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

从微地震精确相对位置研究断层组构和地震力学

• 批准号: 0126184
• 负责人: Allan Rubin
• 金额: $ 26.5万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-01-01 至 2005-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0126184&HistoricalAwards=false
• 关键词: Studies; Fault; Fabrics; Earthquake; Mechanics

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.

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。 通过比较这些地震记录的地震波形，可以获得对地震波相对到达时间的非常准确的估计，并最终获得对地震相对位置的非常准确的估计。 用这种方法得到的相对位置误差，对于相隔几百米的地震，误差可达几十米，对于相隔几十米或更短的地震，经台站随时间变化的校正后，误差可达几十米，比美国地质调查局北方加州地震台网（NCSN）公布的地震目录的相对位置误差小10-100倍。 这种分辨率的提高在断层研究的两个不同领域带来了重要的新见解。 首先，以前无法看到的断层带内部结构已经成像。 其次，由于最近的微震之间的相对位置误差远小于破裂尺寸（数十至数百米），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。 在断裂构造区，微震重定位显示，沿沿着许多蠕动断层的地震活动被组织成平行滑动的“条带”，高几十米到几百米，长可达千米。 在破裂相互作用区，已经观察到，沿着中央圣安德烈亚斯断层沿着的微震余震分布是非常不对称的，在空间和时间上，最近的余震更多地发生在先前主震的西北部，而不是东南部。 这归因于断层两侧物质性质的差异以及这种差异如何影响主震的动力学（优先向东南方向传播）。 如果这种解释是正确的，它是有关地震的危险，因为大地震的方向性集中在地面震动的传播方向。目前的研究致力于开发目录的数千个精确定位的微震在地质利益的地区，解释这些目录的断层力学的含义，并改善重新定位的方法。 正在为NCSN台站确定更多的时间相关校正，并提供给地震学界。 通过台站校正获得的更精确的位置被用于寻找（1）与观测到的微震线理有关的断层倾角的细微变化，以及（2）据信几乎没有滑动的断层上的短线理，以区分线理起源的竞争观点。 在非对称余震分布区域，使用Parkfield HRSN目录，重新定位代码，（具有随时间变化的台站校正）正在进行修改，以常规确定局部跨断层P波和S波速度对比，正在进行不同材料之间界面断裂的数值模拟，以检验对对称性 最后，统计的余震序列的微震不同故障进行了比较的速率和状态摩擦的分析模型的预测，和破裂的相互作用的数值模型的平面断层。 目标是测试模型的速率和状态故障摩擦，并确定不同的故障加载速率和/或表面特性的差异。 因为对于某些故障，余震序列的最早部分可能由于触发事件之后的网络“盲”时间（可能是几十秒）而“丢失”，所以正在处理所有编目事件的近100秒存档波形，以识别和定位通常会触发网络的事件。 这些研究将导致对地震力学的进一步了解。