Geoscience Reference
In-Depth Information
DETECTION OF TSUNAMIS WITH SEA LEVEL SENSORS
Because the seismic signal is the irst observation available to the TWCs, seismic detection
provides the basis for the initial evaluation of the potential for a tsunami. The decision about
the content of the irst message from the TWCs is based solely on seismic parameters and the
historical record, if any, of tsunamis emanating from the neighborhood of the earthquake. How-
ever, as previously noted, this indirect seismic method is limited in the accuracy of its estimates
of the strength of the tsunami, usually underestimating the tsunami potential of large earth-
quakes and tsunami earthquakes. In acknowledgment of this bias, and because forecasters
must err on the side of caution when human lives may be at stake, the TWCs use conservative
criteria to trigger advisories, watches, or warnings based on this initial seismic assessment
(e.g., Weinstein, 2008), as seen in the PTWC's far-ield forecast of the tsunami from the Chilean
earthquake of February 27, 2010 (Appendix J). However, these conservative assessments might
cause unwarranted evacuations, which can cost millions of dollars and might threaten lives. A
TWC must, therefore, not only provide timely warning of a destructive tsunami, but also must
avoid causing unnecessary evacuations with their attendant negative impacts.
The detection and forecasting process requires real-time observations of tsunamis from
both coastal sea level gauges and open-ocean sensors (such as provided by the DART stations).
The combination of the open-ocean and coastal sea level stations, which provide direct
observations of tsunami waves, are important for adjusting and canceling warnings as well as
for post-tsunami validation of models of the tsunami propagation and inundation (U.S. Indian
Ocean Tsunami Warning System Program, 2007). These sea level networks can also detect
tsunamis from sources that fail to generate seismic waves or are generated by an earthquake
on land that generates a sub-aerial and/or a sealoor landslide. Progress to expand the ocean
observing network and advances in oceanographic observing technologies allow the TWCs to
incorporate the direct oceanographic detection of tsunamis into their decision processes.
Conclusion:
An array of coastal and open-ocean sea level sensors is necessary until such
time, in some distant future, when the capability exists of observing the entire tsunami
wave-front in real-time and with high horizontal resolution (e.g., perhaps with satellites) as
it expands outward from its source and comes ashore.
The Tsunami Warning Decision Process Before and
After Enactment of Public Law 109-424
A majority of the funds authorized by the Tsunami Warning and Education Act (P.L. 109-
424) have been used to manufacture, deploy, and maintain an array of 39 DART stations (not
counting the 9 purchased and deployed by foreign agencies; http://www.ndbc.noaa.gov/dart.
shtml), establish 16 new coastal sea level gauges, and upgrade 33 existing water level stations
(National Tsunami Hazard Mitigation Program, 2008; http://tidesandcurrents.noaa.gov/
1mindata.shtml). All these new and upgraded sea level stations, especially the DART sites, have
