Indonesian Earthquake Cluster

Many parts of Indonesia have experienced a number of strong earthquakes located off the coast of Sumatra. The quakes represent a rupture of the boundary between the Australian and Sunda tectonic plates, and are occurring at two locations either side of the city of Bengkulu on Sumatra.

The burst of activity commenced in earnest at 11:10 p.m. on Wednesday 12th September 2007, New Zealand time, with a magnitude 8.4 quake at a depth of 30 km. The shallow quake was located under the Indian Ocean, 130 km south-west of Bengkulu, Sumatra. The quake generated tsunami waves, the highest of which was recorded at Padang, Indonesia at 1:48 this morning, when the sea level rose by 0.98 metres (3.2 feet). An earlier wave at Padang showed a sea level change of 0.56 metres (1.8 feet) at 1:06 a.m. New Zealand time.

Earthquake activity in the area eased, with only 4 aftershocks with magnitudes between 5.2 and 6.0 being reported up to 2:30 this afternoon.

However, the fault zone has continued to rupture, with earthquake activity moving to the north-west of Bengkulu as a series of six foreshocks with magnitudes between 5.2 and 6.0 built up to another major event just over 12 hours after the great quake of magnitude 8. This culminated in a magnitude 7.8 earthquake at 11:49 a.m. on Thursday 13th September 2007, New Zealand time. The quake was located 200 km north-west of Bengkulu at a depth of 10 km.

This second major undersea quake initially triggered tsunami warnings for the Indian Ocean, just as authorities were relaxing as wave gauge data relating to the great quake was being confirmed. The Pacific Tsunami Warning Centre issued a bulletin at 3:02 p.m. advising that it had no reports or measurements of tsunami waves in the Indian Ocean.

Since the second major quake, earthquake activity has moved even further north-west with three quakes with magnitudes between 5.2 and 5.8 at shallow depths 350-odd km from Bengkulu. The activity is continuing, with a shallow magnitude 7.1 earthquake reported 345 km west-north-west of Bengkulu at 3:35 this afternoon.

Since the magnitude 9.1 earthquake and tsunami of the 26th of December 2004, more than 2,000 km of plate boundary has ruptured along the Java Trench under the Indian Ocean, involving collision zones between the Indian and Australian Plates with the Burma and Sunda Microplates. The ruptures have included the magnitude 8.6 Nias Island earthquake of the 28th of March 2005.

The two largest earthquakes in the cluster near Sumatra during the last 24 hours are the world’s largest and 5th largest recorded so far this year.

[Compiled from data supplied by the US Geological Survey and its contributing agencies, and the U.S. National Oceanic & Atmospheric Administration’s Pacific Tsunami Warning Centre.]

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