The zone lies around New Madrid, a town in eastern Missouri that lies on Mississippi River.

A monster quake, since estimated to measure 7.4-8.0 on the Richter scale, ripped across the town on December 16, 1811, causing surface waves across the ground locally and inflicting damage as far as Washington.

Aftershocks continued there for much of the following year, and there were major quakes in 1843 and 1895 measuring 6.3 and 6.7 respectively, making it the most seismic region in the United States east of the Rocky Mountains.

But these events happened when the region was sparsely populated -- New Madrid had a population of just 400, all of whom were reportedly killed.

Today, the region is home to millions of people, which thus makes it vital to try to estimate when the next big quake could hit.

US scientists, reporting in the British science journal Nature on Thursday, say the New Madrid Seismic Zone is deforming rapidly, experiencing rates of strain that are similar to those in notoriously active plate boundaries.

They sowed the area in the late 1990s with a network of monitors that use the Global Position System (GPS), the US satellite navigation network, to record minute movements in the ground.

They also found that the 1811 temblor was so violent that it liquefied the ground. Geological drilling showed previous liquefactions occurred in layers dated to approximately 1450 AD, 900 AD, 300 AD and in 2350 BC.

On that basis, the buildup of seismic strain results in a quake greater than 7.5 roughly every 500 years or so.

The study is headed by Michael Ellis, an associate professor at the Center for Earthquake Research and Information at the University of Memphis, Tennessee.

This knowledge should be factored into construction standards, so that new buildings in Memphis and other cities in the central United States can be protected against big quakes, a commentary published in Nature said.

The New Madrid Seismic Zone is believed to have been formed more than half a billion years ago.

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