Sydney, March 29 (Reuters): A huge undersea earthquake that killed hundreds in Indonesia overnight was a result of increased geological stress caused by last December?s mega-quake that sent a devastating tsunami across southern Asia, seismologists said.

And they warned of a third big earthquake in the area sooner or later.

Last night?s 8.7 magnitude shock was centred 160 km southeast of the epicentre of the 9.0 magnitude December 26 quake off Sumatra?s northern tip. Geoscience Australia said the earlier quake was two to four times larger.

Like the 2004 tremor, the latest one was a vertical earthquake where part of the ocean floor was thrust upwards by another tectonic plate pushing beneath it. ?It appears to be the same plate boundary where the Australian plate is slipping beneath the Sumatran (Sunda) plate,? Geoscience Australia seismologist Phil Cummins said.

Seismologists had been warning of a second earthquake off Sumatra due to the increased geological stress caused by last December?s quake.

Indonesia?s disaster centre said around 1,000 people had been killed on the island of Nias, off Sumatra?s west coast, which took the full fury of the latest quake, one of the eight biggest in the world since 1900. Cummins said a third earthquake was to be expected because geological stress levels would rise further following the latest tremor.

?There is a chance that the next segment further to the southeast could rupture sooner than we expected,? he said.

The US Geological Survey reported that yesterday?s quake was located 2.065 degrees north and 97.010 degrees east. The December 26 quake was located 3.307 degrees north and 95.947 east. Both quakes were shallow, at a depth of 30 km, which heightened the chances of a tsunami.