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 scientific 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 constellation is not being touted as a tool for operational tsunami warning.
Godin (2004) theoretically justified so-called “tsunami shadow” observations (Walker, 1996), namely that the surface of the ocean exhibits a change of appearance during the propagation 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 scientific confirmation 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.
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 verified 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 floor 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 verified that such records could be interpreted quantitatively 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.