Embargoed for release: May 19, 2005, 14:00 Eastern U.S. time (19:00GMT) Copyright, 2005, Graphic News. All rights reserved

Shock and awe of SumatraÕs monster quake

LONDON, May 19, Graphic News: The great Sumatra-Andaman earthquake of December 26, 2004, was an event of stunning proportions, both in its human dimensions -- nearly 300,000 lives lost -- and as a geological phenomenon.

The sudden rupture of a huge fault beneath the Indian Ocean unleashed a devastating tsunami. It was the largest earthquake in the past 40 years, releasing the equivalent to a 100-gigaton bomb, or about as much energy as the United States uses in six months.

Using highly sensitive digital broadband seismometers deployed around the world, seismologists recorded both the huge ground motions from the mainshocks and the tiny motions from small aftershocks around the planet. Their findings are published in the May 20 issue of the journal Science.

ÒThis is really a watershed event,Ó said Thorne Lay, professor of Earth sciences and director of the Institute of Geophysics and Planetary Physics at the University of California, Santa Cruz. ÒWeÕve never had such comprehensive data for a great earthquake, because we didnÕt have the instrumentation to gather it 40 years ago. And then the sheer size of the event is so awesome. It is nature at its most formidable.Ó

Record-setting features include the longest fault rupture ever observed (1,200 to 1,300km, 720 to 780 miles) and the longest duration of faulting (at least 10 minutes). The ground moved up and down by more than 9 centimetres (3.6 inches) as far away as Sri Lanka, 1,600km (1,000 miles), and ground motions greater than 1 centimetre (0.4 inch) occurred everywhere on EarthÕs surface.

The long-distance ripple effects also triggered a swarm of 14 smaller earthquakes near the Mount Wrangell volcano in Alaska, 11,000km (6,800 miles) away, on the other side of the planet.

The new analysis gives the rupture a seismic magnitude of at least 9.1, and possibly as high as 9.3. Earlier estimates had put it at magnitude 9.0. By comparison, the 1960 Chile earthquake was magnitude 9.5, and the 1964 Alaska earthquake was magnitude 9.2. The data from those earlier earthquakes are relatively limited, however, and small differences in magnitude may not be significant, Lay said. 

The earthquake took place along the curving boundary between major plates of the EarthÕs crust, where the Indo-Australian plate plunges beneath the southeastern Eurasian plate. Before the fault ruptured, the edge of the Eurasian plate was being dragged downward by the descending Indo-Australian plate. Released by the rupture of the fault, the edge of the plate sprang back up, uplifting the ocean floor and setting off the tsunami that inundated coastal areas throughout the Indian Ocean. The fault slipped by as much as 15 metres (50 feet) in places, averaging about 10 metres (33 feet) of displacement along the segment off the northwestern tip of Sumatra where the quake was centered. 

From the epicenter, the rupture expanded along the fault at a speed of about 2.5 kilometres per second (1.5 miles per second) toward the north-northwest. But the initial movement of the fault was much less along the northern segment than in the south, which spared much of the coastline in the north from the massive tsunami waves that caused so much destruction further south. Eventually, the northern part of the fault slipped about as much as the southern part, uplifting and tilting the Andaman Islands. The tilting of the islands shows that the northern part must have slipped about 10 metres, but much of that slip occurred gradually, without generating seismic waves.

ÒEven among seismologists, we call this a monster earthquake,Ó Lay said.

After the earthquake and the tsunami came the aftershocks, including the most energetic earthquake swarm ever observed. More than 150 earthquakes of magnitude 5 and greater occurred over a four-day period in late January on faults beneath the Andaman Sea.

Then on March 28, an 8.6-magnitude earthquake struck IndonesiaÕs Nias island, killing almost 1,000 people. This was not an aftershock, but a new rupture of an adjacent segment of the fault. Sixteen days later the Mount Talang volcano on IndonesiaÕs Sumatra Island erupted, causing villagers on its slopes to flee their homes in panic.

Now, concern about additional earthquakes is focused on the next area to the southeast, which last failed in a great earthquake in 1833. Major earthquakes could occur not only on the subduction fault along the plate boundary, but also on a related fault system that runs right down the length of the island of Sumatra. Faulting on that system involves horizontal shearing, similar to the San Andreas Fault in California. 

ÒWhen one part of the fault slides, that loads up the adjacent region and transfers stress. So you have a heightened potential for earthquakes on the adjacent section. The concern is that something like that could happen in Sumatra,Ó Lay said.
/ENDS

Source: Science