Collaborative Research: Vertical seafloor geodesy to accurately image slow slip events in a noisy ocean environment

The largest and most dangerous earthquakes are subduction zone earthquakes where the source of the earthquake is almost completely underwater. The size of an earthquake and the propensity of an earthquake to create large dangerous tsunamis is determined by variations in the plate interface coupling which creates locked and unlocked regions of the fault. Understanding these variations in plate coupling is critical to understanding these dangerous earthquakes. Strain is released along some offshore parts of fault interfaces in occasional slow slip events (SSEs) where the fault moves much as with a normal earthquake, but the stored energy is released so slowly that these events are barely or not detectable on land. Regions between the SSE regions may remain locked, producing the potential for large tsunamigenic earthquakes. There is some evidence that SSEs on one part of a plate interface may increase strain on adjacent parts, triggering large earthquakes. Seafloor pressure gauges can detect SSEs as the seafloor moves upward during a SSE, decreasing the depth of the gauge. A SSE was detected and mapped in the proposed study region offshore New Zealand in 2015 using pressure gauges, as this is currently the only feasible method appropriate for wide application offshore. Those observations were greatly limited by the effects of oceanographic noise from ocean eddies. A much larger experiment is now planned for the same region. By adding oceanographic observations of near-seafloor current and vertical echo sounder data, the research should show this noise source can be much reduced and therefore reveal the spatial and temporal extent of offshore SSEs in greater detail and with better accuracy, improving our understanding of plate coupling relevant to understanding great earthquakes. The project will train a graduate student and postdoctoral researcher and involve them in a large international experiment.

During this large collaborative experiment with New Zealand and Japanese scientists, a large array of ocean bottom geodetic, oceanographic, and seismological instruments will be deployed for two years offshore of the east coast of New Zealand's North Island, where one or more shallow SSEs are expected to occur during the deployment. The joint array would include 44 seafloor absolute pressure gauges (APGs) and 12 current meters and upward looking sonars to test and develop innovative methods to remove contaminating pressure variations that arise within the water column (labeled oceanographic noise). The reduced-noise seafloor data will enable more accurate description of the spatio-temporal evolution of offshore slow-slip events (SSEs). For the first time, an array of 11 APG sensors equipped with a system for removing long term drift from sensor data will be deployed, with potential long term benefit for oceanographic observations and for applying seafloor vertical geodesy at tectonic strain rates. Data from 21 ocean bottom seismometers in the combined array will be used to probe the relationship between earthquakes and tremor and seafloor SSEs. These observations will advance our understanding of offshore variations in plate coupling in subduction zones.

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.