Booming noises that began late in the evening on Sunday, March 19, are said to have been caused by earthquake activity.
Clintonville city officials held a second special meeting Thursday, March 22, to speak to the public and members of the press. The first special meeting was held Wednesday, March 21.
“The mystery is solved-we had an earthquake here,” Kuss said. “We did not discuss it at our meeting on Wednesday night because city staff members were not aware until this afternoon that this was confirmed earthquake activity.”
“The United States Geological Survey (USGS) has only two seismology stations in the area, since earthquake activity is not common to this part of the country. One is in Iowa, and the other is in Lake Butte Des Aire, Wisconsin,” explained City Administrator Lisa Kuss. “There are six additional Incorporated Research Institutions for Seismology (IRIS) portable seismology stations that are in the area, but information from those stations was not rapidly available to the USGS, so they could not determine immediately whether or not the noises and vibrations we heard and felt were due to an earthquake. If the USGS can’t get data from four stations, they can’t classify the activity as an earthquake, since the data is not clearly defined.
“After analyzing data from the IRIS stations, the USGS has determined that at 12:15 a.m. Tuesday, March 20, there was an earthquake in our area that measured a 1.5 on the earthquake magnitude scale,” Kuss stated. “The depth of the quake is believed to be about 3.1 miles, but it could have been more shallow that that, since we were able to hear and feel it.”
Kuss said the booming, shaking and rattling that took place on Monday did register on the various monitoring devices, but there was too much interference to determine the epicenter or magnitude. She also said that according to the USGS, damage to property can be made by earthquakes measuring 5.5 or higher on the earthquake magnitude scale. An earthquake measuring 4 or higher has the capability to crack windows, according to the USGS.
“The USGS has told us that this was a swarm of small earthquakes that took place in a small amount of time,” continued Kuss. “It is not common to feel such small earthquakes, but the rock beneath much of Wisconsin transmits earthquake activity extremely well.”
According to the USGS, earthquake swarms are generally defined as clusters of earthquakes closely spaced in time and area that do not have a defined mainshock. Defining the length of time of an earthquake swarm is often difficult. Swarms are not uncommon on volcanoes and also happen in regions without volcanic activity. The USGS cites a number of earthquake swarms in various parts of the country that have taken place in the last 10 years, including events in Yellowstone National Park (April 2004), Reno, Nevada (February 2008), and El Hierro, nicknamed Isla del Meridiano (the “Meridian Island”), the smallest and farthest south and west of the Canary Islands (July-October 2011).
The USGS also explains the concepts of foreshocks, mainshocks, and aftershocks on its website, usgs.gov. According to the USGS, “foreshock” and “aftershock” are relative terms. Foreshocks are earthquakes which precede larger earthquakes in the same location. Aftershocks are smaller earthquakes which occur in the same general area during the days to years following a larger event or “mainshock”, defined as within 1-2 fault lengths away and during the period of time before the background seismicity level has resumed. Historically, deep earthquakes (>30km) are much less likely to be followed by aftershocks than shallow earthquakes.
Experts say the epicenter of Tuesday’s earthquake has a variance of about 2-3 miles, so officials believe the epicenter could be within the community; however, the USGS says the epicenter may never be accurately pinpointed. Experts feel that the epicenter may not have been located at the center of the reported activity.
USGS officials have also stated that there is no way to know if the earthquakes will happen again. Since the USGS feel that it has accurately determined the cause of the booming and shaking-and since the activity has decreased significantly-city officials say they will not move forward with installing monitoring devices. If the noises increase in frequency or severity, the USGS will likely work with the city to install additional monitoring systems. Various universities may also be called on to help study the occurrences.
“Our main goal has been to keep our citizens safe and provide answers as they become available to us,” Kuss said. “I do think we’re safe. We’ve learned a lot about earthquakes over the past few hours. There is no way to know whether or not it will happen again, but we will keep our residents informed. It was scary, but we’re all still here and we’re all safe. We will use this event to work with emergency management crews and plan our emergency procedures in case of an earthquake.”
Some residents attending Thursday’s meeting were not convinced that an earthquake was to blame for the booming and shaking episodes. One resident claimed that this has been happening for the past six months, stating that earthquakes don’t go on for months.
Kuss responded that residents should report any incident as soon as it takes place so that the events can be investigated and information can be gathered. “The USGS needs specific information, such as the time of each occurrence, in order to investigate these events,” Kuss said. “Please continue to report any incidents immediately to our police department’s non-emergency line at 715-823-3117.”
Other residents wondered why the stronger occurrences seemed to take place in the early morning hours. Many wanted to know where the fault line is. Many were not convinced that there won’t be problems in the future. A few residents who claim to have experienced earthquakes in other states say that what happened in Clintonville didn’t feel the same as the earthquakes they experienced.
“We don’t have all the answers yet, and some questions may not have answers,” said Kuss. “We are working to learn more about earthquakes and we will do our best to determine as much information as we can about the location and cause of this swarm of earthquakes. It’s difficult because earthquakes are not common here, but the USGS will likely continue to assist us with our questions and they will be available to provide data and insight if the earthquakes continue.”
Kuss thanked city employees for their hard work and commended residents for their patience throughout the event. She also thanked the national media for helping draw attention to the events, which spurred assistance from the USGS and other agencies.