Landslides and tsunamis Tsunamis - large, and often devastating, ocean waves - are generated by earthquakes, volcanic eruptions and underwater landslides, each of which is capable, under the right circumstances, of imparting sufficient energy to ocean water to create such waves. However, some earthquakes and volcanic eruptions are known to have generated tsunamis larger than those estimated from the amount of energy that could possibly have been transferred, suggesting that the tsunamis in question were generated not just by the prime events themselves but, in addition, by landslides resulting from the prime events. Thus, for example, earthquake-generating faulting might transmit energy to the water to produce a tsunami but might also lead to a landslide that will add energy to the tsunami or perhaps generate a second one. In this sense, landslides may be seen as central to tsunami generation, which makes the detection of possible tsunami-generating events - and hence tsunami prediction - more difficult than it might otherwise be. For whereas earthquakes, for example, may be detected almost instantaneously, and at great distances away, by seismometers around the world, landslides cannot. Indeed, a tsunami may be the first indication that an underwater landslide has actually taken place. Caplan-Auerbach and others (Geophysical Research Letters, v.28, p.1811, 2001) have therefore been looking at other possible ways of detecting submarine landslides, especially in the context of Kilauea volcano (Hawaii), where, at points at which lava enters the sea, landslides are produced, being partly subaerial events known as bench collapses. Using hydrophones of the Hawaiian Undersea Geo-Observatory (HUGO), some 50 km from the relevant site, Caplan-Auerbach and her colleagues find that they can detect distinctive hydroacoustic signals from the landslides (up to 10 a day). 'Distinctive' here means having spectral characteristics different from those of earthquakes or anthropogenic events. Moreover, some of the larger landslides can even be detected by the Eastern Pacific Hydrophone Array (PMEL) over 5000 km away (Fig. 1). These discoveries suggest that there is now a potentially important way of detecting submarine landslides as they happen and hence of improved monitoring, predicting and modelling of tsunami.
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