Ask most Americans where they expect an earthquake, and you'll likely hear about California. Missouri isn't usually on the list, but a major tremor would be far more devastating there because nobody expects it.

A lot of the U.S., we haven't looked at it thoroughly, said Robert A. Williams, of the Geologic Hazards Team at the U.S. Geological Survey. Williams studied the Christchurch earthquake closely, and said that one thing that made it harder to see where the fault was is that it was in a former river valley, which had eroded away all the evidence.

Earthquakes tend to happen along fault lines, huge cracks in the Earth's crust. Some fault lines are famous, like the San Andreas in California. Some are less so, like the Wabash Fault in Illinois. The San Andreas is a strike slip fault, one that marks an area where two huge pieces of the Earth's crust, the Pacific and North American plates, are sliding by each other. As they pass, they don't always do it smoothly, and when that happens, you get an earthquake. Another type of fault occurs in the center of continental plates. In the U.S., the Wabash Fault in Illinois and New Madrid Seismic Zone in Missouri are the result of ancient cracks that appeared when North America began to break apart 750 million years ago. The rift never fully formed, but it left a zone of weakness that has been a source of earthquakes ever since.

New Zealand, Williams says, is similar to California in the types of faults is has in the area. But geological similarity isn't all that makes an earthquake dangerous. In Christchurch, there was a mix of old and new buildings, and the older ones fared much worse because they were not built up to modern standards. A key part of keeping people safe, he says, is the building code, because newer buildings will usually withstand the shaking and older ones can be retrofitted.

In California, earthquakes are common enough that many people remember being in one. The Loma Prieta quake, which killed 63 people in the San Francisco Bay region, was in 1989, and the Northridge earthquake, which killed 33, was in 1994. But the last big earthquake in the New York City area was in 1884, a magnitude 5 tremor that damaged homes but did not result in deaths. Williams notes that the harder, denser rock in the northeastern U.S. carries the energy of earthquakes much farther and faster than on the west coast. That means a quake of the same intensity as the one in Christchurch -- magnitude 6 or more -- would be far more dangerous.

Earthquakes are less likely (and happen less often) in the Northeast because the area is not as seismically active as California or New Zealand. But the New Madrid Seismic Zone, named for the town of New Madrid, Mo., has produced several intense quakes over the past two centuries. The most famous were in 1811 and 1812, and were of magnitudes between 7 and 8.0. The tremors were felt as far away as New York and Chicago. The last in that series of quakes destroyed the town of New Madrid.

The last major earthquake in the region was in 1968, centering on the Wabash Fault (and resulting in its discovery). The epicenter was about 60 miles west of Evansville and was magnitude 5.4. It was felt in 23 states and caused minor damage to buildings as far as 80 miles away. Here too, a magnitude 6 earthquake would cause much more extensive damage than in a place such as Christchurch, or California.

Robert Yeats, a senior consultant at Earth Consultants International and a former professor of geology at Oregon State University, says the problem is that most people only think about events that happen in a human lifetime. That's a blink of an eye as far as a geologist is concerned, he said. Measuring how often earthquakes happen can be done on several scales. In California they are relatively frequent, so people have building codes that take them into account. In the Midwest, intense ones are few and far between. The New Madrid Seismic Zone, for example, has produced thousands of earthquakes in the last thirty years, but only a small fraction of them were felt because they were not very intense. That means people don't always plan for them.

That said, while predicting earthquakes is impossible to do, it is possible to say where they are more likely to happen. But to do that you need a good map of the geology and seismic characteristics of the country. A complete map doesn't exist yet, but there is one being constructed.

The USArray project has been operating since 2004. It is a network of400 seismographs that have been placed in a banding pattern across the United States. They are about 70 kilometers (43 miles) apart, and are moved every two years. The system is part of the EarthScope project, funded by the National Science Foundation. The whole program will be completed in 2015, and when it is done there will finally exist a more comprehensive seismic map of the U.S.

The quake in Christchurch, New Zealand, happened in a place where building codes are strict and such activity isn't unexpected. But the fault line was one that hadn't been detected before. The USArray project will help detect such faults in the U.S. With that information it will be easier to decide where building codes need to be more strict, and also help scientists to better understand how and why earthquakes happen.

In addition to better studies, there is work being done on detection systems. A startup in California, Seismic Warning Systems, has designed a detector that can give some warning in the event of a tremor, allowing people precious seconds in which to get to a safe place.

Ultimately, Yeats says, the issue is how much money people are willing to spend on reinforcing buildings to deal with events that are few and far between, but devastating when they happen. He notes that there is a known fault line in Seattle where a water treatment plant is being built, but because it hasn't been active for thousands of years, it is considered safe. If the life of the building is 100 years or so, you might not see one. But you can't say for sure. Is that a wise decision? I don't know, he said. Geologists think in terms of millions of years at least, he adds.

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