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Earthquake Activity

Andaman Sea Earthquake Swarm Beginning January 26, 2005

figure 1A large number of earthquakes have recently occurred beneath the Andaman Sea east of the Nicobar Islands, starting 26 January 2005. The earthquake activity is in a broad sense part of the aftershock sequence associated with the great Sumatra – Andaman Islands earthquake of 26 December 2004 and
was likely triggered by stress changes associated with the great earthquake. However, unlike the 26 December main shock, the Andaman Sea activity is not occurring on the western boundary of the Burma plate, where the Burma plate is overthrusting the India plate, but instead is occurring on the eastern boundary of the Burma plate with the Sunda plate, a zone of strike-slip and normal faulting.

Within the overall aftershock sequence of the 26 December earthquake, the episode of Andaman Sea earthquakes is classified as an earthquake “swarm” – an episode of high earthquake activity in which the largest earthquake does not occur at the beginning of the episode and in which the largest earthquake is not substantially larger than other earthquakes of the episode. Worldwide, earthquake swarm activity is commonly associated with plate boundaries like that beneath the Andaman Sea, in which both strike-slip fault and normal faulting occur and where magmatic activity occurs at shallow depths in the earth’s crust.

figure 2The recent Andaman Sea swarm has occurred in an area no more than several tens of kilometers across, within a 300-km long plate boundary that has been persistently active for at least the past four decades. Previous activity in the 300-km long segment has also tended to occur in seismic swarms, though none of the swarms have been nearly as large as the current one. This plate-boundary segment became active within minutes of the 26 December main shock and, prior to the onset of the swarm, was one of the most active parts of the 26 December aftershock region (Figure 1). The swarm activity that began on 26 January nonetheless represents a dramatic burst of seismic activity within the aftershock sequence following the great 26 December main shock (Figure 2).

Although volcanic eruptions are often preceded and accompanied by earthquake swarms, the unusual activity east of the Nicobar Islands is not associated with any known volcanic center. Even in areas near volcanic centers, most earthquake swarms are not associated with volcanic eruptions. These observations suggest that the Andaman Sea swarm will subside with no associated volcanism.

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First Government graphic