NEPTUNE Canada: Project Details

Project Details

West coast “tsunami-meter”

Towards Real-time Observations and Modeling of Tsunamis and Other Infra gravity Waves off the West Coast of North America

Principal investigator(s)

Richard Thomson

Co-investigators

Earl Davis, Kelin Wang, and Garry Rogers (Pacific Geosciences Centre, Natural Resources Canada, Sidney); Josef Cherniawsky, Diane Masson, Alexander Rabinovich, and Isaak Fine (Institute of Ocean Sciences, Fisheries and Oceans Canada, Sidney); Vasily Titov (Pacific Marine Laboratory, NOAA, Seattle); Brian Bornhold (University of Victoria, CEOR)

Discussion & Collaboration

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Abstract

This project will combine observations from an array of high precision, rapid sampling bottom pressure recorders (BPRs) with advanced numerical models to determine the spatial and temporal structure of sea level disturbances associated with tsunami waves and other long waves in the frequency band between 3 and 60 minutes. The project will also investigate sea level fluctuations caused by severe storms (so called “meteorological bombs”) and associated surface waves, and by depressions due to infragravity waves generated by short-period wind waves. Seafloor pressure data recorded at 1-second intervals by the BPRs, combined with simultaneous temperature and salinity data from built-in temperature and conductivity sensors, will help improve models of tsunami wave generation and propagation presently used for tsunami wave studies and forecasting.

On longer time scales, the study will use the proximity of the BPR nodes to TOPEX/Poseidon and Jason-1 (TPJ) satellite altimeter tracks to study the physical relationships between the bottom pressure measurements and surface elevation anomalies measured by an altimeter and to monitor seasonal and interannual wind and buoyancy-driven current variability in this region.

The ultimate goal of this research is to enhance our knowledge and predictive capability for long waves such as tsunamis and other large-scale sea level disturbances in the ocean, by studying their generation, propagation, transformation, run-up and dissipation, starting from a source region, through deep ocean, shelf break and shelf. More immediate objectives include the provision of real-time data as part of the Pacific tsunami warning system, development of mathematical/numerical models of these waves and their verification with the observations. The improved tsunami models will aid in predictions of tsunami waves for the west coast, including those generated by both regional and distant seismic seafloor displacements.

Study Sites/Locations

Highly sensitive bottom pressure recorders (BPRs) with built-in temperature and conductivity sensors will be located at all major nodal sites (except the Middle Valley and ODP889 sites) along the length of the fiber optic cable to record the occurrence of tsunamis and infragravity waves. An array of three additional BPRs with roughly 10-kilometer separation will be placed on the abyssal plain and centered at the ODP1027 node to allow precise determination of deep water tsunami amplitude, direction of propagation, and speed. The array will permit inversion of propagating tsunami and other gravity wave signals, for real-time tsunami warning and for studies of the properties of these waves in the ocean and on continental shelf. The temperature and salinity recorders within the BPRs will also provide long-term information on the effects of climate change on ocean bottom conditions.

Equipment Summary

The bottom pressure recorders consist of high-precision, rapidly recording pressure sensors, coupled with equally high-precision temperature and conductivity (salinity) sensors, capable of measuring millimeter-scale variations in surface water elevation at periods longer than a few seconds. These instruments have been designed and built for the NEPTUNE Canada project by scientists and technicians at the Pacific Geosciences Center (Sidney, BC), with assistance from scientists and technicians at the Institute of Ocean Sciences (Sidney, BC).

How this project will foster collaboration

Results of this study will aid in tsunami, storm surge, and earthquake modeling. Devastating earthquakes and tsunamis from the Cascadia Subduction Zone are expected to cause major damage and loss of life on the West Coast of Canada sometime during the next 500 years. The generation and local amplification of tsunami waves from more frequent distant earthquakes and local submarine slope failures also pose a tsunami hazard for the coast. The devastating tsunami generated by the magnitude 9.0 Sumatra earthquake of 26 December 2004 is a reminder of the hazards presented by tsunamis caused by subduction zone earthquakes. Understanding the dynamics of tsunami propagation and amplification requires the array of seafloor observations provided by this project. The predictive tsunami models, supported by continuous realtime data from the moored array will also be adapted to include tidal prediction and the forecast of surges and harbour seiches from severe storms.

Tsunamis are not the only source of coastal hazard and potential destruction. Although not as spectacular as a major tsunami, storm surges from severe North Pacific storms ("meteorological bombs") are more common, affect large areas and have systematic effects on coastal morphology and shore structures. These storms present considerable hazard to marine traffic and fishing boats, including potential loss of life, with very large wind waves, up to 30 m high, generated near continental shelf break via interaction with other waves, swell and tidal currents. As these low-pressure systems move towards the West Coast, they imprint a sea level signature which, due to ocean dynamics and waves, is not entirely pressure compensated. Near the coast, the winds produce sea level changes due to wind set-up or shelf waves. The latter are more prominent when an atmospheric cyclone moves with the coast on the right. Storm-generated waves also create changes in sea level due to freely propagating infragravity waves, as well as wave radiation stress, which may be of the order of several cm and thus detectable by modern instruments. These low frequency waves are generated by nonlinear processes among wind waves and swell and constitute an important contribution to the deep ocean pressure power spectra of pressure fluctuations from the sea floor. There is considerable interest in large scale observations of sea level variability off the West Coast on seasonal and interannual time scales, its relation to climate change and sea level rise and the driving mechanisms for such changes in this region. More recently, these were observed in satellite altimeter data, which are combined with coastal tide gauge observations to show dramatic changes in sea level, upwelling and circulation during El Niño and La Niña cycles. However, there is a need to distinguish between the steric sea level change, resulting from a change in water density (thermosteric and halosteric effects), and sea level change due to a horizontal volume (mass) flux. The combination of measurements from bottom-mounted pressure sensors and collocated space-based measurements of sea level by an altimeter will help to clarify this issue.