Ocean Observatories Initiative (OOI) Experts Discuss Tsunami Detection
(Click to enlarge) Photo provided by OOI Regional Scale Nodes and Center for Environmental Visualization, University of Washington
In the aftermath of the Great Taiheiyou, Japan, earthquake and resulting tsunami of March 11, media coverage has focused not only on the devastation but on a myriad of questions raised around the world about coastal preparedness, early warning capabilities and seismic monitoring on land and at sea. The Ocean Observatories Initiative (OOI) program has the potential to answer some of those questions.
The OOI, funded by the National Science Foundation, is planned as a networked infrastructure of science-driven sensor systems to measure the physical, chemical, geological and biological variables in the ocean and seafloor. A fully integrated system, OOI will collect data on coastal, regional and global scales and transmit that data in real-time to onshore scientists.
A component of the OOI, called the Regional Scale Nodes (RSN), will be deployed in the ocean west of Oregon. The RSN will have the potential to demonstrate that seafloor-cabled observatories can significantly enhance existing U.S. monitoring capabilities by providing observations in innovative ways, according to senior scientists and engineers working on the OOI program.
A key piece of the existing U.S. Tsunami Hazard Mitigation Program is the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. DART buoys were deployed in the Pacific and North Atlantic Oceans after the devastating Sumatra Tsunami of 2004. DART was deployed to help emergency managers prepare warnings for residents of the U.S. Pacific and Atlantic coasts, as well as neighboring countries, of any incoming tsunami threat by measuring seafloor pressure as a tsunami passes over a seafloor pressure gauge. However, a 2010 National Academy of Sciences National Research Council (NRC) report concludes the DART buoy system has limitations, especially with regard to providing warnings about tsunamis generated near the at-risk shores, and that more must be done to improve buoy reliability and overall warning procedures. Potential uses of the OOI’s sensors for tsunami detection and warning activities also are explicitly identified in that report, “Tsunami Warning and Preparedness: An assessment of the U.S. Tsunami Program and Tsunami Preparedness.”
“The OOI presents a remarkable opportunity to connect land based seismic networks and GPS observations with seismic and pressure measurements offshore using similar sensors and real-time connectivity,” said John Orcutt, the Principal Investigator for the Cyberinfrastructure component of the OOI Program who chaired the National Academy of Sciences NRC panel that conducted the 2010 Tsunami Warning study.
Real-Time Measurements When Minutes Matter
Tsunami detection, warning and preparedness activities for communities near, or at a moderate distance from, the tsunami’s source could benefit from the expansion of technologies for real-time detection, according to the NRC report. It adds that one way to acquire real-time measurements is to collect data using cabled seafloor observatories. These observatories are comprised of a variety of sensors connected to each other and to shore by a seafloor communications cable that serves to deliver power and commands to the sensors, and then to transmit data from the sensors back to onshore data servers. Several types of instruments that can be deployed on cabled observatories are useful for earthquake and tsunami detection, including bottom pressure sensors and broad-band seismometers.
“The NSF OOI system would provide real-time, high-bandwidth connectivity to seismometers and pressure gauges, a capability that the DART buoys can’t offer today,” Orcutt said.
The combination of seismometers and pressure gauges permits discrimination of the earthquake’s motion of the seafloor from the ocean’s response in the form of surface tsunami waves right at the source location, as demonstrated by the Japanese using cabled sensors at the site of the 2003 Tokachi-Oki earthquake. Such discrimination provides extra minutes of warning of the approach of a tsunami at the nearby coast, whereas the DART buoys have to be farther away from the source in order that the faster seafloor seismic waves will be distinguishable from the slower ocean tsunami waves, both of which are recorded by the seafloor pressure gauge at each DART site.
The Cascadia Subduction Zone (CSZ), under the Pacific just west of Washington and Oregon, is of particular interest to experts assessing the tsunami danger to U.S. coasts. The CSZ stretches from Northern California’s Cape Mendocino to southern British Columbia. Based on geological and historical evidence, experts predict that an earthquake along the CSZ could be of the same magnitude as the recent Taiheiyou, Japan, earthquake with a resulting tsunami able to overwhelm coastal communities in a matter of minutes, just as occurred in Japan. This danger increases the need for multifaceted, real-time observing systems such as the OOI.
