High Resolution Studies of the Earthquake Source

这项工作的目的是获得更高的分辨率，更精确的小地震震源参数比目前可用。为了实现这一目标，正在使用比较近距离地震的技术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，但它们提供了我们对更大破坏性地震的震源过程的许多理解。许多地震成核和破裂动力学以及地震触发的模型需要对小尺度行为进行假设，例如，是否存在最小规模的地震或成核块。因此，约束这些参数的观测至关重要。迄今为止，使用包含高频信号的浅钻孔和深钻孔记录的研究表明，静态应力降在我们可以测量的最小地震中是恒定的，但单个测量值的分散性和不确定性很大。地震能量的测量是理解破裂动力学的基础，但其不确定性更大。这些不确定性严重限制了使用这样的观测约束模型的震源process.In这项研究中，新的技术被用来利用紧密定位的地震，以获得更精确的小地震的源参数比在早期的研究。首先分析的数据来自于加州Cajon Pass的深井记录，在那里用地震仪记录了1到3 km深度的当地地震，总共持续了4年。这些地震图仍然是最干净，衰减最小的小地震记录。在以前的工作中，估计了一些在2.5公里深度记录的地震的震源参数，只是为了解决长期存在的缩放争议。从那时起，更精确地测量能量和应力降，以及量化地震图的开始，对于解决地震破裂过程的基本问题的重要性变得清晰起来。加州帕克菲尔德的钻孔网络记录的地震（在那里，小地震应力降存在相当大的争议），以及巴西东北部阿库大坝的高频网络，安装用于监测地震引发的地震活动，也正在进行分析比较。在每一个地点，目录中搜索类似的地震，包括前震和余震。 对具有相同路径效应的成组地震波形和频谱进行了比较，得到了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。这些测量然后被用来改善地震破裂的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，这是任何改进灾害最小化的基础。它资助一名研究生和一名本科生助理，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.