EARTHQUAKE FAILURE SEQUENCES ALONG A CELLULAR FAULT ZONE IN A 3-DIMENSIONAL ELASTIC SOLID CONTAINING ASPERITY AND NONASPERITY REGIONS
EARTHQUAKE FAILURE SEQUENCES ALONG A CELLULAR FAULT ZONE IN A 3-DIMENSIONAL ELASTIC SOLID CONTAINING ASPERITY AND NONASPERITY REGIONS
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DOI：
10.1029/93jb01096
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发表时间：
1993-08-10
影响因子：
3.9
通讯作者：
RICE, JR
中科院分区：
文献类型：
--
作者：
BEN-ZION, Y;RICE, JR
数值模拟的地震破坏序列沿着离散的细胞断层带进行的三维（3-D）模型代表大约中央圣安德烈亚斯故障。该模型由上地壳覆盖下地壳和地幔区域，共同定义了一个弹性半空间与垂直半平面断层。断层包含一个区域，在该区域中，滑动是在受静/动摩擦定律支配的单元格的均匀网格上计算的，而在该区域中，滑动是规定的，以便代表构造载荷、无震断层蠕动和邻近的大地震。计算区域模拟了圣安德烈亚斯断层的一个70公里长，17.5公里深的部分，位于1857年大破裂带的西北部。不同分布的应力下降失败的计算单元被用来模拟凹凸（“帕克菲尔德凹凸”）和nonasperity故障区域。该模型是“固有离散的”，并且对应于断层内几何紊乱的特征尺寸（即，单元尺寸，这里是几百米）比基于滑移弱化或状态演化滑移距离的“成核尺寸”（几十厘米到几十米的量级）大得多。计算网格加载的下地壳，上地幔和蠕动断层区域施加的恒定的板块运动，并施加在1857年和1906年破裂带的“阶梯”滑动历史。利用三维弹性位错理论计算了由于外加荷载和破坏事件沿着计算网格的应力传递。由此产生的位移场的计算区域是兼容的大地测量和地震观测时，粗糙和nonasperity地区的特点是显着不同的平均应力降。对于小事件，模拟故障事件的频率-幅度统计近似自相似，具有B - 1.2（基于破裂面积B（A）的统计的B值约为1），但是对于大于临界尺寸的事件，相对于自相似性强烈增强。这被解释为我们的三维弹性应力传递计算的直接表现;超过一定的破裂面积和强度（地震矩除以刚度）释放值，事件通常是不可阻挡的，它继续增长到一个由特征模型尺寸限制的大小。这种影响是不占元胞自动机和块弹簧模型，其中采用的简化应力传递定律未能正确地扩展与破裂尺寸的增加。模拟结果表明，在所观察到的频率-震级统计的局部最大值对应于相干脆性区的尺寸，如孕震层的宽度或由障碍物界定的断层段的长度。分析表明，一个单一的细胞大小，代表了大约一个单一的规模的几何紊乱，不能诱导自相似性的3-D弹性模型在很宽的幅度范围。因此，可能需要一个覆盖一系列尺度的几何无序表示来生成广泛的自相似古腾堡-里希特统计域。我们的模拟表明，帕克菲尔德粗糙体的故障模式的多样性很大，地震本身定义了一个不规则的事件序列。与许多其他离散断层模型一样，该模型表明，对周期性帕克菲尔德地震和/或从一个事件到另一个事件重复的简单的地震模式的期望是不现实的。
Numerical simulations of earthquake failure sequences along a discrete cellular fault zone are performed for a three-dimensional (3-D) model representing approximately the central San Andreas fault. The model consists of an upper crust overlying a lower crust and mantle region, together defining an elastic half-space with a vertical half-plane fault. The fault contains a region where slip is calculated on a uniform grid of cells governed by a static/kinetic friction law and regions where slip is prescribed so as to represent tectonic loading, aseismic fault creep, and adjacent great earthquakes. The computational region models a 70-km-long and 17.5-km-deep section of the San Andreas fault to the NW of the great 1857 rupture zone. Different distributions of stress drops on failing computational cells are used to model asperity (''Parkfield asperity'') and nonasperity fault regions. The model is ''inherently discrete'' and corresponds to a situation in which a characteristic size of geometric disorder within the fault (i.e., cell size, here a few hundreds of meters) is much larger than the ''nucleation size'' (of the order of tens of centimeters to tens of meters) based on slip weakening or state evolution slip distances. The computational grid is loaded by a constant plate motion imposed at the lower crust, upper mantle, and creeping fault regions and by a ''staircase'' slip history imposed at the 1857 and 1906 rupture zones. Stress transfer along and outside the fault due to the imposed loadings and failure episodes along the computational grid is calculated using 3-D elastic dislocation theory. The resulting displacement field in the computational region is compatible with geodetic and seismological observations only when the asperity and nonasperity regions are characterized by significantly different average stress drops. The frequency-magnitude statistics of the simulated failure episodes are approximately self-similar for small events, with b - 1.2 (the b value of statistics based on rupture area b(A) is about 1) but are strongly enhanced with respect to self-similarity for events larger than a critical size. This is interpreted as a direct manifestation of our 3-D elastic stress transfer calculations; beyond certain rupture area and potency (seismic moment divided by rigidity) release values, the event is usually unstoppable, and it continues to grow to a size limited by a characteristic model dimension. This effect is not accounted for by cellular automata and block-spring models in which the adopted simplified stress transfer laws fail to scale properly with increasing rupture size. The simulations suggest that local maxima in observed frequency-magnitude statistics correspond to dimensions of coherent brittle zones, such as the width of the seismogenic layer or the length of a fault segment bounded by barriers. The analysis indicates that a single cell size, representing approximately a single scale of geometric disorder, cannot induce self-similarity in a 3-D elastic model over a broad range of magnitudes. A representation of geometric disorder covering a range of scales may thus be required to generate a wide domain of self-similar Gutenberg-Richter statistics. Our simulations show a great diversity in the mode of failure of the Parkfield asperity; the earthquakes themselves define an irregular sequence of events. The modeling, like many other discrete fault models, suggests that expectations for periodic Parkfield earthquakes and/or simple precursory patterns repeating from one event to the other are unrealistic.