AIR Analyzes Strong Indonesian Quake

September 3, 2009

A major earthquake struck off the coast of Indonesia’s main island of Java at 2:55 p.m. local time (07:55 UTC) on Wednesday. The U.S. Geological Survey (USGS) originally issued a preliminary magnitude estimate of 7.4 for the event, but has since downgraded it to 7.0. Depth is estimated at 60 kilometers (37.3 miles).

Catastrophe modeling specialists AIR Worldwide said the “tremor occurred about 50 km (31 mi) south of Java (population 130 million), the most populous island in Indonesia and the location of the region’s capital city, Jakarta.

“Local officials have reported that more than 1,300 buildings on the island were damaged or destroyed.” [IJ Ed. Note: The latest reports indicate that at least 44 people were killed by the quake, more than 300 were injured, and many more are missing].

“Today’s earthquake occurred along the Java subduction zone where the Indo – Australia plate subducts under the Sunda plate at a velocity of 6-7 centimeters per year,” explained Dr. Bingming Shen-Tu, principal scientist at AIR Worldwide. “Unlike the Great Sumatra plate boundary zone, west of Java—where large-magnitude earthquakes have been frequent in the past 100 years, the Australia—Sunda plate boundary zone, south of Java, has been relatively quiet during this time. In fact, before this event, scientists were already questioning whether this subduction zone was truly inactive.”

AIR said the depth of this event, as estimated by USGS, suggests that it was not a simple subduction zone earthquake on the plate interface. “The relatively deep focal depth indicates that this earthquake may have ruptured or originated in the subducting oceanic plate,” added Dr. Shen-Tu. “Because of the higher density and strength of the oceanic plate, events occurring in this area can generate more significant ground motion than a typical event occurring along the plate interface or within the crust.”

Ground shaking—which residents reported lasted for a few minutes—was felt as far away as Surabaya, Indonesia’s second biggest city, about 500 kms (300 miles) away, and on the island of Bali, 700 kms to the east. The quake triggered a landslide in a village in the West Java district of Cianjur and power has been cut off in parts of the island. Indonesia’s Bureau of Meteorology issued a tsunami alert but it was withdrawn within 40 minutes of the event.

AIR said it is analyzing the currently available seismological information for this event and will make additional updates as warranted.

Source: AIR Worldwide –

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