为什么自然界中超剪切破裂的地震是如此之少?

Why are there so few supershear rupture earthquakes in real-world nature?

超剪切破裂的地震由于比同震级的亚剪切地震造成更为严重的地震灾害,因而受到广泛重视.可是,自然界中超剪切破裂的地震数量极其稀少;到目前为止,全球只是发现了十几个超剪切破裂的震例.那么是哪些因素造成了超剪切破裂地震是如此之少?这个科学问题,至今未见前人有系统的分析和研究;为此,本文利用有限单元的计算方法,对触发产生超剪切破裂的几种常见力学因素进行定量分析.有限元模拟结果显示,地表作为触发产生超剪切破裂效率最高的因素,但当地表附近具有沉积层、未固结的断层泥等松软介质时,地表附近断层介质是由速度强化的摩擦本构关系所控制,此时亚剪切破裂无法转换为超剪切破裂,近地表的这层薄薄的摩擦速度强化层会有效抑制超剪切破裂的发生.此外,模拟结果还表明,断层上的障碍体、反凹凸体、断层阶区虽然可以促使亚剪切破裂转换为超剪切破裂,但在转化过程中,由于破裂出现时间上的停顿,这样整个断层上的破裂速度就被平均为亚剪切破裂,即远场地震台站接收的地震信息很可能无法辨别局部超剪切破裂的发生.若要产生一个目前可以识别的超剪切破裂地震事件,其条件十分苛刻,通常需要断层的长度足够长,断层几何要足够平直,初始应力水平要足够高,地表的摩擦速度强化层厚度要足够薄,近场要有足够多的地震台站,等.由此可见,上述这些因素导致了自然界中可以辨别的超剪切破裂地震非常稀少.本研究有助于我们深入理解超剪切破裂发生的力学机制,有助于我们更好地评估地震灾害.

The supershear rupture earthquake is more dangerous than the subshear one with the same magnitude,so it has been paid more attention.However,the number of supershear ruptures in nature is extremely rare;so far,only a dozen cases of supershear rupture earthquakes have been identified worldwide.However,the physical mechanism of this phenomenon has not been systematically studied.For this purpose,in this paper,using finite element method(FEM),we carry out quantitative analysis of the familiar trigger factors for the occurrence of supershear rupture.The simulation results show that the Earth′s free surface is the most effective factor for triggering the supershear rupture transition,but when there is a velocity strengthening friction layer(VSFL)near the Earth′s free surface resulted from sedimentary layer,unconsolidated fault gouge and the low level of normal stresses near the free surface,the subshear rupture cannot be transformed into supershear rupture,and supershear rupture will be stifled in this case.In addition,the modeling results also demonstrate that the fault stepover,barrier,and anti-asperity on the fault can promote the transition from subshear rupture to supershear one,but there is a temporal pause in the transition process,so the average rupture velocity of the whole fault is less than the speed of shear waves,that is,the local supershear rupture cannot be distinguished through the seismic signals received by the far-field seismic stations.We found that the conditions for producing a supershear rupture event that can be recognized at present are very tough,such as the length of the fault needs to be long enough,the geometry of the fault needs to be straight enough,and the initial stress level is high enough,the thickness of the VSFL is thin enough,and there are also enough seismic stations in the near field.Thus,all these factors above contribute to the dearth of identifiable supershear rupture earthquakes in nature.This study is helpful for us to deep understand the mechanism of the occurrence of supershear ruptures and the assessment of seismic hazards.