Tuesday night's activity was strong enough to be picked up by the seismic monitoring station at the University of Arizona Geosciences Department. Assistant Professor Eric Kiser says it happened between 8:11 p.m. and 8:12 p.m., "The seismology group looked at that waveforms today, but it is difficult to determine what the source is using one seismic station. Generally, we would expect a sonic boom to not be as impulsive as this signal. A mine blast could be the source, but I would be surprised if they were blasting that late. It would be useful to compare this seismic record to the seismic record next time an event is felt."