An earthquake is ground shaking caused by a sudden movement of rock in the Earth’s crust. Such movements occur along faults, which are thin zones of crushed rock separating blocks of crust. When one block suddenly slips and moves relative to the other along a fault, the energy released creates vibrations called seismic waves that radiate up through the crust to the Earth’s surface, causing the ground to shake.
The Earth is formed of several layers that have different physical and chemical properties. The outer layer consists of several large, irregularly shaped plates that slide over, under and past each other on top of the partly molten inner layer. Sometimes the movement of the plates is gradual. If the plates are locked together, the energy accumulates until it grows strong enough and the plates break free.
Ninety percent of all earthquakes are found at crustal plate boundaries such as the Pacific Plate. Earthquakes can also occur within plates, such as the New Madrid earthquakes of 1811–1812 and the 1886 Charleston earthquake which occurred within the North American plate.
The vibrations produced by earthquakes are detected, recorded, and measured by instruments called seismographs. Seismographs record the motion of the ground during an earthquake. They are installed in the ground throughout the world and operate as seismographic networks. The first seismograph was developed in 1890.
There are many different ways to measure earthquakes. Magnitude is the most common measure of an earthquake’s size. It is a measure of the size of the earthquake source. Intensity is a measure of the shaking and damage caused by the earthquake.
Earthquakes beneath the ocean floor sometimes generate tsunamis. Tsunami waves can travel across the ocean and cause death and destruction among coastal communities.
Greendale Weather provides information about recent earthquakes on our Earthquakes page.
Earthquakes in Wisconsin
Wisconsin has only had one significant earthquake with its epicenter located within the state. This earthquake occurred on May 6, 1947. It was a 4.0 magnitude earthquake which lasted for 40 seconds.
The Milwaukee Sentinel reported:
“Milwaukee shook in the grip of its first recorded earthquake for half a second at 3:25 p. m. yesterday – and can thank its stars that the quake lasted no longer.
The giant tremors, which rocked buildings, knocked dishes off shelves and pictures off walls and put some pedestrians off balance, were felt in surrounding counties, too, but not as strongly.”
While only one earthquake has occurred in Wisconsin, many more have been felt by residents here. The following map and table show earthquakes that have been felt in Wisconsin. The data was obtained from the US Geological Survey (USGS) and from the UW Milwaukee Geosciences Seismic Center. The Seismic Center data includes links to local reports of the earthquakes. Additional descriptions of earthquakes felt in Wisconsin are described by the USGS.
In addition to the earthquakes shown above, there might have been other earthquakes felt in Southeastern Wisconsin. Documentation is lacking. Most notable among these is the series of large earthquakes that occurred in 1811-1812 along the New Madrid Seismic Zone located in Southwest Missouri. This is the most active seismic area near Wisconsin. Additionally, sonic booms, quarry blasts and other non-earthquake events may have been originally reported as earthquakes, but were later attributed to other causes. Events below magnitude 2.0 are also excluded because these are so rarely felt by people and are relatively common events.
One of the factors that enables earthquakes to be felt in Wisconsin is the underlying type and structure of rock is very good at transmitting seismic energy.
Earthquake Risk in Wisconsin and the United States
Wisconsin has a very low comparative expectation of earthquakes (see maps below).
This map shows Wisconsin’s relatively low probability of low-motion earthquakes. The peak acceleration is the maximum velocity experienced by the ground during an earthquake. It’s measured in g, or m/s². Wisconsin is rated at 6%g or less. Most structures not built to resist earthquakes can withstand peak acceleration of up to 10%g.
A map of earthquake risk for the United States shows the lower comparative risk of earthquakes in Wisconsin. The map shows that the nearest area of major earthquake risk is in the area surrounding the New Madrid Seismic Zone,
Cryoseisms (Frost Quakes) in Wisconsin
A cryoseism, also known as an ice quake or a frost quake, may be caused by a sudden cracking action in frozen soil or rock saturated with water or ice. As water drains into ground, it may eventually freeze and expand under colder temperatures, putting stress on its surroundings. This stress builds up until relieved explosively in the form of a cryoseism. Cryoseisms have been reported at various times in Wisconsin. The news stories Frost quakes may be behind Wisconsin cracking sound and Mysterious noises traced to frost quakes document this type of occurrence.
Loud Noises in Wisconsin
In March 2012, the town of Clintonville, Wisconsin (about 40 miles West of Green Bay) experienced a series of “booms”. The cause noises were the subject of a great deal of speculation. As scientists investigated potential sources of the noises, the most likely culprit emerged as a swarm of micro earthquakes in the area. At least one earthquake of a 1.5 magnitude was measured in association with the sound. The news stories Earthquake eyed in mysterious Wisconsin booms and Authorities solve the mystery of town’s odd noises, shaking describe the events.
The Myth of Earthquake Weather
Not having to do with Wisconsin Earthquakes, but pertinent to weather interests is the myth of earthquake weather. There is a misconception that earthquakes occur during “earthquake weather.” The common misunderstanding that earthquakes occur during hot and dry weather dates to the ancient Greeks. A modern theory proposes that certain cloud formations may be used to predict earthquakes; however, this idea is rejected by most geologists Earthquakes take place miles underground, and can happen at any time in any weather. Some recent research has found a correlation between a sudden relative spike in atmospheric temperature 2-5 days before an earthquake. It is speculated that this rise is caused by the movement of ions within the earth’s crust, related to an oncoming earthquake. However, the atmospheric changes are caused by the earthquake, rather than causing it. Furthermore, this relative change would not cause any single recognizable weather pattern that could be labelled “earthquake weather”
For general information about earthquakes, visit the following: