* Astronomy

December 8, 2006
Though categorised as magnitude 7.8, the earthquake could scarcely be felt by beachgoers that afternoon. A low tide and wind-driven waves disguised the signs of receding water, so when the tsunami struck, it caught even lifeguards by surprise. That contributed to the death toll of more than 600 persons in Java, Indonesia.

The general assumption was that if you were near the coast where the earthquake took place, you would feel it and be able to run to higher ground. This event caught people by surprise and showed that its not always that simple - Hermann Fritz, first author of a new Geophysical Research Letters paper about the July 17, 2006 tsunami.

The earthquake was slow rupturing, so it didnt produce strong ground shaking on Java that might have alerted people on the beach.
No local warning was issued for the tsunami waves, which arrived only tens of minutes after the earthquake. Fortunately, the event took place on a Monday. Had the massive waves hit the day before, which was a major national holiday, the popular beach would have been much more crowded and the toll higher.

Warning systems typically dont work very well for locations near earthquakes, where there are only tens of minutes between the earthquake and the tsunamis arrival. Its pretty much a spontaneous self-evacuation. You normally feel the earthquake or see the ocean withdraw. If you hear the noise in the last tens of seconds before it hits, then its just a matter of who makes it and who doesnt - Hermann Fritz, a Georgia Institute of Technology assistant professor who led an inspection team to Java a week after the event.

The survey team, which included scientists from five different countries, interviewed survivors and studied evidence left behind by the tsunami, including debris fields. Beyond the quiet nature of the catastrophe, they discovered evidence of a 21-metre wave that hit a portion of the coastline near the island of Nusa Kambangan, indicating a second event that may have added to the severity of the disaster.
Elsewhere along the 300 kilometres of coastline studied by the International Tsunami Survey team, the waves ranged from 5 to 7 meters.

This event indicates that there was likely a combination of both a tectonic tsunami and a submarine landslide or a canyon failure triggered by the earthquake. The run up was unusually high along one portion of the coast, too much for a 7.8 magnitude earthquake. The only explanation we could think of is that a submarine mass movement triggered by the earthquake could have added to the effect of the earthquake, given the essentially straight coastline with little room for large-scale tsunami focusing - Hermann Fritz, whose research is supported by the U.S. National Science Foundation.

For people in seismically-active areas like Indonesia, an earthquake usually provides the first warning of a tsunami. Whether caused by an earthquake or an underwater landslide, the first visible sign of an oncoming tsunami is often a rapid withdrawal of the ocean that exposes the seafloor or coral reefs. When that appears, the first tsunami wave wont be far behind.

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