为什么自然界中超剪切破裂的地震是如此之少?

Earthquakes with supershear rupture have received extensive attention because they cause more severe seismic disasters than sub-shear earthquakes of the same magnitude. However, the number of supershear rupture earthquakes in nature is extremely rare; so far, only a dozen or so cases of supershear rupture have been discovered globally. Then what factors have caused there to be so few supershear rupture earthquakes? This scientific problem has not yet seen a systematic analysis and research by predecessors; for this reason, this paper uses the finite element calculation method to quantitatively analyze several common mechanical factors that trigger supershear rupture. The finite element simulation results show that the surface is the most efficient factor in triggering supershear rupture, but when there are soft media such as sedimentary layers and unconsolidated fault gouge near the surface, the fault media near the surface is controlled by a velocity-strengthening friction constitutive relationship, and at this time sub-shear rupture cannot be converted into supershear rupture, and this thin layer of friction velocity-strengthening layer near the surface will effectively inhibit the occurrence of supershear rupture. In addition, the simulation results also show that although obstacles, anti-asperities, and fault step zones on the fault can promote the conversion of sub-shear rupture into supershear rupture, during the conversion process, due to a pause in the rupture time, the rupture speed on the entire fault is averaged as sub-shear rupture, that is, the seismic information received by far-field seismic stations is very likely unable to distinguish the occurrence of local supershear rupture. To produce a currently recognizable supershear rupture earthquake event, the conditions are very harsh, usually requiring the fault length to be long enough, the fault geometry to be straight enough, the initial stress level to be high enough, the thickness of the surface friction velocity-strengthening layer to be thin enough, and there to be enough seismic stations in the near field, etc. It can be seen that these factors mentioned above have led to very few recognizable supershear rupture earthquakes in nature. This research helps us to deeply understand the mechanical mechanism of supershear rupture and to better assess seismic disasters.

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.