Geo Jocks: Chilean Tsunami

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Deep below the surface of the Pacific Ocean, 100 miles off the coast of Chile, lies a monstrous, 6,000 km-long gouge in the sea floor called the Atacama Trench. It delineates an active tectonic plate boundary: a fault line between the South American plate and the subducting Nazca plate, which slides under continental South America at 65–80 millimeters a year. As the Nazca plate plunges into the earth, it occasionally becomes stuck. When it slips free of being stuck on the plate above it, the results are some of the largest earthquakes — and tsunamis — in the world.

A magnitude 8.3 quake struck at the shallower, westerly end of the subduction zone Sept. 16 off the shore of Illapel, Chile. The quake shook cities as far away as Argentina and sent a tsunami radiating across the Pacific Ocean — waves were three meters higher than average along the Chilean Coast. It also triggered a tsunami warning for over 25 countries, including New Zealand, Russia and Japan. In Chile, 1,800 people were left without drinking water and hundreds of thousands without power. But thanks to a new tsunami warning system that predicts the height and location of tsunami waves within seconds of the earthquake, 1,000 Chileans from cities along the coast were evacuated and the death toll was an estimated 11 people, a low count in comparison with the 2010 Chilean earthquake, which killed over 500.

The warning system — a mathematical model devised by a team led by Sebastián Riquelme at the University of Chile, Santiago — predicts tsunami height using characteristics of the subduction zone’s structure and the style of the earthquake. Their success has the potential to affect anyone living near a subduction zone. Riquelme and his colleagues have drafted formulas for several other subduction zones with a well-defined structure, including Peru, Nicaragua and Japan.

The formula is unique for each area and for each style of earthquake. The estimated amplitude of the tsunami wave is based on the angle that the subducting plate is dipping into the crust, the “strike” (orientation of the fault), the fault length, slip of the fault, mechanism of the earthquake (deformation in seismic wave-generating area) and its proximity to the sea floor.

Although the formula is simple, it is speedy and has the potential to save many lives. The next step is to improve the speed at which monitors pick up on characteristics of the earthquake itself, according to Riquelme and colleagues. In the near future, international teams plan on installing GPS units that will make fast and accurate measurements by measuring ground displacement.

These upcoming advances will be welcomed by Chileans, who are no strangers to earthquakes. One third of the earth’s seismic energy is released in the slipping subduction zone along South America’s west coast, and the largest earthquake ever recorded (magnitude 9.5) occurred in Chile in 1960. A combination of improved seismic technology and more robust infrastructure can save lives in Chile and seismically hazardous zones across the world.