Plate Tectonics in Action

The sliding of one tectonic plate beneath another, called subduction, was graphically illustrated by the series of earthquakes that zippered along the Indian Ocean near the Java Trench off the coast of Sumatra this week.

Earthquake activity near Sumatra has eased dramatically following two days of intense activity that had geologists, seismologists and tsunami experts constantly re-evaluating risks and recalculating quake data earlier this week.

There was no short-term warning of the massive earthquake which struck off the coast of Sumatra shortly before midnight New Zealand time on Wednesday night. Earthquakes had occurred throughout South East Asia during the previous five days but at scattered locations, with a pair of magnitude 5 quakes in the Nias region and another pair in the Molucca Sea. Single magnitude 5 quakes had been reported in the Sunda Strait and the Sulawesi Sea, and two 4th magnitude events in the Bali Sea.

At 11:10 p.m. New Zealand time on September 12th, the massive magnitude 8.4 quake struck 130 km south-west of Bengkulu, Sumatra at a depth of 34 km, triggering a series of earthquakes that would ripple along several hundred kilometres of the tectonic plate interface marked by the under-sea Java Trench.

Over the next two days a series of 44 earthquakes would strike at five distinct locations off the coast of Sumatra. Initially, two clusters of earthquakes centred 150-odd km south-west and 100-200 km west-north-west of Bengkulu were active. The south-west cluster presented a series of aftershocks following the magnitude 8 quake, while the west-north-west cluster presented a series of one 6th magnitude five 5th magnitude foreshocks before the main event of magnitude 7.9 at 11:49 a.m. on September 13th, New Zealand time.

Within 2 hours a new centre of activity opened up further north-west, centred about 160 km south-west of Padang. An abrupt series of five 5th magnitude foreshocks preceded a magnitude 4.8 quake which was followed by the main event of the cluster, a magnitude 7.0 earthquake at 3:35 p.m. New Zealand time on the afternoon of Thursday September 13th.

As this cluster entered an aftershock phase, a new centre of activity centred around 180 km west of Bengkulu opened up. This centre has been remarkably consistent, producing five quakes with magnitudes between 4.9 and 5.2 before it settled down at 1:30 yesterday afternoon (Friday).

As the plate boundary reacted to the rapidly altered stresses, earthquakes from these four centres of became intermingled. Up until mid-day today (Saturday), the cluster south-west of Bengkulu has produced eight earthquakes between magnitude 4 and 8, the cluster north-west of Bengkulu has produced thirteen earthquakes with magnitudes between 4 and 7, the cluster south-west of Padang has produced sixteen quakes with magnitudes between 4 and 7, and the cluster west of Bengkulu five quakes with magnitudes between 4 and 5. A smaller cluster centred about 55 km west of Bengkulu has contributed another two 5th magnitude earthquakes.

In addition to these 44 earthquakes, the wider region has experienced sporadic events. Activity reached its peak on September 13th when 27 quakes of 5th magnitude or greater rocked the region.

The rapid migration of activity along the plate interface had tsunami experts scrambling to identify the risks from the shallow earthquakes. The Pacific Tsunami Warning Centre found itself managing five major events that could produce tsunami waves over a period of less than two days. The centre is hampered by the low number of wave gauges available to monitor sea levels in the Indian Ocean area. It issued 17 bulletins for Pacific and Indian Ocean areas, on two occasions not long after cancelling tsunami “watches” from previous nearby events.

The Warning Centre’s new website struggled to handle the peaks of visitors as the events unfolded. The site features a meter, similar to our fire danger signs with their settings for low, medium, high and extreme fire danger. Over the period, the meter reached high tsunami risk on at least two occasions, before settling back.

As seismologists and tsunami experts at the Warning Centre and US Geological Survey have taken time off to catch up on sleep, revision work has continued apace. The iterative process of analysing more data on each earthquake is being used to fine tune information on magnitude, location and depth.

The largest event of the series has settled at magnitude 8.4 and stands as the largest earthquake to have struck the world to date in 2007. The magnitude 7.9 and 7.0 quakes rank fifth and eleventh, respectively. During 2006, The US Geological Survey reported ten quakes of magnitude 7 or greater, eleven in 2005, thirteen in 2004 and fourteen in 2003.

[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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