Collaborative Research: Imaging the Continental Lithosphere with Earthquake Sources

该项目的主要主题是利用远处地震的地震体波作为照明源，对大陆岩石圈的地下特征进行成像。通过这样做，首席调查人员正在将地震成像的范围扩大到甚至连最厚的大陆地壳到地幔岩石圈的深处，同时保持良好的分辨率，以解决一级地质问题。他们正在验证和验证两项新技术：1)一种消除(去卷积)震源引起的复杂情况的新方法；2)通过一种称为“迁移”的过程，将数据转换成距离和深度上正确维度的图像。第一种方法保留了地震数据的所有三个组成部分--否则很难实现这一壮举。为了进行迁移，他们使用所谓的高斯光束方法来成像横向变化的复杂地质结构。为了调查大陆岩石圈的关键目标，他们从卡斯卡迪亚活动大陆边缘的数据开始。这一高质量的数据集是公共领域的，已经成为使用其他技术的几项研究的主题，因此是一个自然的基准。这项研究的主要重点是最近记录的一组数据集，该数据集来自喜马拉雅-西藏造山带--世界上最高和最大的活动大陆-大陆碰撞带--的非常令人兴奋的地震剖面。去年年底，陈刚刚结束了三年的野外工作；在平均海拔5000米的地方。在近800公里的距离内，高攀登阵列覆盖了喜马拉雅山脉通过西藏中部的所有主要构造单元。尽管有许多障碍，但还是实现了密集的站点间距，即使在高喜马拉雅山脉上也是如此，距离最近可达3公里，但从未超过8公里。此外，许多车站位于西藏最偏远的地区，背景噪音非常低。Hi-Rate数据集还没有进入公共领域，他们希望通过将前所未有的数据集与新技术相匹配，在最宏伟的碰撞带的深层解剖方面产生及时、令人兴奋的结果。为了进行比较研究，还确定了两个潜在的次要目标。在每一种情况下，大规模的地质环境都类似于主要目标之一，并从宽带阵列收集了高质量的数据。研究人员正在为理解超大陆是如何通过大陆碰撞而组装的做出了根本性的贡献。高攀登阵列总共部署了220多次，是迄今为止范围最广的宽带地震实验。在不久的将来，这个数据集只会因为美国地球望远镜阵列的完成而黯然失色。为此，这项工作是及时的，因为预计会有其他数据集来探测大陆岩石圈。特别是，美国阵列的移动部件有可能调查北美大陆的全谱地质环境。随着现在可以从密集分布的宽带地震阵列获得数据，曾经严格属于勘探地震学领域的进展现在可以被纳入使用震源的地震成像技术中。为此，该项目中的技术处于研究的前沿：事实上，只有地震源可以同时满足两个严格的要求：1)照明源必须足够强，足以穿透整个岩石圈；2)成本和后勤必须在学术环境中负担得起。该项目预计将产生广泛的社会影响。一个自然产物是喜马拉雅地震带的改进图像--世界上一个主要的地震孕育断层系统，那里居住着数百万人，包括阿富汗、巴基斯坦、印度、尼泊尔和孟加拉国的一大部分人口。对喜马拉雅中部孕震构造的更好成像为在地质灾害具有全球影响的区域进行地震危险性分析提供了基本限制。此外，正在努力让本科生和研究生中的多文化和少数民族学生参与进来，并将这项工作纳入本科生和研究生教学以及公共宣传方案。就技术进步而言，该项目纳入了最初由石油行业为勘探能源而开发的地震成像技术。随着工作的进展，预计工业和学术研究之间的互动和相互反馈将会增加。

The main theme of the project is to image subsurface features of the continental lithosphere by using seismic body-waves from distant earthquakes as the source of illumination. In doing so, the Principal Investigators are extending the range of seismic imaging to depths below even the thickest continental crust into the mantle lithosphere, and at the same time maintaining good resolution to address first-order geologic problems. They are verifying and validating two new techniques: 1) A novel approach to remove (deconvolve) complications arising from earthquake sources; and 2) through a process called "migration", to transform data into images of correct dimensions in distance and depth. The first method preserves all three components of seismic data -- a feat otherwise difficult to achieve. For migration, they are using the so-called Gaussian beams approach to image laterally varying, complex geologic structures. To investigate key targets of the continental lithosphere, they are starting with data from the Cascadia active continental margin. This high-quality dataset is in the public domain and has been the subject of several studies using other techniques, thus is a natural benchmark. The primary focus of the research is to a recently recorded dataset from a very exciting seismic profile ("Hi-CLIMB") across the Himalayan-Tibetan orogen -- the highest and the largest active continent-continent collision zone in the world. Chen just concluded three years of field work late last year; at elevations averaging 5,000 meters above sea-level. Over a distance of almost 800 km, the Hi-CLIMB array covers all major tectonic units of the Himalayas through central Tibet. Despite numerous obstacles, a dense station-spacing was achieved, as close as 3 km and never over 8 km, even over the high Himalayas. Furthermore, many stations are in the most remote part of Tibet where background noise is exceedingly low. The Hi-CLIMB dataset is not yet in the public domain and they expect to produce timely, exciting results regarding the deep-seated anatomy of the most majestic collision zone by matching an unprecedented dataset with new techniques. For comparative studies, two potential secondary targets are also identified. In each case, the large-scale geological setting is analogous to one of the primary targets and high-quality data from broadband arrays have been collected. The researchers are making fundamental contributions to understanding how super-continents are assembled through continental collision. With a total of over 220 deployments, the Hi-CLIMB array is the most extensive broadband seismic experiment to date. In the near future, this dataset will only be eclipsed by the completion of the USArray of EarthScope. To this end, the work is timely in anticipation of other datasets to probe the continental lithosphere. In particular, the mobile component of the USArray has the potential of investigating a full spectrum of geologic settings over the North American continent.With data now available from densely spaced, broadband seismic arrays, advances that were once strictly in the domain of exploration seismology can now be incorporated into seismic imaging techniques using earthquake sources. To this end, the techniques in this project are at the leading edge of research: Indeed, only earthquake sources can satisfy two stringent requirements simultaneously: 1) The source of illumination must be strong enough to penetrate the entire lithosphere; and 2) the cost and logistics must be affordable in an academic setting.The project is expected to have broad societal impacts. One natural product is the improved images of the Himalayan seismic belt -- a leading seismogenic fault system of the world where millions of people reside, including a large fraction of population in Afghanistan, Pakistan, India, Nepal and Bangladesh. Better imaging of seismogenic structures in the central Himalayas provides essential constraints for seismic hazard analysis in a region where geologic disasters have global repercussions. Furthermore, efforts are being made to involve multi-cultural and minority students at both the undergraduate and graduate levels, as well as the inclusion of this work into undergraduate and graduate teaching, and public outreach programs. In terms of technical advances, the project incorporates aspects of seismic imaging originally developed by the petroleum industry for exploration of energy resources. As the work progresses, increased interactions and mutual feedback are expected between industrial and academic research.