Ice Quake

Madison. A tremor caused by shifting ice on Lake Mendota was recorded by a geology department seismometer at 12:50 p.m. Thursday, Jan. 31, 2008, and lasted approximately two or three seconds. UW Police recieved dozens of calls about the rumbling disturbance.
The above picture is from the UW geology department seismometer in Weeks Hall,recorded at 12:50 p.m. on Jan. 31, 2008.
Ice quakes, usually accompanied by loud cracking noises, are caused by large shifts in ice and are most commonly triggered by drastic temperature changes.
Thanks to Mr.Dembroski.