The Hindu Business Line : `Quake rupture patch compares to size of California'

IN its observations on Sunday's killer earthquake off northern Sumatra, the US Geological Survey has said that the `rupture patch' involved compared to the size of California.

"Earthquakes rupture a patch along a fault's surface. Generally speaking, the larger the rupture patch, the larger the magnitude of the earthquake. Initial estimates based on the aftershock distribution show the magnitude 9 Sumatra-Andaman Islands earthquake ruptured a patch of fault roughly the size of California, and modelling of the seismic waves show that most of the slip occurred in the southern 400 km of the patch.

"For comparison, a magnitude 5 earthquake would rupture a patch roughly the size of New York City's Central Park," the agency said on its Web site. Preliminary estimates of the size of the rupture that caused the temblor were obtained from the length of the aftershock zone, the dimensions of historical earthquakes and a study of the elastic waves generated by the quake. Aftershocks suggest that Sunday's earthquake rupture had a maximum length of 1,200-1,300 km, parallel to the Sunda trench, and a width of over 100 km.

The maximum displacement on the rupture surface between the two plates was estimated at 20 metres. Regarding the maximum displacement of the sea bottom above the earthquake source, it said that, in places, the block of crust beneath the sea floor and overlying the causative fault may have moved by 10 metres to the west-southwest and got uplifted by several metres.

At the source of the quake, the interface between the India plate and the Burma plate dips about 10 degrees to the east-northeast. The subducting plate dips more steeply at greater depths. Since 1900 and prior to the December 26 earthquake, the largest earthquake along the subduction zone from southern Sumatra to the Andaman Islands occurred in 2000 and had a magnitude of 7.9.

A magnitude 8.4 earthquake occurred in 1797, a magnitude 8.5 in 1861 and a magnitude 8.7 in 1833. All three had ruptured sections of the subduction zone that was to the south of the recent earthquake.

"Interestingly, the 1797 and 1833 quakes are believed to have ruptured roughly the same area with only 36 years separating the events. Paleoseismic evidence shows that great earthquakes or earthquake couplets occur about every 230 years," the Web site said.

Being a major earthquake, scientists took time to accurately measure the magnitude of Sunday's temblor. "In the case of the M9.0 Sumatra-Andaman Islands earthquake, the standard methods were inadequate for measuring the very low frequency energy produced and had to be modified. This delayed the final determination of the magnitude until the next day."

According to the Web site, the earthquake would have redistributed tectonic stresses along and near the boundary between the India plate and the Burma plate. In some areas, this redistribution would be such that it shortens the time before the next big earthquake compared to what would have been the case had the temblor not occurred.

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`Quake rupture patch compares to size of California'