Offshore Geophysical Monitoring of Cascadia for Early Warning and Hazards Research,
by Schmidt, D. A., W. Wilcock, P. Bodin, F. Gonzalez, M. Harrington, R.J. LeVeque, D. Manalang, E. Roland, and J. Vidale (2018),
Workshop Report, Seattle WA, April 3-5, 2017.

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Executive Summary. Subduction zones produce the largest earthquakes on our planet and initiate devastating tsunamis that can inundate coastal communities. However, our ability to monitor and study the offshore sources of these events is significantly limited by the logistical and technical challenges of making geophysical observations on the seafloor. Several countries, such as Japan and Canada, are now making investments in offshore infrastructure to better monitor the seafloor in risk-prone areas. Real-time data from the seafloor can be used to provide refined earthquake and tsunami early warnings to coastal populations, which will directly help to save lives, reduce injuries, and allow companies and municipalities to enact automated systems to protect infrastructure. In addition, sustained offshore sensor networks can provide observations of subduction zones that will enable new scientific insights into the geological processes and thus help to characterize and mitigate the natural hazards.

This meeting focused on the Cascadia subduction zone, where large earthquakes (up to magnitude 9) and tsunamis occur regularly through geological time and where real time offshore observations are limited in their spatial footprint to multidisciplinary scientific cable observatories off central Oregon and Vancouver Island. It provided a forum for interested scientists and engineers to discuss the scientific and societal motivation for an offshore geophysical sensor network extending the length of the subduction zone, the technical requirements for such a network, and the merits of various engineering approaches. The meeting also considered how the existing seafloor infrastructure in Cascadia could be leveraged to jumpstart this effort.

Through a series of plenary presentations and small group breakouts, workshop participants discussed the merits of such a system and identified a number of scientific, engineering, and policy recommendations. An offshore sensor network can improve the timeliness, reliability, and accuracy of earthquake and tsunami early warnings. While an offshore sensor array would greatly improve our monitoring capability and scientific understanding of the offshore faults, the greatest benefit to society is the early warning capability, particularly for providing an accurate warning and estimate of an incoming tsunamis. Any system design should be focused on a clear set of objectives, such as early warning. Most participants felt that a phased implementation strategy is likely the best way to develop and test the technologies, demonstrate success, and motivate the completion of a margin-wide sensor network; however, others articulated that a push for a full-scale system is the better strategy for success.

Future work is needed to understand whether it is possible to deconvolve the simultaneous movement of the seafloor and sea surface height from seafloor pressure sensors in order to inform real-time assessments of the incoming tsunami or whether additional instrumentation is required to measure seasurface elevation. Additional modeling studies are needed to optimize the spacing and mix of instrumentation given scientific and/or hazard objectives. There are a number of emerging technologies that might provide the required observations at reduced cost, although further development and testing is needed before such technologies could be deployed on a large scale. Additional offshore geophysical experiments and data sets are needed to inform how best to optimize and design a seafloor system for such questions as where instruments are ideally located in relation to likely earthquake sources and to minimize risks from turbidity currents. A strong leadership team and broad coalition of stakeholders are needed to advance any long-term effort to deploy a seafloor sensor network that extends along the full length of the Cascadia Subduction zone.

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