“Real-time connectivity is a very big deal when the event is close to the coastline — certainly the biggest threat to the U.S. is the Cascadia fault zone, which broke last in 1700,” Orcutt said. “The fault is very similar to the fault that just broke in Japan and similar to the earlier 2004 Sumatra earthquake, not to mention the recent event in Chile. The Japanese do have a seafloor cable for earthquakes and tsunami warning and understand the value of warnings as early as possible to save lives ashore.”
The OOI RSN component is planned for deployment in 2012-2014 across Oregon’s continental shelf and slope and over the Juan de Fuca tectonic plate. The current plan for the RSN provides for two sites on the CSZ off Oregon where seismometers and pressure sensors will be collocated. The cabled array is designed to accommodate expansion, with additional funding, of additional multi-sensor sites along the CSZ off Grays Harbor, Wash., in order to optimize the benefits of the cabled sensors for tsunami detection and warning.
“We are enthusiastic about the potential of the cabled system off Washington and Oregon to improve our understanding of the physical coupling between the seafloor triggers such as earthquakes and landslides and the resultant tsunamis that have so commonly threatened coastal societies throughout human history,” said Douglas Luther, an OOI RSN Project Scientist based at the University of Hawaii. “These kinds of fundamental, multi-sensor, real-time measurements can translate into much enhanced strategies to protect coastal communities around the entire Pacific Rim where analogous tectonic environments are known to have similar hazard potential.”
Timing Is Everything
Currently, given the distances of likely tsunami sources to the closest coastal or open-ocean sea level sensors, it can take up to an hour or more to confirm a tsunami forecast and potentially even longer to forecast the size, the NRC report says. This is not only of concern in the case of strong near-field earthquakes, but also in the event of an underwater landslide or in the event of what is referred to as a “slow” earthquake. Both events generate only a small amount of ground shaking but could trigger a tsunami of much greater amplitude than would be expected. In this circumstance, official warnings, rather than environmental cues, may be the only way to notify people of the imminent tsunami danger. The report pointed to the Meiji Sanriku tsunami of 1896 in northeast Japan as an example of a devastating slow earthquake event. While that earthquake measured magnitude 7.2, it ruptured the seafloor relatively slowly and generated such weak ground shaking that few people were concerned about the potential for a tsunami. However, more than 22,000 people perished in the huge tsunami that followed.
Cabled ocean observatories now operating along North American coasts include NEPTUNE Canada, off the coast of British Columbia and the Monterey Accelerated Research System (MARS) in the Monterey Bay off the central California coast. In calling for the development of more rapid and accurate warnings of local tsunamis, the NRC report recommends that the Tsunami Warning Centers coordinate with NEPTUNE-Canada and OOI observatory managers to ensure access to their seismic and bottom pressure data in near real-time. Data interpretation tools jointly applied to the seismic and bottom pressure data should be developed to realize the most rapid tsunami detection possible, the report adds.
The OOI Program is managed and coordinated by the OOI Project Office at the Consortium for Ocean Leadership, in Washington, D.C., under contract from the National Science Foundation. Ocean Leadership is responsible for construction and initial operations of the OOI network. Three major Implementing Organizations are responsible for construction and development of the overall program. Woods Hole Oceanographic Institution and its partners, Oregon State University and Scripps Institution of Oceanography, are responsible for the coastal and global moorings and their autonomous vehicles. The University of Washington is responsible for the cabled seafloor systems and moorings west of Washington and Oregon. The University of California, San Diego, is implementing the cyberinfrastructure component. Rutgers, The State University of New Jersey, and its partners University of Maine and Raytheon Mission Operations and Services, is responsible for the education and public engagement software infrastructure.
Read more about the Ocean Observatories Initiative here. Access the National Academy of Science’s NRC report “Tsunami Warning and Preparedness: An assessment of the U.S. Tsunami Program and Tsunami Preparedness” here.
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