中小震级事件产生地表破裂的震例分析

ANALYSIS ABOUT THE MINIMUM MAGNITUDE EARTHQUAKE ASSOCIATED WITH SURFACE RUPTURES

以往的地震统计研究中,由于中小地震产生地表破裂的震例缺乏,多数研究者认为在大多数情况下,当地震震级M>6时才会产生地表破裂。基于上述统计认识,古地震探槽中破裂的出现也认为更可能是由M 6以上地震造成的。收集了1950—2014年全球范围内有明确记录产生地表破裂的56个中小强度震例,发现伴有地表破裂地震的震级下限可能在5级左右,极端情况下震级可以低至3.6级。同时,从理论和经验的角度,探讨地表破裂的控制因素,发现震源深度较浅是中小强度地震产生地表破裂的1个重要原因,此外高热流值和拉张的构造环境、摩擦强度较弱的活动断裂也是低震级事件破裂达到地表的有利条件。因此,虽然中小震级地震产生地表破裂的概率较低,但并非完全不可能。古地震探槽解译时,也不能绝对地认为只要有破裂出现,就一定是6.5级以上地震所为,而忽视古地震探槽揭示中等震级地震事件的可能性。

Statistical study of earthquakes in the past, due to the small-medium size magnitude earthquake associated with surface rupture are rare, considers that only the earthquakes beyond magnitude 61/2 could produce surface ruptures in the most cases. Identification of paleoseismic events is also often based on this assumption. In this paper, we summarized 56 historical moderate size earthquakes worldwide, which have clearly documented about surface ruptures from 1950 to 2014. Resuhs show that the magnitude lowest limit of the earthquake associated with surface rupture may be lower than the 61/2, probably is about 5, even can be as low as 3.6 under extreme conditions. Additionally, from the view of theory and practice, this paper explored the effect of control factors on surface rupture, so as to indicate that the shallow focal depth is one of the most important factors for small-medium size earthquake associated with surface rupture, also included are the high heat flow values, tensile tectonic environment and active fault with weak friction strength. Although the probability that small magnitude earthquake produces surface rupture is low, it is not impossible. In the interpretation of paleoearthquake events, it also cannot overgeneralize that the corresponding earthquake magnitude must be 6.5 or greater as long as the fracture appeared, while ignoring the possibility of some moderate size earthquakes.