the facts and evidence observed on many active Quaternary faults in the continent of China, such as the active Tancheng-Lujiang fault, the active Xianshuihe fault, the North Qilianshan fault and the active Altun fault. The total amount of displacements produced by Quaternary faulting during different time intervals often exhibits a multiple increase of accumulation which, as proved by lots of facts and actual evidence, is caused by a great number of abrupt slip events, but not by continuous or uniform creep. According to the analysis of chronological data of slip events, it is found that they often occur concentratively in some definite time intervals or reach on some definite stratigraphic horizons. This reflects that there indeed exists grouping of rupturing and slip events. Faulting migration may take place along an active fault, and also along a fault belt within an active region. It causes ruptures further developed and propagated forward and inactive preexisting faults reactivated, and results in the continuous increase and extension of fault lengths, and at the same time making the fault lines smoothened or complicated. The tendency and direction of migration of Quaternary faulting follow certain principles, for instance, rupturings along the NE-trending faults mostly migrate from southwest to northeast, while along the NW-trending faults from southeast to northwest. Evaluations of seismic risk are established from predicting and estimating the potential activity by means of studying and understanding the behaviours of the earthquake-generating faults. To carry out this work data such as the parameters of the times, places, periodic intervals of fault slips and palaeoearthquake events, and the fault slip rates are needed. It is considered that understanding of the nonuniformity of faulting will improve the accuracy and reliability of the evaluation of faulting intensity and seismic risk. How to determine correctly the existence of rupturing events or palaeoearthquake events and how to improve the accuracy of dating of these events are two key problems.