Towards the Understanding of Deep Crustal Faulting and Fluid Movement through the Analysis of Long Period Earthquakes at Clearlake, CA

火山对人类社会构成严重威胁，但也可能是地热能的来源。清湖火山田是位于加州的一个火山蒸汽田。它拥有许多地热发电厂。虽然它已经有11000年没有喷发了，但它显示出火山型地震活动，如震颤和长周期（LP）事件。强震事件是由流经地壳裂缝和断层的流体产生的。研究这些事件可以更好地理解流体运动和断层动力学。这在经济上和更好地评估地震和火山灾害方面都很重要。然而，来自LP事件的地震信号很小，很难检测和分析。在这里，研究人员调查了清湖火山场的这些事件。他们使用地震学，研究地震产生的地震波并在地球上传播。这些波在地表被探测并分析。研究人员利用现有的数据。他们结合了最先进的分析技术，以更好地识别和定位LP事件。他们研究地震事件的动态特性（应力下降），它们的震级，如果可能的话，研究产生这些地震的震源机制。改进的LP事件目录、高分辨率位置和震源机制用于研究该地区的流体输送系统。该项目的成果——包括改进LP事件分析的方法和发展混合方法——可以应用于其他火山系统。这项为期一年的计划亦支持学术界与业界的合作，以及向公众推广教育。这项研究是基于现有的地震波形数据目录，这些数据已显示包括LP事件。这些事件与圣安德烈亚斯断层系统其他地方的非火山震动和低频地震进行了比较。研究人员使用自适应波形模板匹配搜索连续波形数据集，以识别与目录中已有事件相关的LP事件。他们开发了基于波形相似性的新模板，以进一步查询数据并检测和编录新的LP事件。他们利用双差定位法编制了一份高分辨率的目录。利用谱建模方法对探测到的地震事件进行分析，估计标量地震矩和角频率；目的是评估中低压地震可能的尺度特性。该团队还利用三分量波形数据从矩张量反演中确定震源参数。LP事件很小，很难建模，因此需要努力改进方法和开发混合方法，以提取更可靠的结果。由于其量级较小，研究人员需要探索综合反演方案，利用波形和第一运动极性的源型反演，可能将方法扩展到利用体波振幅比。多管齐下的分析结果将用于研究清湖火山田的潜在断裂结构和流体输送系统。它还将允许评估是否有必要继续进行便携式阵列部署，以提高覆盖范围，以更好地调查中低压地震的地震活动性。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。

Volcanos pose serious threats to human societies yet can be sources of geothermal energy. Clear Lake Volcanic Field is a volcanic steam field located in California. It hosts numerous geothermal power plants. Although it has not erupted for 11,000 years, it exhibits volcanic-type seismic activity such as tremors and long-period (LP) events. LP events are produced by fluids surging through cracks and faults in the Earth’s crust. Studying these events allows better understanding fluid movement and fault dynamics. This is important economically and to better assess seismic and volcanic hazards. However, seismic signals from LP events are small and challenging to detect and analyze. Here the researchers investigate these events at Clear Lake Volcanic Field. They use seismology which is the study of seismic waves generated by earthquakes and travelling through the Earth. The waves are detected at the surface and analyzed. The researchers leverage on existing data. They combine state-of-the-art analytical techniques to better identify and locate the LP events. They examine the dynamic properties of the events (stress drops), their scaling with magnitude, and when possible the focal mechanisms of the earthquakes that produced them. The improved catalog of LP events, high-resolution locations, and focal mechanisms is used to investigate the fluid transport system in the region. The project outcomes – which include refined methods and development of hybrid approaches to improve LP event analysis – could be applied at other volcanic systems. The one-year project also supports a collaboration between academia and the industry, and educational outreach to the public. The study is based on existing catalog of seismic waveform data that have shown to include LP events. These events are compared to non-volcanic tremor and low-frequency earthquakes at other places along the San Andreas fault system. The researchers search the continuous waveform data set using adaptive waveform template matching to identify LP events related to events already in the catalog. They develop new templates - based on waveform similarity - to further interrogate the data and detect and catalog new LP events. They use the double-difference location method to develop a high-resolution catalog. The detected events are analyzed using spectral modeling methods to estimate the scalar seismic moment and corner frequency; the goal is to assess possible scaling properties of the LP earthquakes. The team also utilizes three-component waveform data to determine focal parameters from moment tensor inversion. LP events are small and challenging to model and therefore require efforts toward the refinement of methods and the development of hybrid approaches to extract more reliable results. Because of their small magnitude, the researchers need to explore combined inversion scheme that utilizes a source-type inversion of both waveforms and first-motion polarities, possibly expanding the method to utilize body wave amplitude ratios. The results of the multipronged analysis will be used to investigate potential fault structure and the fluid transport system of the Clear Lake Volcanic Field. It will also allow evaluating whether continued study is warranted with a possible portable array deployment to improve coverage to better investigate LP earthquake seismicity.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.