CAREER: Stress, deformation and seismicity in continental interiors: Testing the "weak zone" hypothesis

大多数地震都发生在构造板块的边缘。然而，大陆内部偶尔会发生破坏性的大地震。地震发生在远离构造板块边界的地方的原因还不是很清楚。人们提出了几种可能的解释，这些解释已经争论了几十年。在这里，研究人员验证了一种主要的解释：大陆内陆的“薄弱地带”可能会导致应力积累，从而导致地震。为了探索这种可能性，他们使用了远距离地震产生的地震波的记录。这些波在地壳和地幔中传播。它们被用来研究地球内部，就像超声波用于医学成像一样。该团队调查了澳大利亚和美国可能存在薄弱地带的几个地区。它分析记录以确定地震波是否在穿过地球的路径上损失了能量。事实上，当波通过弱区时，它们会损失大量可以被探测到的能量。这使得可以绘制出可能的薄弱地带的位置，从而在大陆内部应力聚集的地方。这些数据被用作地球动力学模型的输入，该模型再现了大陆板块和薄弱地带的力学。为了测试这些模型，使用统计分析工具将它们的输出--即模拟的地震位置和特征--与真实地震的记录进行比较。该项目为一位早期职业科学家提供了支持，该科学家来自《科学》杂志中代表性不足的群体。它还为明尼苏达大学双子城分校的博士后助理和研究生提供培训。这项研究的各个方面与一个教育组成部分相结合，该组成部分使用声音和音乐来介绍信号分析和地球成像的概念。研究人员从横向流变变化的角度来测试“薄弱地带”假说。这项工作包括三个阶段：1)地震成像(主要是远震衰减)将限制研究地区粘性强度的横向变化；2)地球动力学模拟将确定在给定区域应力场的情况下，成像的强度异常将在哪里导致偏应力增强；3)Molchan误差分析将定量测量预期偏应力预测孕震区的能力。第三阶段将包括对结果进行统计意义检验。模型预测的主应力的相对大小也将与观测值进行比较。研究地区包括澳大利亚、美国东部和美国中南部。选择目标地区是因为可以获得必要的数据。初步成像还表明，这些地区的地震活动符合弱带假说。对于美国中南部地区，重点是诱发地震活动；团队在那里测试薄弱地带的存在是否可以解释俄克拉荷马州诱发地震活动的不成比例的高发生率。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。

Most earthquakes happen around the edges of tectonic plates. Yet, large and damaging earthquakes occasionally happen in the continental interiors. The reasons why earthquakes occur away from tectonic plate boundaries is not well understood. Several possible explanations have been proposed and they have been debated for decades. Here, the researchers test one of the leading explanations: "weak zones" in the continental interior may lead to the buildup of stress, resulting in earthquakes. To explore this possibility, they use records of seismic waves produced by distant earthquakes. These waves propagate through the Earth’s crust and mantle. They are used to investigate Earth’s interior in a similar way that sonography is used in medical imaging. The team investigates several regions where weak zones may exist, in Australia and the United States. It analyzes records to determine if seismic waves have lost energy on their path through the Earth. Indeed, when the waves pass through a weak zone, they lose a significant amount of energy which can be detected. This allows mapping the locations of possible weak zones, hence where stresses build up in continental interiors. These data are used as input for geodynamic models which reproduce the mechanics of continental plates and weak zones. To test the models, their outputs – i.e., modeled earthquake locations and characteristics - are compared to records of real earthquakes using statistical analysis tools. The project provides support for an early career scientist from a group underrepresented in Science. It also provides training for a postdoctoral associate and a graduate student at the University of Minnesota – Twin Cities. Aspects of this research are integrated with an educational component that uses sound and music to introduce concepts in signal analysis and Earth imaging.The researchers test the "weak zone" hypothesis in terms of lateral rheological variations. The work includes three phases: 1) Seismic imaging (primarily teleseismic attenuation) will constrain lateral variations in viscous strength in the studied areas; 2) Geodynamic modeling will determine where the imaged strength anomalies would lead to enhanced deviatoric stresses, given the regional stress field; 3) Molchan error analysis will quantitatively measure the ability of the expected deviatoric stresses to predict the seismogenic regions. Phase 3 will include testing of the results for statistical significance. The relative magnitudes of the principal stresses predicted by the modeling will also be compared to observations. The studied regions include Australia, the Eastern U.S. and the South-Central U.S. Target areas were chosen because of the availability of the requisite data. Preliminary imaging also shows that seismicity is consistent with the weak zone hypothesis in these regions. For the South-Central U.S., the focus is on induced seismicity; the team test there whether the existence of a weak zone can explain the disproportionately high occurrence of induced seismicity in Oklahoma.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.