Constraining earthquake stress drop and mantle attenuation from teleseismic body-wave spectra

大多数地震发生在板块构造汇聚带内。发生在深度大于50公里（30英里）的地震是由于沉入地幔的海洋板块的破裂事件造成的。这些深层地震的特征与浅层地震相似，但又有所不同。观测结果表明，深地震和浅地震一样，都伴随着沿平面断层的摩擦滑动。然而，目前还不清楚断层是如何在发震地幔中普遍存在的极端压力下断裂的，这种压力高达大气压力的25万倍。因此，造成深地震的机制已经争论了几十年。在这里，研究人员使用新的地震波分析来研究这些机制。地震波穿过地幔和地壳，远离产生它们的地震。当它们到达地表时，它们可以用来调查地球深处的事件。在这里，研究小组分析了波谱，类似于天文学家分析恒星的电磁波谱。它验证了深地震以高破裂速度破坏小断层面的假设。研究人员记录了地壳（浅深度）和地幔（深深度）断层剪切应力状态的差异。该项目利用国家对地震基础设施的投资，如IRIS全球地震台网（GSN）和Earthscope USArray。它为一名早期职业女性科学家和一名女研究生提供支持。它还为密歇根大学安娜堡分校的几名本科生提供地震学、地球物理学、科学计算和大数据分析方面的培训。在这里，地震破裂过程的分析与地震波的传播是紧密联系在一起的，因为震源的复杂性和沿波路径岩石的非均质性都形成了地震波谱。该团队采用了一种新的两步谱比方法，旨在最佳地分离远震纵波和s波谱中的源和传播效应。利用长达40年的IRIS数据库，研究人员选择了附近的地震对，以确定地壳和地幔中数百次大地震的破裂持续时间和应力下降。通过对GSN和USArray的地面运动记录，他们估计地震波的衰减和散射如何影响波谱。通过在这两个步骤中一致地处理频谱，他们可以比较震源特征，并用稳健的统计数据绘制应力降随地震深度、震级和构造板块边界变化的图表。此外，他们量化了沿几条传播良好的路径的波传播，以增加对地球上波散射和衰减变化的新限制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Most earthquakes occur within plate-tectonic convergence zones. Earthquakes which occur at depths larger than ~50 km (30 miles) result from rupture events in oceanic plates that are sinking into the Earth’s mantle. These deep earthquakes have similar characteristics than shallow earthquakes, yet they are different. Observations suggest that deep earthquakes involve frictional sliding along planar faults, as the shallow ones. Yet it is still unclear how faults can break at the extreme pressure prevailing in the seismogenic mantle, reaching up to 250,000 times the atmospheric pressure. The mechanisms responsible for deep earthquakes have thus been debated for decades. Here, the researchers investigate these mechanisms using new analyses of seismic waves. Seismic waves travel away from the earthquakes that generated them, through the Earth’s mantle and crust. When they reach the surface, they can be used to investigate events in the deep Earth. Here, the team analyze the wave spectra, analogous to how astronomers analyze the electromagnetic spectra of stars. It tests the hypothesis that deep earthquakes rupture small fault planes with high rupture speeds. The researchers document the differences in shear-stress states of faults in the crust (at shallow depths) and in the mantle (at large depths). This project leverages national investments in seismological infrastructure, such as the IRIS Global Seismic Network (GSN) and the Earthscope USArray. It provides support for an early career female scientist and a female graduate student. It also provides training in seismology, geophysics, scientific computing, and big-data analytics for several undergraduate students at University of Michigan - Ann Arbor. Here, the analysis of the earthquake rupture process and seismic wave propagation are tightly linked, as both the earthquake source complexity and the heterogeneous properties of rock along the wave paths shape seismic wave spectra. The team follows a new two-step spectral-ratio approach aimed at optimally separating the source and propagation effects in teleseismic P-wave and S-wave spectra. Using the four-decade long IRIS database, the researchers select pairs of nearby earthquakes to determine rupture durations and stress drops of several hundred large earthquakes in the crust and mantle. From pairs of GSN and USArray ground-motion recordings, they estimate how seismic wave attenuation and scattering affect wave spectra. By processing the spectra consistently in both steps, they can compare the source characteristics and chart with robust statistics how stress drop varies with earthquake depth, magnitude, and tectonic plate boundary. Furthermore, they quantify wave propagation along several well-travelled paths to add new constraints on variations of wave scattering and attenuation in the Earth.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.