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zone could be made available within 30 minutes of the initiation of a similar event. Although
networks or arrays like Hi-Net are rare, a similar or even more capable array is currently being
implemented across the continental United States, funded by the NSF EarthScope program.
Today's high-speed, high-capacity networks coupled with large-capacity computing facilities
such as cloud computing provide the technologies for implementing an early warning system.
The compressional wave velocity is high (>8 km/s) and will provide fault images more quickly
than the hydrophone approaches discussed below. The technique used for acoustics, however,
is similar to seismic back-projection.
Conclusion: The P-wave duration and back projection methods appear robust and
applicable to high-frequency records. These methods have some advantages over the
W-phase approach because they can provide constraints on the rupture length and
duration and do not rely on having seismometers with a stable long-period response.
Recommendation: The committee recommends that NOAA and the TWCs consider
the use of arrays and networks such as Hi-Net and EarthScope Array National Facility
to determine rupture extent and moment of great earthquakes. The networking and
computational requirements are signiicant and would need to be included in TWC
upgrades in the future.
Hydroacoustic Monitoring of Underwater Geophysical Events
Sound wave (“hydroacoustic”) signals can propagate a great distance within a waveguide
in the ocean, termed the sound ixing and ranging channel (“SOFAR channel”). This propaga-
tion was discovered during World War II, and immediately following declassiication scientists
began exploring the possibility of using hydroacoustic signals generated by large earthquakes
(the so-called T phases) for the purpose of tsunami warning (Ewing et al., 1950). With the
development of the UN International Monitoring System of the CTBTO, several state-of-the-art
hydrophone stations have been deployed in the world ocean, offering an opportunity for com-
plementary use in the context of tsunami warning. Each station comprises three hydrophones
separated by approximately 2 km to provide some directionality at low frequencies.
By placing hydrophone sensors within the SOFAR channel, a scientist can “listen” to
sealoor seismic, tectonic, and volcanic events occurring at a great distance. The potential of
using hydroacoustic techniques to monitor underwater landslides has yet to be fully explored,
but it may represent the best approach for detecting unsuspected underwater landslides, as
occurred in the 1998 Papua New Guinea (PNG) tsunami (Okal, 2003). However, that detection
represents to this day a unique, unrepeated occurrence. Furthermore, the PNG landslide was
identiied as such because its hydroacoustic signal was too weak for its duration, in violation of
earthquake scaling laws. At the same time, T phases can be used to complement the identiica-
tion of anomalously slow events, such as tsunami earthquakes, because hydroacoustic signals
include very high frequencies (3 Hz and above) and their energy bears the imprint of the earth-
quake at very short periods (Okal et al., 2003).
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