Linking seismological observables and dynamic simulations of microseismicity to constrain models and improve observations

地震通常是在构造板块运动的作用下，在地球内部预先存在的断层上快速滑动或动态破裂而产生的。虽然大的破坏性破裂很少发生，但通常被称为微震活动的小得多的事件发生得更频繁，其中绝大多数人感觉不到。然而，由这些小得多的地震引起的地表运动可以被称为地震仪的敏感仪器记录下来，并进行分析以揭示它们的位置和性质。事实上，微震活动为我们了解地壳的应力环境、结构和物理性质及其随时间的变化提供了一个独特的窗口。例如，在加利福尼亚州，大多数微震活动发生在预计会发生大型破坏性事件的同一深度范围内。然而，目前记录的微震活动的地面运动是基于几十年前相对简单的震源模型进行分析的。最近在计算、实验室实验和观测方面的快速进展使我们能够创建更现实的数值模型，来解释这些微震破裂可能如何发生。这项研究将使用新的微震活动数值模型来增进对地壳性质的了解和改进，这些信息是通过微震活动研究的重要窗口获得的，同时促进我们对断层力学和微震活动的现实来源的理解。由此产生的对地震物理的洞察将有助于基于物理的地震危险性分析的模拟能力。该项目将通过产生和分析其动力学模型的合成远场地震图，促进对几种科学和实用上重要的微震源类型的理解，如前震事件、重复地震、非均质斑块破裂和流体诱发事件。其目标是：(1)确定合成谱与以简单模型为基础的目前使用的表达式的吻合程度，利用现有的地震学方法确定震源参数，并研究它们与可从这些模型确定的实际震源性质之间的关系；(2)寻找表明有关震源比基本地震学假设更为复杂的地震学特征，并可能区分几种特征类型的震源；(3)研究是否有可能利用更符合所确定的具体地震学特征的模型，为这类震源建立震源参数估计的替代办法。这项研究将检查不同类型的源是否可以与当前使用的频谱拟合表达式中的一组典型参数相关联，或者其中一些是否需要更高级的方法，如双角频率函数。它还将研究不同类型震源在焦球上的光谱特性如何变化，并考虑P波和S波谱之间的差异，不同类型的震源可能会有所不同。这项研究将潜在地使观测科学家能够在具有合适的台站覆盖范围、结构知识和数据质量的地区进行更详细的地震学研究。拟议活动的结果将广泛传播。一些研究生和本科生，包括来自科学界代表性不足的群体，将在开展拟议的跨学科活动方面获得宝贵的研究和教学经验。此外，更多的学生将通过研究人员教授的两门相关课程的课程项目来了解这项研究中涉及的问题。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Earthquakes are typically generated by rapid slip, or dynamic rupture, on pre-existing faults inside the Earth, loaded by tectonic plate motion. While large, destructive ruptures occur only rarely, much smaller events, often called microseismicity, occur much more often, with the vast majority of them not felt by people. However, the surface motion due to these much smaller events can be recorded by sensitive instruments called seismometers and analyzed to reveal their location and properties. In fact, microseismicity provides us with a unique window into the stress environment, structure, and physical properties of the Earth's crust and their changes over time. In California, for example, most of microseismicity occurs in the same depth range that is expected to host large, destructive events. However, the ground motions recorded for the microseismicity are currently analyzed based on relatively simple models of earthquake sources from several decades ago. The rapid recent progress in computing, laboratory experiments, and observations has enabled us to create much more realistic numerical models of how these microearthquake ruptures might occur. This study will use new numerical models of microseismicity to advance understanding and improve information about the properties of the crust, gained through the important window of microseismicity studies, while advancing our understanding of fault mechanics and of realistic sources of microseismicity. The resulting insight into earthquake physics will contribute to the physics-based simulation capability for seismic hazard analysis. These results may benefit the society at large.The project will advance understanding of several scientifically and practically important types of microearthquake sources, such as foreshock-like events, repeating earthquakes, ruptures of heterogeneous patches, and fluid-injection-induced events, by producing and analyzing the synthetic far-field seismograms of their dynamic models. The goals are to (i) determine how well the synthetic spectra are fit with the currently used expressions based on the simple models, determine source parameters using the existing seismological approaches, and study how they relate to the actual source properties that can be determine from these models; (ii) look for seismological signatures that would indicate that the sources in question are more complex than the basic seismological assumptions, and potentially distinguish between several characteristic types of sources; and (iii) investigate the possibility of creating alternative approaches to seismological source-parameter estimates for such sources, using models more consistent with the specific seismological signatures identified. The study will examine whether different types of sources can be associated with a typical set of parameters in the currently used spectral fitting expressions, or whether some of them require more advanced approaches such as two-corner-frequency functions. It will also look at how the spectral properties for different types of sources vary over the focal sphere and consider differences between P- and S-wave spectra, which may be different for different types of sources. The research will potentially enable observational scientists to conduct more detailed seismological studies in areas with suitable station coverage, knowledge of the structure, and data quality. The results of the proposed activities will be broadly disseminated. Several graduate and undergraduate students, including from groups that are underrepresented in science, will gain valuable research and teaching experience carrying out the proposed interdisciplinary activities. In addition, a wider group of students will be educated about the issues involved in this research through course projects in two relevant courses taught by the researcher.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.