Unbalanced rocks in New York and Vermont provide boundaries for earthquake tremors.

An unstable rock in Westfield, Vermont. USGS seismologist Thomas Pratt stands on top. A two-meter-long measuring stick was provided to measure the size. Photo credit: Devin McPhillips

Five massive boulders, brought down by glaciers and balanced precisely on bedrock in upstate New York and Vermont, could help pinpoint the region's long-term maximum earthquake intensity.

Seismologists examine the fragility of unstable rocks to determine how much shaking is needed to dislodge them. The age of rock formations provides information about how long it has been since a particular area experienced that level of shaking.

Data from a cluster of five seismic reactors in New York and Vermont are generally consistent with the average level of seismic ground shaking in the region predicted by the U.S. Geological Survey's 2023 National Seismic Hazard Model, according to a report in the Bulletin of the Seismological Society of America.

One such earthquake, measured on the Blue Ridge Road in New York, suggests that the average hazard in the immediate area may be declining. According to Devin McPhillips and Thomas Pratt of the USGS, the “persistence” of these earthquakes could rule out sources of magnitude 7.0 earthquakes in much of the Adirondacks and southern Lake Champlain Valley.

However, the PBR's aggregate data allow for an earthquake of magnitude 6.5 to 7.0 in the northern Adirondacks or Lake Champlain Valley, the authors note.

The researchers also created a map showing the minimum distance a potential large earthquake source could be located without causing a particular volcano to collapse. Maps like this could help pinpoint the location of active faults in the region.

Pratt said seismologists have used other geological features that record vibrations, such as liquefaction in the New Madrid and central Virginia seismic zones, to help determine seismic hazards in the eastern United States. But much of the previous work using PBRs to determine earthquake hazards comes from places like New Zealand, Australia and the western United States.

“I think the main reason is that this field is just starting to mature. The limited work to date has focused on the most dangerous and risky places,” McPhillips said. “Another possible reason is the early focus on unstable, equilibrium rocks that were formed by weathering and erosion of core rocks. This process doesn’t create many of the unstable rocks in the Northeast, but it turns out that other processes do.”

All of the glacial rocks studied by McPhillips and Pratt are irregularly shaped glacial boulders—large rocks that have been picked up by glaciers and left behind. The large rocks are balanced on bedrock that has been eroded and cut into grooves by the glaciers. The last ice sheet retreated from the area between 15,000 and 13,000 years ago, suggesting that the glacial rocks are roughly the same age.

The rugged, forested landscape made it difficult to locate vulnerable nuclear reactors in the area, McPhillips added, “so we needed local knowledge.”

“The project actually started when Tom found these hobbyist books documenting interesting rocks. [New York hiking guide] Russell Dunn was particularly helpful in New York, and Jan and Christy Butler in Vermont have put together an excellent guide there. Climbers have also gotten some useful information on forums like Mountain Project.”

The researchers measured the fragility of five of these rocks after scanning them using ground-based lidar sensors, field observations, and seismic response. After identifying the rocks, pillars, and contact points between rocks, McPhillips and Pratt were able to calculate the probability of a rock falling from its place as a function of maximum ground acceleration and the ratio of maximum ground speed to maximum ground acceleration.

The findings should help better define the seismic hazard in this part of the eastern United States, the researchers said. Northern New York and northwestern Vermont have higher seismic hazards than most other parts of the eastern United States, due to high historical earthquake rates.

This history includes the 1944 magnitude 5.7 Massena earthquake, the 1983 magnitude 5.1 Newcomb earthquake, and the 2002 magnitude 5.3 Au Sable earthquake in New York. Nearby, the Western Quebec Seismic Zone and the Charlevoix Seismic Zone hosted a 7.3 to 7.9 magnitude earthquake in 1663.

McPhillips was pleased to see that their data were consistent with the USGS seismic hazard model, especially since “we were able to calculate shaking intensity constraints over time periods several orders of magnitude longer than any previously available data.”

“I look forward to finding more PBRs – and we will likely be able to do better than the five we started with in this paper,” he added.

For more information: Devin McPhillips et al, Precariously Balanced Rocks in Northern New York and Vermont, USA: Ground-Motion Constraints and Implications for Fault Sources, Bulletin of the Seismological Society of America (2024). DOI: 10.1785/0120240069

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