Studies of Fault Fabrics and Earthquake Mechanics from the Precise Relative Locations of Microearthquakes

具有相似震源(位置)和震源机制(断层面和滑动方向)的微震(小于3级的地震)在给定的地震台站产生看起来非常相似的地面运动。通过比较这些地震的记录地震波形，可以非常准确地估计地震波的相对到达时间，并最终非常准确地估计地震的相对位置。利用这种方法获得的相对位置误差对于相隔数百米的事件为数十米，并且在对随时间变化的台站变化进行校正后，对于相隔数十米或更小的事件，相对位置误差为数米。这些误差比公布的美国地质调查局北加州地震台网(NCSN)目录中的误差小10-100倍。分辨率的提高在断层研究的两个不同领域带来了重大的新见解。首先，断层带内部以前看不到的结构已经被成像。其次，由于最近的微震之间的相对定位误差比破裂尺寸(几十到数百米)小得多，这种分辨率允许使用数千个事件的数据集来研究地震相互作用。在断裂构造方面，微震定位表明，许多蠕动断裂的地震活动被组织成滑动平行的“条带”，高达几十米到几百米，长度可达几公里。在破裂相互作用区，观察到圣安德烈亚斯断裂中部微震的余震分布非常不对称，最近的余震(在空间和时间上)发生在前一次主震的西北部，而不是东南部。这归因于断层上物质性质的反差以及这种反差如何影响主震的动力学(优先向东南传播)。如果这一解释是正确的，它与地震灾害有关，因为大地震的方向性集中在传播方向上的地面震动。目前的研究致力于建立地质名胜区数千次精确定位的微震目录，从断层力学的角度解释这些目录，并改进定位方法。正在为NCSN台站确定更多与时间相关的改正，并向地震界提供。通过台站校正获得的更精确的位置正被用来寻找(1)与所观察到的微地震线条有关的断层倾角的细微变化，以及(2)据信滑动很小的断层上的短线条，以区分关于线条起源的相互竞争的观点。在不对称余震分布方面，正在使用Parkfield HRSN目录将数据集扩展到较小的主震规模，正在修改重新定位代码(具有随时间变化的台站改正)，以常规确定局部跨断层P波和S波的速度对比，并对不同材料之间界面上的破裂进行数值模拟，以检验对对称性起源的解释。最后，将不同断层上微震余震序列的统计量与速率和状态摩擦分析模型的预测以及平面断层上破裂相互作用的数值模型进行了比较。其目的是测试速率和状态断层摩擦模型，并确定不同断层的加载速率和/或表面属性的差异。对于一些故障，余震序列的最早部分可能已经因为触发事件(可能是几十秒)之后的网络“盲”时间而“丢失”，所有编目的事件的近百个S存档的波形正在被处理，以识别和定位通常会触发网络的事件。这些研究将使人们对地震的机制有进一步的了解。

Microearthquakes (earthquakes smaller than magnitude ~3) that have similar hypocenters (locations) and focal mechanisms (fault planes and slip directions) produce ground motions at a given seismic station that appear very similar. By comparing the recorded seismic waveforms of these earthquakes, it is possible to obtain very accurate estimates of the relative arrival times of the seismic waves, and ultimately very accurate estimates of the relative locations of the earthquakes. Errors in relative location obtained using this method are tens of meters for events separated by hundreds of meter, and, after correcting for time-dependent station changes, meters for events separated by tens of meters or less. These errors are a factor of 10-100 smaller than those in the published US Geological Survey Northern California Seismic Network (NCSN) catalog. This increase in resolution has led to significant new insights in two different areas of fault studies. First, structures internal to the fault zone that previously could not be seen have been imaged. Second, because the relative location errors between the nearest microearthquakes are much smaller than the rupture dimensions (tens to hundreds of meters), this resolution allows studies of earthquake interaction using datasets of many thousands of events. In the area of fault structure, microearthquake relocation shows that the seismicity along many creeping faults is organized into slip-parallel "streaks", tens to a few hundred meters tall and up to kilometers in length. In the area of rupture interaction, it has been observed that the distribution of aftershocks of microearthquakes along the central San Andreas fault is very asymmetric, with many more of the nearest aftershocks (in space and time) occurring to the northwest of a prior mainshock than to the southeast. This is attributed to the contrast in material properties across the fault and how this contrast affects the dynamics of the mainshock (preferential propagation to the southeast). If this explanation is correct, it is relevant to seismic hazards because directivity of large earthquakes concentrates ground shaking in the direction of propagation.Current research is devoted to developing catalogs of thousands of precisely-located microearthquakes in regions of geologic interest, interpreting these catalogs in terms of their implications for fault mechanics, and improving the relocation method. More time-dependent corrections are being determined for NCSN stations and made available to the seismological community. The more precise locations obtained with the station corrections are being used to search for (1) subtle changes in fault dip associated with the observed microseismic lineations and (2) short lineations on faults believed to have little slip, to distinguish between competing ideas for the origins of the lineations. In the area of asymmetric aftershock distributions, the dataset is being extended to smaller mainshock sizes using the Parkfield HRSN catalog, the relocation code (with time-dependent station corrections) is being modified to routinely determine the local across-fault P-wave and S-wave velocity contrasts, and numerical modeling of rupture on the interface between dissimilar materials is being undertaken to test the explanation proposed for the origin of the symmetry. Finally, the statistics of aftershock sequences of microearthquakes on different faults are being compared to the predictions of analytic models of rate-and-state friction, and to numerical models of rupture interaction on planar faults. The goals are to test models of rate-and-state fault friction and to determine differences in loading rates and/or surface properties of the various faults. Because for some faults the earliest portion of the aftershock sequences might have been "lost" because of the network "blind" time following a triggering event (perhaps a few tens of seconds), the nearly 100 s archived waveforms of all cataloged events are being processed to identify and locate events that would ordinarily have triggered the network. These studies will lead to further insights into the mechanics of earthquakes.