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Experts sound off on Wisconsin mystery quakes

Musing about Earth’s inner workings has become a booming business in Clintonville, Wisconsin, where residents have been awakened regularly since 19 March by a series of loud and mysterious noises, sometimes accompanied by earthquake-like shaking.

Located in the midst of farm country about 50 kilometres west of Green Bay, the small town captured the national spotlight as reports of the sounds began to mount. This prompted scientists at the US Geological Survey (USGS) to check seismic readings, leading to the discovery that a small 1.5-magnitude earthquake occurred in the region at about a quarter past midnight on 20 March (seismograph pictured above). But that has hardly put the matter — or the town — to bed, because the noises are still being heard, as recently as last night.

David Hill, a USGS geophysicist based in Menlo Park, California, is putting his money on a series of mini-earthquakes as the likely explanation. Although not all such quakes cause loud noises, if they occur at shallow depth, the impact of the seismic waves hitting the surface can, quite literally, go ‘boom’.

Hill points out that Clintonville lies on firm granite bedrock, which is an excellent transmitter of seismic waves. “As a result, the waves are less attenuated,” he says. “They have a higher amplitude when they hit the surface.”

But Clifford Thurber, a seismologist at the University of Wisconsin–Madison who served as a consultant for the city, is still on the fence. “I won’t be amazed if it turns out to be earthquakes, but it could also be a near-surface event, such as rocks fracturing beneath the surface due to erosion from flowing water.”  So far, the booms have only been heard within in a small, cigar-shaped area that encompasses most of Clintonville, Thurber says, suggesting an origin that is close to the surface, a possibility that falls within the uncertainty of the USGS seismic data.

In Clintonville, most (but not all) reports of noises and minor shaking come in at night, when there’s less ambient noise and it’s easier to hear low-frequency sounds from seismic activity. “It also might be a clue to the source of the disturbance that we don’t know how to interpret,” says Thurber.

Around 20 earthquakes have been recorded in Wisconsin since 1892, all with magnitudes less than 4.0 , says Dave Hart, a geophysicist with the Wisconsin Geological and Natural History Survey. “So earthquakes happen, just not frequently or with much strength,” he says. Hart speculates that the Clintonville ‘swarm’ of quakes is caused by glacial rebound, in which the removal of stress from sheets of ice that once covered the Earth causes landmasses to rebound, creating more seismic strain and causing rock to shift.

The past year has been notable for earthquakes in unlikely places, including a 5.8 tremor in Virginia on 23 August that rattled monuments in Washington DC.

According to Hill, a series of booming sounds similar to those heard in Wisconsin occurred in coastal North Carolina in January 2011.  Another episode was recorded in Alaska in 1947. But Thurber thinks that the Clintonville noises most strongly resemble sporadic rumblings in Moodus, Connecticut, that date as far back as the European colonization of North America.

Hart worked with Thurber to find similar low-magnitude events based on seismographic data at a station in nearby Shiocton, Wisconsin, but only a few correlated with the Clintonville Police Department’s call record. Thurber plans to sift through surface-wave data and model seismic waves to flag similar events that have occurred throughout the week in hopes of pinpointing the depth of the activity.

At this point there’s no telling what has triggered all the activity, but shifts in groundwater or temperature are potential culprits. “To me the mystery there is, why did it start this week?” says Thurber. “If the noises continue, we can study it over time and maybe find out what the source is, but if not, then we may never know.”

Seismograph courtesy of Clifford Thurber, University of Wisconsin-Madison.

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