Most readers will remember the appalling destruction and loss of life resulting from the Indian Ocean tsunami in December, 2004. That tsunami was caused by a magnitude 9.2 earthquake in a subduction zone near Sumatra, which produced waves up to 30 meters high, and affected coastal areas thousands of miles from the earthquake’s epicenter.
The disaster pointed up two problems in defending coastal areas against tsunamis:
- Detection There are existing networks of pressure sensors on the ocean floor, which can detect tsunamis by the change in weight of the water column above the sensor. However, sensor networks are expensive to install, and only five countries have them (Australia, Chile, Indonesia, Thailand, and the US); their coverage is far from complete.
- Warning Even when a suspected tsunami is detected, existing mechanisms for disseminating warnings may not be up to completing the job in a timely enough way.
According to an article in New Scientist, a group of researchers at the US National Oceanographic and Atmospheric Administration [NOAA] has come up with a new approach to the first problem, detection. They believe that existing undersea communications cables could be used to detect the passage of tsunamis, by using sensors to measure the change in electric field caused by the passage of an unusually large quantity of salt water (which of course contains many electrically-charged ions). Their simulations indicate that the induced voltage might be on the order of 0.5 volt, which ought to be detectable after correcting for background noise.. This information would not provide any directional indication, but might be quite useful when integrated with data from other sources.
This is quite a clever idea, and might be a valuable way to augment the information gathered by the existing sensor networks without great incremental expense. However, the second part of the problem, getting the warning information distributed in time, is still a tough nut, especially in those poor countries that have especially vulnerable coastlines.