Geoscience Reference
In-Depth Information
are available in near-real time from a variety of sources (e.g., http://sopac.ucsd.edu; Schmidt et
al., 2008). Recently, NSF Geosciences elected to undertake the improvement and densiication
of seismic and geodetic stations in the Cascadia region including the enhancement of near-real
time access to GPS (http://www.oceanleadership.org/2010/nsf-cascadia-initiative-workshop/).
Sweeney at al. (2005) have demonstrated that centimeter-level horizontal accuracy can be
achieved on the sealoor using GPS coupled to sealoor geodetic monuments using acoustic
methods. These technologies might be extended to verify offshore displacements predicted by
accurate coastal GPS stations.
Permanent GPS stations should be incorporated into the tsunami warning program and
expanded, if needed, to provide tsunami prediction capabilities. Although Cascadia is one of
the most critical sites for U.S. tsunami warning in the near-ield regions, Alaska and the Carib-
bean are also critical sites. There are few new technologies that promise such revolutionary
approaches for improving tsunami warning, especially in the near-ield region.
Conclusion:
GPS geodesy, exploiting near-real-time data telemetry from permanent
geodetic stations, holds great promise for extending the current seismic networks to
include capabilities for measuring displacements in the coastal environment for great
and mega-earthquakes. Displacements onshore can potentially be used to infer offshore
displacements in times as short as ive minutes in an area such as the Cascadia Fault Zone.
Recommendation:
NOAA should explore further the operational integration of GPS data
into TWC operations from existing and planned GPS geodetic stations along portions of
the coast of the United States potentially susceptible to near-ield tsunami generation
including Alaska, Cascadia, the Caribbean, and Hawaii. Where GPS geodetic coverage is not
adequate NOAA should work with NSF and the states in extending coverage including the
long-term operation and maintenance of the stations.
Observation of Tsunami Wave Trains with Satellite Altimeters
Satellite altimeter measurement of the ocean's surface height, in use since 1978, consists
of measuring (with a precision of a few centimeters) the deformation of the surface of the
ocean by precisely timing the relection of a radar beam emitted and received at a satellite. Its
capability to detect a tsunami was proposed following the 1992 Nicaragua tsunami (Okal et al.,
1999), and it achieved a deinitive detection following the 2004 Sumatra tsunami, with a signal
of 70 cm in the Bay of Bengal (Scharroo et al., 2005; Ablain et al., 2006). (See also the preceding
topic, “Continuous GPS Measurements of Crustal Movement.”)
Although the method has obvious promising potential in the ield of tsunami warning, two
major problems presently hamper its systematic use: (1) delayed processing of the data, which
in the case of the 2004 event was made available to the scientiic community several weeks
after the event, and (2) the presently sparse coverage of the earth's oceans by altimetry satel-
lites. In lay terms, the satellite has to be over the right spot at the right time; in the case of the
Sumatra tsunami, the passage of two satellites over the Bay of Bengal as the tsunami propa-
