地震成因软沉积物变形记录的地震强度研究进展

The use of soft-sediment deformation structures as proxies for paleoseismic activity and shaking: A review

确定地震震级对中长期地震预报、震后应急救援和地震危险性评价具有重要意义。古地震学是研究地质记录中的地震事件,特别是它们的位置、时间和震级大小。然而,传统由地表破裂参数确定的古地震震级仍存在不确定性(大多数地震事件不会导致地表破裂,或位移小于0.3 m),尤其是由湖泊沉积记录的古地震事件。为了解决未发现明显位错地震震级问题,本文依据软沉积物变形构造的类型和形式,对确定地震震级/强度的方法(经验估算、最大液化距离、扰动层厚度、软沉积物变形类型,经验公式,快速沉积砂层厚度)进行总结和讨论,并分析其理论基础、优缺点、误差大小、适用性、存在问题等。并以中东死海盆地利桑组晚更新世湖相沉积中的震积岩(混杂层)和岷江上游萝卜寨晚第四纪湖相沉积中地震成因的液化底劈为例,利用上述6种方法推断,其代表的震级分别为M5.5~6.5和M6.0~7.0,进一步证实了前人的研究结果。这6种方法的结合,为确定地震震级/强度,特别是湖泊沉积中的地震事件提供了一种新的、相对便捷的方法。该研究可为基于地表破裂参数确定的古地震震级提供可靠的参考,为更好地认识构造活跃地区的地震活动性和危险性提供数据支持。

Objectives: Quantifying the magnitude of an earthquake is very important for long-term and medium-term earthquake prediction,post-earthquake emergency rescue and seismic hazard assessment. Paleoseismology is the investigation of past earthquakes in the geological record,in particular their location,timing and size. Uncertainties remain in the paleoearthquake magnitudes determined by traditional surface rupture parameters,especially because most seismic events do not result in surface ruptures. This study is in order to address the problem of magnitudes evaluation of earthquakes that did not reveal major dislocations.Methods: This paper deals with the methods used to determine the seismic shaking based on the types and forms of soft-sediment deformation structures,including,maximum liquefaction distance,thickness of disturbed layer,empirical formulae,and thickness of rapidly-deposited sand layer. Then discuss and analyze these methods in terms of their theoretical basis,advantages and disadvantages,accuracy,applicability and problems. We chose two case studies: first,a typical seismics-related deposit( liquefied layer and dsirupted layer) represented by a seismite in the late-Pleistocene Lake Lisan section near Masada in the Dead Sea Basin;and second,the liquefied diapir triggered by an earthquake in the Luobozhai late-Quaternary lacustrine sediments in the upper reaches of the Minjiang River,east Tibet.Results: The six methods listed above are employed to determine earthquake magnitudes associated with the seismics-related deposit and liquefied diapir,yielding magnitudes of M 5.5 ~ 6.5 and M 6.0 ~ 7.0,respectively.Conclusions: The combination of the six methods,provided a new and relatively convenient method for determining seismic shaking,especially in lacustrine sediments. This study can serves as a valid reference for comparing methods of calculating the magnitude of a paleoearthquake based on surface rupture parameters,and provides a better understanding of the long-term seismic activity and risk in tectonically active regions.