Geoscience Reference
In-Depth Information
gated across was a lucky coincidence. Thus, making satellite altimetry operational for tsunami
warning requires geostationary satellites over the ocean basins of interest, or a dense array of
low earth orbit (LEO) satellites, with either set-up providing data availability in near-real time. In
fact, Iridium Communications, Inc. is designing its second generation of LEO communications
satellites (called Iridium NEXT), which are expected to be fully deployed by 2016 and will carry
scientiic payloads such as altimeters for sea height determination, including observation of
tsunamis (http://www.iridium.com/About/IridiumNEXT/HostedPayloads.aspx). The planned
constellation of 66 satellites suggests that a tsunami created anywhere in the world could be
observed close to the moment of inception. At the present time, however, the NEXT constella-
tion is not being touted as a tool for operational tsunami warning.
Tsunami-Induced Sea-Surface Roughness and “Tsunami Shadows”
Godin (2004) theoretically justiied so-called “tsunami shadow” observations (Walker,
1996), namely that the surface of the ocean exhibits a change of appearance during the propa-
gation of a tsunami. In simple terms, the tsunami creates a coherent change in sea-surface
slope, inducing turbulence in wind currents at the surface, which in turn results in enhanced
roughness of the sea- air interface. Godin et al. (2009) further showed that the phenomenon
was detectable in the form of anomalous scattering in the radar signal from the JASON
satellite altimeter, during its transit over the wavefront of the 2004 Sumatra tsunami in the
Bay of Bengal. This remarkable scientiic conirmation and physical explanation of what had
amounted to anecdotal reports provides some promise as a complementary means of near-
real-time tsunami detection. In its reported form, the method suffers from the same limitations
as satellite altimetry, namely the need to have a satellite at the right place at the right time. On
the other hand, it may be feasible to develop a land-based detector of sea-surface roughness
using over-the-horizon radar technology.
Direct Recording of Tsunami Waves by Island Seismometers
Another notable observation made in the wake of the 2004 Sumatra event was that the
actual tsunami wave was detectable on horizontal long-period seismometers located on
oceanic islands or on the shores of continental masses (e.g., Antarctica) (Yuan et al., 2005). Okal
(2007b) later veriied that such signals could be extracted from past events (e.g., Peru, 2001),
and showed that the recordings expressed the response of the seismometer to the combined
horizontal displacement and tilt of the ocean loor during the passage of the tsunami wave,
the latter having such large wavelengths (typically 300 km) that the structure of a small island
can be neglected. In particular it was veriied that such records could be interpreted quantita-
tively on this basis, which amounts to saying that near-shore seismometers can play the role of
tsunameters deployed on the high seas for tsunami detection. The present network of island
seismic stations (see Figure 4.1) thus has the potential of increasing the density of the tsunami
(sea level) detection network, at essentially no cost, since the stations already exist.
