The Echo
Alexandria, Minnesota
June 9, 1993

Quake was 'business as usual'
by Dennis Dalman

In geologic terms, Friday's Minnesota earthquake was "business as usual," a mere quiver of the ancient bedrock that lies beneath the state.

"It could be compared to a slight creaking of floor boards in a house," said Dr. Val Chandler, geophysicist with the Minnesota Geologic Survey at the University of Minnesota.

If data trends can be relied upon, Minnesotans can expect a magnitude 4 earthquake once every decade or so and a magnitude 5 earthquake about every century. That's small potatoes compared to the Californian record.

So far, Chandler has heard no reports of anything other than minor rumblings or clattering dishes caused by the Friday quake. Minnesota's quakes, typically, cause very little damage because unlike the seething geologic plates under California or Japan, the structure beneath the upper Midwest is cold and "dead" rock. Chandler termed it a "mixed bag" of rock -- granite and volcanic -- that was created billions of years ago.

There are cracks in the bedrock that periodically give way, but the shock waves tend to be dissipated over a wide area.

Because of the movement of giant crustal plates on the earth, the American continent has been moving ever so slightly westward for millions of years. This massive westward nudging could be enough to cause the Midwestern faults to grind and slip -- enough to cause quakes, Chandler noted.

The Friday quake, he said, was likely caused by a fracture zone ripping only an inch or two along a relatively short area -- maybe a few feet, certainly less than a mile. In less stable earthquake zones, rock masses can grind past each other for many feet over many miles along the fault line.

But Midwestern quakes can be felt over three or four times the size of affected areas of unstable-zone quakes, Chandler added. That's because shock waves are conducted easily through such stable bedrock over wider areas. Consequently, the damage in continental quakes can be far more widespread, even if such damage is minor.

Most of continental America is so stable, rock-wise, that a devastating quake is almost unthinkable. However, and this is a big however, a peculiarly unstable area in the southern Missouri area could someday cause horrendous destruction. It's called the "New Madrid" fault. Over a period in 1811 and 1812, it slipped massively several times, causing such eerie damage that geologists are still awed by its power. In cataclysmic upheavals, the series of quakes rearranged rivers, emptied lakes, raised up new hills and caused soft-sand geysers to gush into the air. At that time, there was scant population in the area. It was only later that geologists figured out how massive the quake had been.

The New Madrid fault, said Chandler, is a spectacular exception to the otherwise-stable American continental crust.

Are earthquakes and volcanoes on the increase in recent years?

No, said Chandler. They might seem to be more common but that's only because there are many more people living on more areas of earth -- people who feel and report tremors or eruptions. In addition, instantaneous communication and jet-age travel can spread the news of geologic phenomenon much more quickly and thoroughly.

An analysis of geologic data for the last 100 or so years shows that there might, in fact, be a slight recent "downturn" in such unsettling earthly activity.

Can animals really sense earthquakes before they occur?

Yes, said Chandler, scientists speculate that animals might be extremely sensitive to certain sound waves that travel through the ground before a quake happens. The animals, at such times, tend to act skittish, nervous. Even cockroaches and fish have been known to exhibit confused behavior before quakes.

But observing animal behavior cannot, at least so far, be used as reliable earthquake indicators. For one thing, by the time animals react, it's usually too late to take precautionary action, although in some cases the odd behavior was observed several days before a quake. For another thing, animals don't always react skittishly. Some quakes, apparently, don't send out "warning" sound waves.

"It's a real hit-and-miss thing," said Chandler.