And great earthquakes shall be in divers places, and famines, and pestilences; and fearful sights and great signs shall there be from heaven…
Milkshakes: unusual earthquakes strike Wisconsin
By Scott K. Johnson Published Thursday 29 March 2012
Some of the Wisconsin locals might have experienced a little shaking lately.
It was a genuine small-town mystery that briefly put Clintonville, Wisconsin in the national spotlight. Late on March 18, folks in the city of 4,700 just west of Green Bay (and a couple hours north of my current base of operations) suddenly flooded 911 with reports of unsettling booms and shaking. Callers described the noises as being similar to jackhammers, rattling pipes, rumbling thunder, or slamming doors. Authorities scrambled to identify the source. Gas lines were checked for signs of leaks or other anomalies, the same for sewers and water mains. Planes surveyed the county for plumes of smoke. The landfill was checked for signs of a methane explosion. The dam was inspected for structural damage. The military was asked about exercises. Everything checked out, and there were no reports of industrial accidents, either.
Thoughts naturally turned to earthquakes, even though Wisconsin is about as seismically active as a sloth is fast. The United States Geological Survey (USGS) wasn’t reporting any events in the region, so that route of inquiry didn’t go far. The reports stopped coming around in 10:00 am, but the booms returned with nightfall like the mysterious assailants in The 13th Warrior.
Some in the town grew uneasy as officials stood baffled. Others had some fun with it. Theories poured in, including sinkholes (unlikely given the sandstone bedrock), fracking (unlikely given Wisconsin’s total lack of fracking operations), meth lab explosions, secret underground bunkers, worms from the movie Tremors, and aliens.
On the third night, the city put out some cameras and microphones, hoping to capture some clues. Only a few booms were reported and none were caught on the recordings. Early that evening, though, booms and shaking were reported in another town—Montello, about 65 miles southwest of Clintonville. Reports slowed after the first couple evenings, though a number of calls came in again late on the 27th.
On March 22, news that the USGS had identified a magnitude 1.5 earthquake in the seismic records from March 20th began circulating. Such small earthquakes are difficult to tease apart from noise in seismographs, and are only detected through focused effort. This was only possible because of the EarthScope project, which deployed a dense network of transportable seismometers slowly stepping their way across the country, providing high resolution records as they go. Wisconsin is currently within the coverage of the transportable array, so a number of seismometers were able to detect the earthquake in Clintonville.
I sat down with Alex Teel, a graduate student in Seismology at the University of Wisconsin- Madison, to get a look at some of the seismic data. A little data processing to filter out low frequency noise made the magnitude 1.5 quake stand out pretty clearly, but going hunting for more events was another story. By the time the seismic energy reaches the nearest seismometer, the amplitude of shaking is about the same as background noise (like industry and road traffic). In fact, one of the most obvious events in the record was the March 20 magnitude 7.4 earthquake in Oaxaca, Mexico. At least one seismograph contained signals of microearthquakes at the time that shaking was reported in Montello, however.
Teel said, for a number of reasons, it was difficult to confidently pin down a depth for the Clintonville earthquake. Additional analysis may make that possible. There are no known faults in the rock there, as the deep bedrock is largely mysterious. Such rocks are often complicated (a lot can happen in 1.5 billion years), and it’s difficult to study something you can’t see. The recent seismic activity had never been detected before in that area (you have to go back to 1947 to find the last report of an earthquake anywhere in Wisconsin). But it could force the rock to yield some of its secrets.
Why would a shake make a boom?
There are a couple reasons why such small earthquakes could have such an effect. While Clintonville sits atop glacial sediment and sandstone, you don’t have to go too deep to hit 1.5-billion-year-old granite. As we explained after Virginia’s magnitude 5.8 earthquake last August, seismic energy is better transmitted by old, dense rocks. Secondly, the earthquake epicenter just happened to be directly beneath the town.
But what about the loud booms reportedly accompanying the shaking? This curious phenomenon is not unheard of, particularly with small earthquakes. Moodus, Connecticut has apparently been experiencing them for centuries. In fact, the name of the town comes from a Native American word meaning “place of bad noises.” Residents there have heard booms associated with earthquakes smaller than magnitude 1, which were only detectable when a network of seismometers was set up in the area for that purpose.
And there are other examples, such as New York’s Lake Seneca, where the booms are known as “Seneca Guns.” Earthquake booms appear to be caused by seismic P-waves, which are compression waves (like sound), passing out of the ground and into the air. In an article published in Seismological Research Letters, USGS geologist David P. Hill wrote that in some cases “it seems that the free surface acts as a giant woofer in response to the incident P-wave”.
Quakes far from plate boundaries
These events made for an auspicious time to be reading a paper on earthquakes that occur far from tectonic plate boundaries, published recently in Geophysical Research Letters. The paper reviews the little we know about these rare quakes and lays out the reasons why they’re so difficult to study.
The paper focuses on the recent Virginia earthquake, but there have been many other sizeable earthquakes in areas of North America that do not lie near a tectonic boundary. In 1929, a magnitude 7.2 earthquake off the coast of Newfoundland caused a submarine landslide that severed telegraph cables and created a tsunami, killing 28 people. Of course, the legendary earthquakes near New Madrid, Missouri in 1811 and 1812 (which included several around magnitude 7.5) serve as a potent reminder that intraplate earthquakes can mean business. Earthquakes around magnitude 7 also occurred near Charleston, South Carolina in 1886 and in Baffin Bay in 1933.
If these areas are so far removed from tectonic plate boundaries, why the earthquakes? First of all, the stress caused by the movement and collision of plates is not limited to the boundaries—some of it is distributed throughout the plate. That stress can be released at weak spots, such as old faults. The eastern US, for example, has many such faults associated with the creation of the Appalachian Mountains in the distant geologic past.
Another source of stress is the vertical motion of the crust caused by the growth and retreat of ice sheets. Massive ice sheets, like the one that covered large portions of North America during glacial periods, actually depress the crust. Just like placing a weight on a floating sponge will cause it to sit lower in the water, an ice sheet makes continental crust sit a little lower in the mantle. When you melt away an ice sheet, the crust slowly rebounds upward. This warping of the crust creates stress that can also trigger movement on faults. The authors say today this is much more significant farther north where the rate of vertical motion is greater. South of the Great Lakes, the contribution is minor.
Intraplate earthquakes are tough nuts to crack. While plate boundary earthquakes are related to large-scale structures that are intensely studied, intraplate quakes often occur in areas where the geology is poorly understood (as is the case in Clintonville). Events large enough to be detected are also incredibly infrequent, making it difficult to connect multiple earthquakes in the same area.
In addition, when one section of a fault relieves stress during an earthquake, it usually causes stress to shift down the fault line. In terms of determining areas of seismic hazard, the area where you know an earthquake has occurred may, paradoxically, be the least likely to experience one in the near (or distant) future. When you know very little about the faults and stresses involved, effectively preparing for future events can be a formidable task.
About earthquakes along the east coast of the US, the authors write, “A natural question to ask is whether, given these challenges, passive margin earthquakes are a minor curiosity about which research is unlikely to yield results commensurate with the effort involved. Our sense is that the problem is worthy of study precisely because we do not understand how large earthquakes occur where idealized plate tectonics predicts they should not. The eastern US’s population density and the need to expand the nation’s energy portfolio, perhaps via offshore drilling and further development of nuclear power, make understanding continental margin earthquakes even more significant.”
As for Clintonville, Wisconsin, the booms continue but nerves are settling. City officials even have t-shirts for sale that read “I Survived the 1.5”.