Ocean Sciences Meeting Report

Commercial Underwater Cable Systems Could Reduce Disaster Impact

Workshop on SMART Cable Applications in Earthquake and Tsunami Science and Early Warning; Potsdam, Germany, 3–4 November 2016

By , , and Rhett Butler

Every minute counts in the business of tsunami early warning because tsunami waves often arrive less than 30 minutes after offshore earthquakes. Because most massive subduction zone quakes occur offshore, offshore observations are extremely valuable for quickly detecting and characterizing potential tsunamis. At the same time, unnecessary evacuations are costly and can endanger lives, so false warnings must be minimized.

The current Deep-ocean Assessment and Reporting of Tsunamis (DART) system uses ocean bottom pressure sensors to detect ocean-crossing tsunamis. The DART sensors are too sparse and too distant from shore to provide local warnings, and other real-time solutions like dedicated submarine detection cables come with a hefty price tag. Comprehensive coverage of all endangered subduction zones is out of reach using these systems, particularly in the developing world, but another approach that adds new capabilities to an existing resource could be a significant step in the right direction.

Tsunami detection figure
The color scale shows the time lag between tsunami generation and detection for possible earthquake epicenters along subduction zones in the Pacific Ocean that could be achieved by installing sensors on the cable repeaters (black dots are 500 kilometers apart; separation in actual systems is 50 kilometers) along several existing submarine cables. The paths crossing the South Pacific Ocean are notional future paths, whereas the others are existing routes that are renewed every 10 years or so. Dark gray triangles show existing DART tsunameter buoys, and the light gray triangles represent seismic stations and mainland and island stations that measure sea level. Credit: Nathan Becker and Stuart Weinstein, Pacific Tsunami Warning Center, NWS, NOAA

Today, submarine telecommunications cables cross the world’s oceans, and many run through or parallel to margins threatened by subduction zone earthquakes. The cables that currently form this network are not sensing their environment; however, these cables are routinely replaced every 10 to 15 years. Installing suitably modified repeaters along future cables, spaced at nominal 50-kilometer (31-mile) intervals, could provide power and bandwidth for sensors along these cables.

Last November, a group of research scientists, practitioners from earthquake observatories and tsunami warning centers, and engineers gathered for a workshop in Potsdam, Germany, to discuss the viability of a new early warning system that uses enhanced telecommunications cables to create a Science Monitoring and Reliable Telecommunications (SMART) network capable of detecting tsunamis and shaking from great earthquakes. They further discussed how SMART cable sensor arrays would support research into tsunami excitation and propagation, the physics of great earthquakes, and the structure of Earth.

Given the needs of operational earthquake observatories and tsunami warning centers, attendees were excited about the concept of SMART cable systems equipped with accelerometers, pressure gauges, and temperature sensors. This concept is being advanced by a joint task force of the International Telecommunication Union, the World Meteorological Organization, and the Intergovernmental Oceanographic Commission of the United Nations Educational, Scientific and Cultural Organization. The Potsdam workshop followed two prior NASA workshops focused on applications in climate research and oceanography.

The original signal (black) registered by pressure gauges of Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) station B08 during the 2011 Tohoku-Oki earthquake and tsunami barely records the tsunami at 07:00 to 07:30 Coordinated Universal Time (UTC). Band-pass-filtered signals are shown in red and green with the frequency ranges as indicated. High-frequency measurements show acoustic waves (red), whereas water column oscillations following seismic activity are best shown at intermediate time periods (green). The signal from the “gravitational wave” range (blue) is amplified tenfold and clearly shows the tsunami. Times corresponding to the main seismic event of Mw 9.0 and the first strong aftershock of Mw 7.9 are indicated. Sensors on SMART cables would allow the separation of tsunami signals in the ocean bottom pressure record as shown here, whereas the 15-second sampling interval of a standard DART system precludes this. Credit: Physics of Tsunamis, Role of the Compressibility of Water and of Nonlinear Effects in the Formation of Tsunami Waves, 2nd ed., 2016, p. 236, Boris W. Levin and Mikhail A. Nosov, © Springer International Publishing Switzerland 2016. Used with permission from Springer.

In one of the studies presented at the meeting, models showed that a few cables crossing the Pacific could reduce the time to detection of potentially tsunami inducing earthquakes by approximately 20%. The time to detection of the actual tsunami wave would be similarly reduced. Furthermore, the linear sensor arrays enabled by the SMART cables allow direct measurements of the tsunami wavefield. Such dense sampling could reduce the dependence on seismological networks and allow researchers to characterize tsunamis triggered by submarine landslides or other nontectonic sources.

Workshop participants identified several potential targets for a small demonstration system, including existing cabled seafloor observatories. The participants agreed the demonstration systems should be deployed in a manner equivalent to commercial cable-laying operations to demonstrate the viability of the SMART cable vision and to deliver valuable science data.

—Frederik Tilmann (email: [email protected]), Deutsches GeoForschungsZentrum (GFZ), Potsdam, Germany; Bruce M. Howe, Ocean and Resources Engineering, University of Hawai‘i at Mānoa, Honolulu; and Rhett Butler, Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu

Citation: Tilmann, F., B. M. Howe, and R. Butler (2017), Commercial underwater cable systems could reduce disaster impact, Eos, 98, https://doi.org/10.1029/2017EO069575. Published on 23 March 2017.
© 2017. The authors. CC BY 3.0