Geoscience Reference
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tions that currently average a little over one year before failure, compared to a four-year design
lifetime. Additional open questions include dependence of U.S. tsunami warning activities on
sea level data supplied by foreign agencies and on sea level data derived from U.S. and foreign
gauges that do not meet NOAA's standards for establishment, operation, and maintenance.
Looking to the future, the committee concludes that the numbers, locations, and prioritiza-
tions of the DART stations and coastal sea level gauges should not be considered static, in light
of constantly changing iscal realities, survivability experience, maintenance cost experience,
model improvements, new technology developments, and increasing or decreasing interna-
tional contributions. The committee inds of great value NOAA's continual encouragement
and facilitation of researchers, other federal and state agencies, and nongovernmental orga-
nizations (NGOs) who utilize their sea level observations for novel purposes. The committee
believes that stations with a broad user base have enhanced sustainability.
The committee is optimistic that continued enhancements to the sea level monitoring
component of the U.S. Tsunami Program can measurably mitigate the tsunami hazard and pro-
tect human lives and property for far-ield events. The committee's recommendations for the
DART and coastal sea level gauge networks fall under the following categories: (1) assessment
of network coverage; (2) station prioritization; (3) data stream risk assessment and data avail-
ability; (4) cost mitigation and cost prioritization; and (5) sea level network oversight.
Similar to open-ocean tsunami detection, tsunami forecast modeling has only recently
become operational at the TWCs, as described below. The committee anticipates that further
development and implementation of numerical forecast modeling methodologies at the TWCs
will continue to help improve the tsunami warning enterprise.
As described below, the rapid detection of a tsunami striking within minutes to an hour,
either for the purpose of providing an initial warning or for conirming any natural warnings
that near-ield communities have already received, will likely require consideration of alterna-
tive detection technologies, such as sensors deployed along undersea cabled observatories
and coastal radars that can detect a tsunami's surface currents tens of kilometers from the
shore. Finally, examples of other new technologies and methodologies that have the potential
to improve both estimation of earthquake parameters and tsunami detection are discussed at
the end of this chapter.
DETECTION OF EARTHQUAKES
All initial tsunami warnings are based on rapid detection and characterization of seismic
activity. Because of the fundamental differences in nature between the solid earth in which an
earthquake takes place and the luid ocean where tsunami gravity waves propagate, the vast
majority of earthquakes occurring on a daily basis do not trigger appreciable or even measur-
able tsunamis. Nevertheless, some smaller earthquakes could trigger submarine landslides that
can result in local tsunamis. It takes a large event (magnitude >7.0) to generate a damaging
tsunami in the near-ield and a great earthquake (magnitude >8.0) to generate a tsunami in
the far-ield. However, the generation of a tsunami is affected not only by the magnitude of an
