A UW-Madison news release reports that this afternoon's brief tremor may be attributable to an ice quake. According to the release, dozens of employees in buildings along the Lake Mendota shoreline phoned UW police and the facilities staff to ask about the shaking, which occurred at about 12:50 p.m., lasted for two or three seconds and registered on a seismometer at the geology department.
The release goes on to state that ice quakes (or a cryoseism) are often synchronized with loud cracking noises, "are caused by large shifts in ice and are most commonly triggered by drastic temperature changes" similar to the significant thermometer variations of recent days, and may result in pressure ridges or other fractures in the ice.
Such events are not without precedent. There's a lot of expansion and contraction happening out on the big ice sheets that cover Madison's lakes in winter. One of the most dramatic such occurrences happened a little before noon on Jan.15, 1948, when seismographs measured a tremor at 3.8 on the Richter scale.
That's not much by the standards of California or other active earthquake zones, but press accounts of the time noted that the shaking was "accompanied by loud reports of breaking ice fields" and "had the sound and force of a blast." An account in the American Journal of Science ascribed the quake to "an ice fracture on Lake Mendota," and noted that it "was of sufficient intensity to shake some plaster off the ceiling of an office and to crack the sewer drain of one fraternity house."
The day after that tremor of 60 years ago, a group of geology students discovered "a four-foot overthrust in ice 1-1/2 feet thick."
Today's shake-up was minuscule compared to the reports from 60 years ago: UW-Madison geology Prof. Cliff Thurber estimates today's brief tremor at less than 0.2 on the Richter scale -- enough to be noticed, but well short of calamity.
The UW-Madison news release noted no reports of damage as a result of today's tiny quake.