High Resolution Studies of the Earthquake Source

这项工作的目的是获得比目前可用的更高分辨率、更精确的小地震震源参数。为了实现这一目标，使用比较近距离地震的技术来分析最高质量的现有数据，以提高相对和绝对震源参数的精度。小地震本身并不构成重大的自然灾害，但它们使我们对更大破坏性地震的震源过程有了更多的了解。许多地震成核和破裂动力学模型以及地震触发都需要对小尺度行为的假设，例如，是否存在最小规模的地震或成核斑块。因此，约束这些参数的观测是至关重要的。迄今为止，使用含有高频信号的浅孔和深孔记录的研究表明，静态应力降是恒定的，直到我们可以测量到的最小地震，但单个测量中的分散和不确定性很大。地震能量的测量是理解破裂动力学的基础，但它具有更大的不确定性。这些不确定性严重限制了利用这些观测来约束震源过程模型。在这项研究中，使用了新技术，利用近距离地震的优势，获得比早期研究更精确的小地震震源参数。首先分析的数据来自加利福尼亚Cajon Pass的深钻孔记录，在那里用地震仪记录了1到3公里深度的地震，总共持续了四年。这些地震记录仍然是最清晰、衰减最小的小地震记录。在以前的工作中，震源参数估计了一些记录在2.5公里深度的地震，只是为了解决长期存在的尺度争议。从那时起，对于解决有关地震破裂过程的基本问题来说，更精确地测量能量和应力下降，以及对地震记录的开始进行量化的重要性已经变得清晰起来。加州帕克菲尔德的钻孔网络记录的地震（在那里，关于小地震应力下降存在相当大的争议）和巴西东北部阿库大坝的高频网络记录的地震也正在进行分析比较。阿库大坝是为了监测水库诱发的地震活动而安装的。在每个地点，都要搜索类似地震的目录，包括前震和余震。将假定具有相同路径效应的群集地震的波形和频谱进行比较，以获得高分辨率的相对震源参数。还对波形的起始点进行了详细的研究，以调查地震成核过程，并确定，例如，它是否随震级而不同。这些测量结果可用于改进地震破裂成核和动力学模型的约束条件。这项工作将提高我们对震源过程的理解，这是最小化危险的任何改进的基础。它资助一名研究生和本科生助理，以加强教育。

The aim of this work is to obtain higher resolution, more precise source parameters of small earthquakes than are currently available. To accomplish this, the highest quality existing data are being analyzed using techniques that compare closely located earthquakes to improve the precision of both relative and absolute source parameters. Small earthquakes do not in themselves present a major natural hazard, but they provide much of our understanding of the source processes of larger damaging earthquakes. Many models of earthquake nucleation and rupture dynamics, and earthquake triggering require assumptions about small-scale behavior, for example, whether there is a minimum sized earthquake or nucleation patch. Observations to constrain these parameters are therefore critical. To date, studies using shallow and deep borehole recordings containing high frequency signals have shown that the static stress drop is constant down to the smallest earthquakes we can measure, but the scatter and uncertainty in the individual measurements are large. Measurements of seismic energy, which are fundamental to understanding the rupture dynamics have even larger uncertainties. These uncertainties severely limit the use of such observations to constrain models of the earthquake source process.In this study, new techniques are used that take advantage of closely-located earthquakes to obtain significantly more precise source parameters of small earthquakes than in earlier studies. The data analyzed first are from the deep borehole recording at Cajon Pass, California where local earthquakes were recorded with seismometers between 1 and 3 km depth over a total of four years. These seismograms remain the cleanest, least attenuated recordings of small earthquakes available. In previous work source parameters were estimated for some of the earthquakes recorded at 2.5 km depth, simply to address long standing scaling controversies. Since then, the importance of more precise measurements of energy and stress drops, as well as quantification of the onset of the seismograms has become clear for solving fundamental questions about earthquake rupture processes. Earthquakes recorded by the borehole network at Parkfield, California, (where there is considerable controversy about small earthquake stress drops), and a high frequency network at Acu Dam, northeastern Brazil, installed to monitor reservoir-induced seismicity, are also being analyzed for comparison. At each location, the catalogues are searched for similar earthquakes, including foreshocks and aftershocks. The waveforms and spectra of the clustered earthquakes, which can be assumed to have identical path effects, are compared to obtain high-resolution relative source parameters. Detailed study of the onsets of the waveforms is also performed to investigate the earthquake nucleation process and determine, for example, whether it differs with magnitude. These measurements are then used to improve the constraints on models of the nucleation and dynamics of earthquake rupture.This work will improve our understanding of the earthquake source process, which is fundamental to any improvements in hazard minimization. It funds a graduate student and also undergraduate assistants to enhance education.