Geoscience Reference
In-Depth Information
NSF/IRIS funding operates 41 of the total 150 GSN stations through this mechanism. There can
be no assurance that this funding will be sustained at current levels in the future. GSN stations
have been operating since the mid-1980s (see Appendix G); much of their hardware is out of
date and increasingly dificult to maintain. Operations and maintenance budgets regularly
decrease and, except for events like the 2004 tsunami, modernization funds are generally not
available to boost the data return rates including the necessary hardware. The more modern
NSF EarthScope Transportable Array (with more than 400 telemetered broadband stations),
for example, boasts data return rates in excess of 99 percent. Unfortunately, the TWCs could be
among the most vulnerable of the IRIS clients in a constrained budget environment, because
the TWCs are among the users needing some of the most remote seismic stations, which are
dificult, hence expensive, to maintain.
To meet the requirements for detection of near-ield tsunami events, the TWCs have
supplemented existing seismic networks with their own local stations. The WC/ATWC maintains
a network of 15 sites throughout Alaska, and most stations were upgraded to satellite commu-
nications and broadband seismometers after 2005 (National Oceanic and Atmospheric Admin-
istration, 2008a). The PTWC, in collaboration with other partners, is also working to enhance an
existing seismic network in Hawaii to improve tsunami and other hazard detection capabilities
through a Hawaii Integrated Seismic Network (Shiro et al., 2006).
NOAA's Tsunami Program Strategic Plan (2009-2017; National Oceanic and Atmospheric
Administration, 2008b) recommends that the TWCs “monitor critical observing networks,
establish performance standards, and develop a reporting protocol with data providers” (e.g.,
the USGS and the NTHMP) and effect “complete upgrades of Alaska and Hawaii seismic . . .
networks.” The committee agrees with these recommendations; however, to be strategic with
limited resources, it is essential to determine and prioritize seismic stations that are critical
to tsunami warning (e.g., oceanic stations in known tsunamigenic source regions or within
30°-50° from potential tsunami source areas to allow the more rapid determination of the
tsunami potential).
Algorithms for Estimating an Earthquake's Tsunami Potential
Once data from the seismic networks have been received, the data are analyzed by the
TWCs to determine three key parameters for evaluating tsunamigenic potential: location,
depth, and magnitude of an earthquake. Algorithms for determining the geographical
location and depth of an earthquake source from seismic arrival times are based upon the
concept of triangulation (U.S. Indian Ocean Tsunami Warning System Program, 2007). With
the network of stations available to the TWCs, automatic horizontal locations are routinely
obtained within a few minutes of origin time with accuracy on the order of 30 km. This is
more than satisfactory to determine tsunami source locations, given the fact that earthquakes
of such high magnitudes have much larger source areas. The three seismic parameters are
used for issuing the initial bulletin. The focal mechanism characteristics are later obtained
through moment tensor inversion of broadband seismic data if the data quality is adequate
