University of Wisconsin-Madison students and staff (including myself) were taken by surprise early Thursday afternoon by a resounding crack and a weak but detectable quake. What made the earth shake in this city where earthquakes are all but unheard of? University geologists say it was most likely an ice quake, caused by shifting and cracking of the ice over Lake Mendota, on whose shores the campus is situated.
The ice quake, or cryoseism, was a result of the large temperature changes which the Madison area saw this week. According to a report from the Isthmus, the quake’s magnitude was estimated by UW-Madison geology professor and seismologist Cliff Thurber to have rated about a .2 on the Richter scale, and a university press release states that the quake, which occurred at 12:50 on Thursday, lasted 2 to 3 seconds. While the quake was strong enough to be felt at least by workers in buildings closest to the lake, I was unable to detect the shaking; the sound of the ice cracking, however, was loud enough to seriously disrupt my thoughts as I prepared to teach an afternoon class! No damage was reported in the university press release. The university’s press release was appended on Saturday to include an image of a seismograph of the quake and a picture of the crack which formed in the lake near Memorial Union.
Through the campus grapevine, I’ve been informed that the ice quake was the most dramatic one anyone at the university Limnology building (which is located on the edge of Lake Mendota itself, and hosts researchers who study fresh water bodies like lakes and rivers) could remember in the last decade; however, sixty years ago, campus was rocked by an ice quake that measured 3.8 on the Richter scale. According to an account by a geology grad student at the time, Charles C. Bradley, which was published in the American Journal Of Science, the quake caused some minor damage, cracking the sewer drain of one fraternity house and shaking plaster from the ceilings of an office.
Bradley was teaching a beginning geology course at the time and utilized the unusual event to capture his students interest; using a city map and first-hand accounts from students of where they were and what they felt when the quake hit, his class constructed a map of intensity for the quake, and used the information to determine that it origin was somewhere in Lake Mendota. Unaware that the quake had been caused by shifts in the lake ice, the students went to the lake to try to locate the epicenter of the quake and found where the ice had cracked; the stress induced by temperature changes in the ice had caused the formation of four-foot-overthrust in 1.5 foot thick ice.
David Medaris. “Lake Mendota ice quake shakes UW and Madison.” “http://www.thedailypage.com/daily/article.php?article=21322”
Jill Sakai. University of Wisconsin-Madison Press Release: “Photo, seismograph from “Mendota Ice Quake” posted.” http://www.news.wisc.edu/14680
“The Science and Art of Charles Crane Bradley.” http://www.geology.wisc.edu/alumni/ccbradley/graduate.html