Rock Doc

I once experienced a small earthquake when I was visiting the San Francisco Bay area in California. The natives thought little of the temblor but I was impressed that the ground beneath my feet could suddenly and without warning start to shake.

Later, when I majored in geology in college, I learned that my native Northwest is also at risk for earthquakes, as is much of Alaska. Another part of the country with a history of large quakes is called the New Madrid Seismic Zone. It's a pretty large region centered near where the states of Arkansas, Missouri, Illinois, Tennessee and Kentucky come together in the lower Midwest.

Some earthquake-rich areas are easier for geologists to understand than are others. In the Pacific Northwest, for example, major tectonic plates are coming together. Their movement guarantees earthquakes from time to time. In California, quite famously, the San Andreas fault marks the place where two plates are moving past each other. This leads to shallow earthquakes that can be particularly destructive. But there are also regions of the country -- like the New Madrid area -- where major quakes can occur away from plate boundaries.

To understand the New Madrid Seismic Zone in the middle of the continent, we first need to review a bit of history. Mother Nature was heard from in a big way in late 1811 and early 1812 in that region. According to a U.S. Geological Survey website, during that time the area experienced three very large quakes with magnitudes over 7.

The way that geologists now think of the quakes is basically this: the first mega-quake occurred on December 16 of 1811. The second quake occurred about a month later, on January 23, 1812, and the third two weeks after that, on February 7, 1812. But those quakes were not isolated. There were numerous other quakes that geologists now interpret as likely aftershocks. The aftershocks may have been as big as magnitude 6 or 6.5. That means the "aftershocks" would count as large quakes in their own right by human standards. Numerous smaller aftershocks also shook the region.

There are written accounts from people living in the lower Midwest at the time of the big quakes that describe ground movement that went on and on. Structures in St. Louis were damaged. In short, it was not a good place to be when the Earth decided to release enormous amounts of energy that were pent up within it.

From that time down to the present there have been small temblors in the New Madrid Seismic Zone. One question that has arisen for geologists is whether to think of these quakes as long-term aftershocks of the major events of 1811-1812, or to think of them as something else. It's an important question because if the quakes have been aftershocks, there might be little stress building within the Earth in the area. That would be good news for everyone living in the lower Midwest.

Recently two researchers published a piece in Science about their efforts to understand the long history of quakes in the region. Morgan Page and Susan Hough of the U.S. Geological Survey used computer modeling of aftershocks to analyze what's been happening in the New Madrid area. They found that there haven't been many quakes of "moderate" size - that is, in the approximately magnitude 6 range -- but there have been a lot of small quakes in the region.

This pattern, the scientists argue, isn't consistent with the idea that all the quakes are aftershocks of the events of 1811 and 1812. Instead, the recent history of quakes in the area suggest that ongoing Earth processes continue to generate stress in the region. And that means some energy will likely need to be released -- perhaps in another quake on the scale of those that hit in the early 1800s.

Geologists are so far unable to make specific predictions of when quakes will occur. But it seems likely, if Page and Hough are right, that one day the lower Midwest will have to cope with a major quake. It's not good news, but it's a risk we need to face squarely -- and it highlights the importance of preparedness for individuals, families and municipalities.

Dr. E. Kirsten Peters, a native of the rural Northwest, was trained as a geologist at Princeton and Harvard. This column is a service of the College of Agricultural, Human, and Natural Resource Sciences at Washington State University.