Collaborative Research: Imaging the Continental Lithosphere with Earthquake Sources

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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.