Collaborative Research: Deployment of Seafloor Optical Fiber Strainmeters for the Detection of Slow Slip Events

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Convective forces deep below Earth’s surface cause tectonic plates (continents and seafloor) to slowly move at many centimeters (several inches) each year. In places, the plate boundaries must slide past one another to accommodate this motion. Sometimes this occurs smoothly, but sometimes the plate edges are stuck together by friction and don't slide at all until enough stress builds to the point where the plates slip past each other suddenly in an earthquake. Over the past two decades, a different form of tectonic boundary slip has been studied in which built up stress is relaxed episodically, as in an earthquake, but at a much slower rate taking days, weeks, months, or even years to gradually slip. These events are called "slow slip events" or "slow earthquakes." Because the motions are gradual they do not generate seismic shaking (as normal earthquakes do), making them far less dangerous. However, slow slip events occurring near areas that are frictionally stuck may trigger large earthquakes, a phenomenon that requires further study. This study focuses on searching for slow slip events in the offshore, shallow part of the Cascadia subduction zone, which lies offshore the western United States stretching from northern California to north of the Canadian border. Here an oceanic tectonic plate is colliding with North America, producing very large destructive earthquakes and tsunamis every few hundred years. The lack of seismic shaking associated with slow slip events makes these events difficult to detect, especially offshore. On land they are evident in precise, continuous GPS records which show plate motions of several centimeters that start and stop over a few weeks. Such motions are much more difficult to observe on the seafloor because GPS signals do not penetrate sea water. Consequently alternative methods must be devised to detect offshore slow slip events. This research project will record very precisely the lengths of optical fibers stretched across the seafloor near the Oregon coast. If a slow slip event occurs in the region near the optical fibers, the associated optical length change will be recorded by a battery powered laser system at the end of each optical fiber. Studying such events offshore will help to build models of their influence on the timing and location of great earthquakes, and may lead to future advances in earthquake forecasting. The project supports the training of a student.Widespread deployments of GPS sensors in the past decade have helped identify Slow Slip Events (SSEs), especially near subduction zone faults in Cascadia, Costa Rica, Japan, and New Zealand. Understanding SSEs presents an opportunity to gain new insights into the mechanism governing locking and unlocking of subduction zone and other faults, and may be important in assessing the hazard levels presented from potential great earthquakes and tsunami. In particular, SSEs occurring at the downdip limit of the strongly locked zone may pose a risk of triggering large earthquake ruptures. It is therefore critical to search for SSEs occurring at the base of the locked zone, which in Cascadia (as in most subduction zones) lies offshore. In this study, two orthogonal optical fiber strainmeters will be installed on the seafloor above the Cascadia subduction zone to detect offshore SSEs. A recent study of the cumulative effect of SSEs in Cascadia using onshore GPS data indicates possible offshore slow slip at the base of the locked zone which is hypothesized to occur simultaneously with onshore SSEs. However, the detection of this slip using onshore GPS is very weak. Confirming the presence or absence of offshore slow slip in Cascadia is important for understanding the potential role of SSEs in influencing the timing and location of the next great earthquake. While GPS networks have sufficient sensitivity to map the location of SSEs onshore, they do not cover that portion of the crust under the oceans, nor are they able to distinguish the timing of sub-events because of the need for averaging over daily periods. In contrast, optical fiber strainmeters can be deployed offshore and have their best signal to noise ratio at shorter periods, complementing onshore GPS both in location and frequency band. Because SSEs evolve in complex patterns indicative of propagating stress fronts, it is important to resolve, both in scale and time, the deformation signals in order to understand more fully the evolution of the rupture plane. In conjunction with GPS, the availability of highly sensitive and stable strainmeters offshore will enable such characterizations. The measurements in this study will be timed with an expected onshore SSE to capture the hypothesized offshore slip, testing this model and others that address the extent of an offshore locked zone. In addition to the detection of SSEs, a number of other studies will become feasible following the deployment of this offshore strainmeter, including investigation of strain from traveling seismic waves and tidal observations for inferring local Earth structure. In addition, the work will advance the technology of optical fiber sensors and likely find applications in other disciplines.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.