Experimental simulation of earthquake rupture processes

当地壳中的断层突然破裂时，就发生了地震。在小地震中，限制断层的岩石块会滑动很小的距离，在大地震中则会滑动几米。在这种滑动过程中，断层减弱，而这种减弱提供了能量来继续地震破裂。显然，这是一个复杂的过程，其中断层的力学性质在几秒钟内急剧变化。探索这些过程的经典实验方法是在恒定速度或非常慢的速度下进行的;这些条件都与天然地震的破裂无关。我们提出了一种新的实验方法，预计将揭示与地震相关的关键参数。这些实验将利用我们位于俄克拉荷马州大学的地震实验室的独特的能源供应和电力控制能力。这些能力的影响在初步工作中得到了证明，这些工作成功地模拟了地震期间的断层行为，震级高达8级（类似于1906年旧金山弗朗西斯科地震）。为什么这些实验很重要？首先，一般来说，更好地了解地震过程是减少地震危险的关键。其次，本文的研究对于建筑结构设计中的地震动计算具有重要意义。这些计算是基于地震期间断层的力学行为，更准确和相关的计算需要更好地了解这种行为。 理论模型表明，地震破裂是一个复杂的过程，具有非平凡的运动学和动力学性质的演变（例如，Tinti等人，2005年）。最近的实验模拟了实际地震的速度历史（例如，Sone and Shimamoto，2009; Fukuyama and Mizoguchi，2010; Liao，Chang and Reches，in revision for EPSL）表明，在低速、短距离摩擦实验中确定的本构关系不一定适用于可变的、高滑动速度的天然地震。本文的目的是通过对实验断层施加类似于地震荷载的荷载来模拟地震破裂过程，并推导出在这些条件下的本构关系。 实验将使用高速旋转剪切设备进行，该设备在滑动速度高达1 m/s、法向应力高达30 MPa和大滑动距离下沿沿着岩石块施加摩擦滑动。该装置具有两个独特的功能：（1）通过存储在飞轮中的有限能量（高达10^7 J/m^2）加载实验故障（Chang等人，2012）;以及（2）控制施加在断层上的功率密度的实时反馈系统（功率密度=滑动速度 * 剪切应力）。这些方法允许应用任何加载历史来揭示瞬态地震荷载下的断层响应。这两种方法的初步结果显示出很强的相似性断层行为的理论模型的断层强度（弱化和加强），滑动速度，能量耗散和上升时间的演变。我们将把观察到的本构关系与断层弱化机制联系起来，为此，我们将用超微观方法（AFM，XRD，SEM和TEM）表征断层性质。

An earthquake occurs when a fault in the earth crust ruptures abruptly. The rock blocks that bound the fault, slip for tiny distance in small earthquakes, and up to a few meters in large events. During this slip, the fault weakens and the weakening provides energy to continue the earthquake rupture. Obviously, this is a complex process in which the mechanical properties of the fault drastically change within seconds. Classical experimental approaches to explore these processes are conducted at constant velocity or at very slow velocity; neither of these conditions is relevant to the rupture of natural earthquakes. We propose a new experimental approach that is anticipated to reveal key parameters that are relevant to earthquakes. These experiments will utilize the unique capabilities of energy supply and power control of our earthquake laboratory in the University of Oklahoma. The impact of these capacities was demonstrated in the preliminary work that succeeded in simulations of fault behavior during earthquakes up to magnitude M=8 (similar to 1906 San Francisco earthquake). Why are such experiments important? First, in general, better understanding of earthquake processes is essential for the reduction of seismic hazard. Second, this study is particularly important for the calculations of seismic ground shaking, which are used for building structural design. These calculations are based on the mechanical behavior of faults during earthquakes, and more accurate and relevant calculations require better knowledge of this behavior. Theoretical models suggest that earthquake rupture is a complex process with non-trivial evolution of kinematic and dynamic properties (e.g., Tinti et al. 2005). Recent experiments that simulated the velocity history of actual earthquakes (e.g., Sone and Shimamoto, 2009; Fukuyama and Mizoguchi, 2010; Liao, Chang and Reches, in revision for EPSL) show that constitutive relations which were determined in low-velocity, short-distance friction experiments do not necessarily hold for variable, high slip-velocity of natural earthquakes. The present objective is to simulate the earthquake rupture process by loading experimental faults similarly to earthquake loading, and to derive the constitutive relation at these conditions. The experiments will be conducted with a high-speed rotary shear apparatus that applies frictional sliding along rock blocks under slip-velocity up to 1 m/s, normal stress up to 30 MPa, and large slip distance. This apparatus has two unique capabilities: (1) Loading the experimental fault by a finite amount of energy (up to 10^7 J/m^2) that is stored in a flywheel (Chang et al., 2012); and (2) A real-time feedback system that controls the power-density applied on the fault (power-density = slip velocity * shear-stress). These methods allow the application of any loading history to reveal the fault response under transient earthquake loading. The preliminary results of both methods show strong similarities to theoretical models of fault behavior in terms of the evolution of fault strength (weakening and strengthening), slip velocity, energy dissipation, and rise-time. We will link the observed constitutive relations to the mechanisms of fault weakening, and for this purpose, we will characterize the fault properties with ultramicroscopic methods (AFM, XRD, SEM, and TEM).