New Madrid Seismic Zone: Aftershocks or Future Threat?

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Over a century after the New Madrid earthquakes, the marks were still visible on the landscape. Image: USGS Photographic Library

The New Madrid Fault, in the Midwest, has shaken Missouri residents recently  – are they in for more quakes, or were these isolated events?. This seismic zone has quite a history, and research shows that the region’s tremors may not be over.

New Madrid Quake Storm: 1811-1812

In the period between December 1811 and February 1812, a series of devastating earthquakes struck the New Madrid area of what was later to become the state of Missouri. The first, on 16 December, struck in Northern Arkansas; the epicentres of the second and third, on 23 January and 7 February, were in or close to the town of New Madrid itself.

There are no accurate records of deaths, although eyewitness accounts detail enormous devastation and terror. In the early days of settlement the population of the Missouri region was relatively sparse; today the number of people within the same region is much higher and a far greater number of people would therefore be at risk in the event of a New Madrid earthquake.

The magnitudes of these tremors, which predated the development of seismometers, are uncertain – but scientists estimate that the largest was between M7.7 and M8.7 and the smallest between M7.0 and M8.4. This makes them comfortably the largest to have occurred in the eastern part of the United States. Even on the lower estimates, ranks the New Madrid quakes close to some of the larger tremors recorded along the much more seismically-active San Andreas Fault Zone, if not quite as large as the tremors which can occur along the Alaskan and Cascadia subduction zones of the north and west Pacific.

The New Madrid Seismic Zone

Seismicity in the New Madrid Seismic Zone. Image credit: USGS

At first glance, the New Madrid region appears innocuous, even to the seismologist. A map of major world earthquakes shows a strong concentration along the boundaries of the Earth’s tectonic plates and. although the central and eastern parts of the US, like many other continental interiors, experience occasional minor tremors, such locations rarely see any seismic activity remotely approaching the magnitude of that of 1811-12. So what happened to destroy the town of New Madrid – and could it happen again?

Although the central United States is currently part of a continental interior, this wasn’t always the case. The Mississippi Valley overlies an ancient tectonic margin – a deeply-buried rift around 100km wide and doglegged in form — which extends from northern Arkansas before trending north west into Missouri.

Although originally formed as long ago as 540 million years, the fault was active much more recently (in geological terms) well into the Tertiary period, less than 65 million years ago. And, as the magnitude of the New Madrid earthquakes indicated, the rift has not finished moving yet.

Aftershocks or Active Movement?

Seismic activity around New Madrid differs from a typical aftershock pattern. Image credit: USGS

Major earthquakes frequently generate aftershock sequences which can continue to reverberate for decades and more after the main event. Seismologists have argued that the ongoing seismic activity in the area, which has persisted over the past two centuries, has produced a pattern of small earthquakes and level of seismic activity which the USGS describes as “a higher rate than elsewhere in the central United States.

Recently, however, scientists studying the pattern of aftershocks have reached a different – and, for the residents of the New Madrid seismic zone, potentially disturbing – conclusion.

In a study published in January 2014, seismologists Morgan Page and Susan Hough concluded that the pattern of activity observed within the seismic zone may not comprise aftershocks but may instead represent continuing seismic activity along the rift – leaving open the possibility of larger, more damaging earthquakes in the future.

The Future For New Madrid

The New Madrid earthquakes are much more than just a seismological curiosity. Their existence, reinforced by the ongoing smaller-magnitude tremors, demonstrates a clear threat of earthquakes in the region. Looking further back in time, there’s evidence of other major tremors in the past.

Even before Page and Hough’s report, seismologists were involved in detailed monitoring and investigation of activity in the zone. In 2009 the USGS published a summary of those investigations which concluded that: “The preponderance of evidence leads us to conclude that earthquakes can be expected in the future as frequently and as severely as in the past 4,500 years.”

Clearly, such warnings are a concern for the residents not just of New Madrid but of surrounding areas. Equally clearly, we cannot prevent those earthquakes. As the USGS seismic hazard paper concludes: “Such high hazard requires prudent measures such as adequate building codes to protect public safety, and ensure the social and economic resilience of the region to future earthquakes.”

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