Quake rates among most powerful

By Roland Pease
BBC Science

At an estimated magnitude of 8.7, the latest earthquake off Indonesia was about 5-6 times weaker than the one that triggered December's tsunami.

But it is still massive by historical standards, rating among the eight strongest since 1900.

The movement was along a line around the coast of Sumatra, to the south of the long fault that gave way three months ago.

Earlier this month, researchers had warned that the earlier movement had added stress to this region, increasing the chance of an earthquake there.

This zone last ruptured in 1861 and was already considered at risk.

The expectation had been that an earthquake here would cause a tsunami. The fact that it has not may indicate that the seismic movements were all below the earth's surface - only if the seafloor itself moves will it create the devastating surge of a tsunami.

Improving the science of tsunamis will be an urgent task for researchers. There is still concern that another fault in the region, running down the middle of the island of Sumatra, was stressed by December's earthquakes.

New calculations will be needed to estimate the effect of the most recent quake on that fault.