Geoscience Reference
In-Depth Information
In this respect, hydroacoustic signals play a complementary role in tsunami warning
because they travel slowly (1,500 m/s). However, de Groot-Hedlin (2005) and Tolstoy and
Bohnenstiehl (2005) demonstrated that it was possible to use ocean hydrophones to track the
rupture of the 2004 Sumatra event from the original epicenter to the termination more than
600 km to the north. The hydrophones were 2,800 and 7,000 km from the epicenter and acous-
tic propagation required 31-78 minutes while the fault itself ruptured for more than 8 minutes.
The information would not be useful for alerting nearby communities but could have provided
meaningful warnings for Sri Lanka and more distant countries.
Other properties of T phases can shed some interesting, but again complementary, light
on properties of the seismic sources, for example, their duration, along lines similar to the
τ
1/3
method described earlier. Salzberg (2008) has also proposed to precisely constrain hypocentral
depth using the decay of very high frequency (20-80 Hz) T phases from the parent earthquakes.
Once such techniques reach an operational status, they could contribute to tsunami warning.
An additional aspect of SOFAR hydrophone sensors is that they can record pressure
variations accompanying the passage of the tsunami, and in this sense could supplement the
network of DART buoys, as their sensors (in both cases pressure detectors) essentially share
the same technology, with the only difference being that the latter are deployed on the ocean
bottom. However, within the context of the CTBTO, the Integrated Maritime Surveillance (IMS)
sensors have been hard-wired with drastic high-pass ilters (with a corner frequency of 10 Hz),
and the main spectral components of the 2004 Sumatra tsunami (around 1 mHz) were re-
corded only as digital noise (Okal et al., 2007). The use of software rather than hardware ilters
for any future deployment of hydrophones in the SOFAR could be extremely valuable to the
tsunami community. The cabled NSF OOI Regional Scale Nodes (RSNs) to be deployed off
Washington and Oregon and the existing NEPTUNE-Canada network (see above) could sup-
port both bottom pressure gauges as well as hydrophones in the SOFAR channel for enhancing
tsunami research and warning in the Cascadia area.
Continuous GPS Measurements of Crustal Movement
When combined with seismic data, continuous global positioning system (GPS) measure-
ments of displacement have proven to be powerful in studying continental earthquakes; for
example, in illuminating the processes of earthquake after-slip, creep, and viscoelastic deforma-
tion. Continuous GPS can provide a map of the three-dimensional deformation incurred at the
surface in the proximity of the epicenter as a result of the earthquake rupture. It provides a
resolution to the problem of the long-period component of the seismic source by simply allow-
ing measurement during a time window long enough to be relevant to tsunami generation
even for nearby sources.
GPS and broadband seismic measurements differ substantially in that GPS geodetic mea-
surements provide distances between neighboring stations, while individual seismometers are
affected by applied forces and signals are proportional to acceleration. Normally, the output
of a seismometer is “shaped” to be proportional to velocity above some frequency (1/360 Hz
for an STS-1; Appendix G). Because earthquakes cannot apply a constant force at zero fre-
