The study will undertake a systematic search for very low frequency earthquakes (VLFE), along the entire margin of Cascadia subduction zone, from the trench to the down-dip edge of the transition zone. The sources of VLFEs will be located and characterized to better understand the physics of fault slip, particularly at the edges of the locked zone. The study will better characterize the nature of seismic radiation and their spatiotemporal variability along the fault zone in the crust. It is also holed that it will improve our understanding of the full spectrum of the seismic radiation during slow slip episodes, physical mechanism governing slow earthquakes, and their relationship with regular seismicity. It may also shed light on the implications of the slow seismic activities on the nucleation of large damaging earthquakes. The study will form an early career research project for a new postdoc under the mentorsuip of the PI.
Broader impacts include support of career development of a new young investigator. The study will examine the seismicity of a subduction zone that can potentially produce great damaging earthquakes. It reflects some of the major goals of the NSF-GeoPRISMS and the Cascadia Initiative, and makes good use of the Amphibious Array. Results from this study may help in planning focused interdisciplinary experiments, and guide the selection of target areas for future research.
Very low frequency earthquakes (VLFEs) are enigmatic shallow seismic signals that have previously been found in Japan and associated with slow slip on subducting plates. In this project we used seismic data from the Northwest United States to detect these events offshore North America for the first time. We focused on a period of known episodic tremor and slip in August 2011 and searched over all the available data in the Puget Bay area. We robustly detected five very low frequency earthquakes. All have slip orientations consistent with the long-term plate motion. Interestingly, the very-low frequency earthquakes migrated along the plate boundary with the tremor (See Primary Figure). The observations suggest that both are manifestations of the same slip phenomenon and very low frequency earthquakes have a potential to be an additional window into the evolution of stress on this plate boundary. In addition to the research goals, this postdoctoral fellowship was designed to facilitate the career development of Abhijit Ghosh, a recent PhD. Dr. Ghosh expanded his skill set to include inverting for moment tensors and was introduced to an analogous field site in the Aleutian Islands during the course of the fellowship. The connection to the Aleutians has instigated a new, currently funded major collaboration with Alaska Volcano Observatory. More significantly, Dr. Ghosh has built on this experience to move on to an Assistant Professor position at the University of California, Riverside where he supported his first postdoc, Eduardo Huesca, from this grant. Graduate student Stephanie Taylor at UC Santa Cruz also supported the Cascadia data analysis.
