Chile earthquakeThe Chile quake was followed by a smaller tsunami than originally expected

Published 2 March 2010

The tsunami which followed Saturday’s earthquake in Chile was smaller than originally expected because the earthquake ruptured only a relatively small segment of fault at the boundary between the Nazca and South American tectonic plates — around 350 kilometers; the length of fault rupture determines the distance at which a tsunami begins to lose energy

How to explain the fact that the earthquake in Chile on Saturday, one of the biggest the world has felt in the past century, created a much smaller tsunami across the Pacific than originally feared?

The magnitude-8.8 earthquake did created large tsunami waves along parts of Chile’s coastline, and reports suggest the town of Constitución was worst affected by the wave.

Kate Ravilious Kate Ravilious that locations further afield, however, were more or less spared by the tsunami. Waves smaller than 1.5 meters struck Hawaii and Japan, for example, causing very little damage. By contrast, a magnitude-9.5 earthquake in 1960 spawned a tsunami that claimed more than 200 lives in Japan, Hawaii, and the Philippines.

Tim Henstock of the National Oceanography Center at the University of Southampton, United Kingdom, speculates that the reason might be that Saturday’s earthquake ruptured a relatively small segment of fault — around 350 kilometers. The length of fault rupture determines the distance at which a tsunami begins to lose energy. By comparison, the magnitude-9.1 earthquake that generated the 2004 Indian ocean tsunami ruptured around 1600 kilometers of fault.

Chile’s recent tsunami was also highly focused. “It was quite a directed tsunami, rather than a ‘stone in a pool’ type propagation,” explains Simon Haslett of the University of Wales, Newport. The tsunami was severe at the coast nearest the epicenter, and westward at the Juan Fernandez Islands, but the energy and height were lost quickly in other directions, he says.

Furthermore, the relatively deep origin of the earthquake — 35 kilometers — may have minimized the uplift on the sea floor that displaced the water. “The Chile quake was smaller and deeper than the Indian Ocean quake of 2004, so less energy was released and, most importantly, less of this reached the surface,” says Bill McGuirep of University College London.