Concentrating on Vermont, New Hampshire, and even Ontario, Canada, many earthquakes have shaken these states. The intensity of these earthquakes is measured in two ways. The first is our most current form of categorizing them, the Richter scale. This is the logarithmic scale that measures the magnitude of the earthquake at its source (epicenter). The second way of measuring earthquakes was the Modified Mercalli intensity scale (MMI), which was the predecessor to the Richter scale. The MMI scale measured earthquakes on the basis of damage and had values that ranged from Intensity I (not felt), to Intensity XII (total destruction), and there was even an Intensity IX.

Vermont has recorded around three earthquakes in its history. Since they occurred after the invention of the Richter scale, they were assigned both Mercalli and Richter scale intensities.

New Hampshire feels a good deal of earthquakes originating outside of the state. One of the largest recorded quakes of this kind occurred on November 18, 1929. It was a magnitude of 7.2 and it was centered on the Great Banks of Newfoundland. Another strong and yet distant earthquake was on November 1, 1935. It originated in Timiskaming, Canada, and was felt over 2,600,000 square kilometers in the United States and Canada.

Southern Ontario is generally thought of being relatively seismically inactive. However, it is unaware to most people that along the Ottawa and the St. Lawrence Valleys, southwest across Lake Ontario, the Niagara Peninsula, and Lake Erie there exists a zone of high earthquake risk. The St. Lawrence Valley actually used to be a mid-continental rift zone creating extensional zones in the past. Within these areas strange rock trends are observed. These rock trends are referred to as rock popups. Rock popups are small faults and folds in the surface rocks that produce small upheavals of bedrock about one to two meters in height. These rock popups are only seen at rock exposures at outcrops or at excavations or quarries. Mining and quarry blasting releases pent up stress in the bedrock and this is why some of these popups occur. It seems that it is still possible for more small earthquakes to occur and for more stress to buildup.