Detection of Tsunamis from Changes in Ocean Surface Roughness

The need for a reliable system for early tsunami detection and warning was made painfully clear by the over two hundred thousand lives lost to the tsunami generated by the 9.3 magnitude Sumatra-Andaman earthquake that swept across the Indian Ocean on 26 December 2004 (Stein & Okal, 2005; Lay et al., 2005; Titov et al., 2005). The tsunami claimed the lives of over 220,000 people and despite a lag of up to several hours between the earthquake and arrival of the tsunami in some locations, the majority of victims were given little or no warning of the impending threat. While tsunamis occur much more frequently in the Pacific Ocean and a tsunami warning system has been in place in the region for many years, no such system was in place in the Indian Ocean and the communications infrastructure was not adequate for issuing widespread warnings at the time of the Sumatra-Andaman tsunami. An early and dependable assessment of a tsunami threat requires detection of the tsunami wave in the open ocean away from the shore (Lautenbacher, 2005; Levin & Nosov, 2005; Bernard et al., 2006; Schindele et al., 2008). In the open ocean, however, the wave amplitude of the tsunami is small (generally less than one meter) and it is only as it approaches the shore that the tsunami rapidly grows in amplitude. Given the expansiveness of the ocean, sensors capable of detecting the tsunami must have very broad coverage. In addition to detecting the tsunami early enough to provide adequate warning, the method of detection must be reliable with few false warnings. If coastal populations go to great lengths to move to safe areas only to find out later such an evacuation was unnecessary, they may be less likely to heed warnings in the future. By complementing traditional seismic data and point measurements as provided by the Deep-Ocean Assessment and Reporting of Tsunamis (DART) buoys network (Gonzalez et al., 2005; Bernard et al., 2006), satellite observations of tsunami manifestations can potentially improve the accuracy and timeliness of tsunami forecasts (Levin & Nosov, 2005; Synolakis & Bernard, 2006; Geist et al., 2007; Wei et al., 2008; Behrens et al., 2008), increase