Collaborative Research: Imaging the Continental Lithosphere with Earthquake Sources

该项目的主题是利用遥远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。在这样做的过程中，首席研究人员将地震成像的范围扩展到甚至最厚的大陆地壳以下的深度，进入地幔岩石圈，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)一种消除地震源引起的（反卷积）并发症的新方法；2)通过“迁移”过程，将数据在距离和深度上转换成正确尺寸的图像。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。对于迁移，他们正在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。为了研究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。这个高质量的数据集属于公共领域，并且已经使用其他技术进行了几项研究，因此是一个自然的基准。研究的主要重点是最近记录的数据集，这些数据来自一个非常令人兴奋的地震剖面（“Hi-CLIMB”），横跨喜马拉雅-西藏造山带——世界上最高、最大的活跃大陆-大陆碰撞带。去年年底，陈刚刚结束了为期三年的野外工作；在平均海拔5000米以上的地方。在近800公里的距离上，Hi-CLIMB阵列覆盖了穿过西藏中部的喜马拉雅山脉的所有主要构造单元。尽管有许多障碍，但仍然实现了密集的站距，接近3公里，从未超过8公里，即使在高喜马拉雅山上也是如此。此外，许多台站位于西藏最偏远的地区，那里的背景噪音非常低。Hi-CLIMB数据集尚未进入公共领域，他们希望通过将前所未有的数据集与新技术相匹配，及时产生有关最宏伟碰撞区域深层解剖的令人兴奋的结果。为了进行比较研究，还确定了两个潜在的次要目标。在每种情况下，大规模的地质环境都类似于主要目标之一，并且从宽带阵列收集了高质量的数据。研究人员正在为理解超级大陆是如何通过大陆碰撞形成的做出根本性贡献。Hi-CLIMB阵列共部署了220多个，是迄今为止最广泛的宽带地震实验。在不久的将来，这个数据集只会因为USArray of EarthScope的完成而黯然失色。为此，这项工作是及时的，预计其他数据集探测大陆岩石圈。特别是，USArray的移动组件具有调查北美大陆全谱地质环境的潜力。由于现在可以从密集的宽带地震阵列获得数据，曾经严格限于勘探地震学领域的进展现在可以结合到使用震源的地震成像技术中。为此，本项目的技术处于研究的前沿：事实上，只有震源才能同时满足两个严格的要求：1)光源必须足够强，能够穿透整个岩石圈；2)费用和后勤必须在学术环境中负担得起。预计该项目将产生广泛的社会影响。其中一个自然产物是喜马拉雅地震带的改进图像。喜马拉雅地震带是世界上主要的发震断层系统，数百万人居住在这里，其中包括阿富汗、巴基斯坦、印度、尼泊尔和孟加拉国的很大一部分人口。在一个地质灾害具有全球影响的地区，更好地成像喜马拉雅山中部的发震构造为地震危险性分析提供了必要的限制。此外，正在努力让多文化和少数民族学生在本科和研究生阶段都参与进来，并将这项工作纳入本科和研究生教学以及公共宣传项目。在技术进步方面，该项目结合了石油工业最初为勘探能源而开发的地震成像技术。随着工作的进展，预计产学研之间将增加互动和相互反馈。

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.