Imaging the earthquake

Seismologists are using a new method to detail the processes that unfolded during the disastrous Sumatra-Andaman earthquake in the Indian Ocean on December 26, 2004.

Scientists at Scripps Institution of Oceanography at the University of California, San Diego, in collaboration with scientists at the University of California, Los Angeles, have developed a new method for imaging how the earth ruptured during the quake, which is providing a fresh perspective of the massive event.

The new method traces seismic waves back to their original rupture source. It makes use of the Japanese Hi-Net array, consisting of about 700 high-quality seismometers, as antennae to track the seismic sources.

"If you were at a pond and dropped a pebble, you would see a ripple appear in the water. If another person only saw the ripple, they could still probably guess where you dropped the pebble by tracing the rings back to the centre. That's exactly what we are doing. We are looking at how the ring of seismic waves is approaching the array to find out where the rupture is occurring," said Scripps’ Miaki Ishii.

"It's similar to some ideas that have been used in the past, but as far as we know it's the first time that it has been applied to directly image the rupture of a large earthquake," added Scripps’ Peter Shearer.

The resulting images from the Sumatra-Andaman event paint a detailed picture, starting at an epicentre just west of northern Sumatra. A powerful burst of rupture energy is seen 80 seconds later as the quake progresses northwest. A second significant burst occurs after another 220 seconds, west of the Car Nicobar region. The entire event lasts for about eight minutes and ends at the northern Andaman archipelago.

To view the video, click here: http://scrippsnews.ucsd.edu/video/sumatra_vid.mov

The new findings also show that the event extends father north than initially reported. At 500 seconds and 1,300 kilometers, the Sumatra-Andaman event is longer than the three other largest recorded earthquakes: the 1957 Aleutian earthquake, the 1960 Chile earthquake and the 1964 Alaskan earthquake.

Because their method capitalizes on an earthquake's first-arriving seismic energy, called "P waves," the authors say their model could be implemented in a real-time system in which an accurate estimate of the length and duration of great earthquakes could be obtained within 20 to 30 minutes of the earthquake initiation.

The scientists note that the method's performance depends on the distance of the seismic array from an earthquake.

"The existing global seismic network could provide enough information to produce useful results for earthquakes anywhere in the world," said Shearer. "This would give people a much faster idea of the size and extent of large earthquakes. We are trying to work with other scientists to develop ways that they could use this method in a worldwide warning system."