Unstable rocks in New York and Vermont limit earthquakes

Five boulders delivered by a glacier and now carefully balanced on rock pedestals in upstate New York and Vermont may help determine the long-term maximum shaking intensity of earthquakes in the region.

Seismologists study the fragility of unstable rock formations to determine how strong the tremors would be to dislodge them from their anchorage. The age of the rock formations provides information about how long ago a particular region experienced such tremors.

According to a report in Bulletin of the Seismological Society of AmericaData from the five PBRs in New York and Vermont are generally consistent with the average earthquake ground shaking magnitude for the region predicted by the U.S. Geological Survey's 2023 National Seismic Hazard Model.

Only one PBR, measured on the Blue Ridge Road in New York, indicated a possible reduction in the moderate hazard for the surrounding region. The continued “survival” of this PBR could rule out magnitude 7.0 earthquake sources from much of the Adirondack Mountains and southern Lake Champlain Valley, according to Devin McPhillips and Thomas Pratt of the U.S. Geological Survey.

However, the authors point out that the PBR data overall would allow for an earthquake of magnitude 6.5 to 7.0 in the northern Adirondacks or Lake Champlain Valley.

The researchers also created a map showing the minimum distance at which a plausible large earthquake source could be located without causing a particular PBR to overturn. Maps like this can help determine the location of active faults in the region.

Seismologists have used other geologic features that record shaking, such as liquefaction in the New Madrid and central Virginia seismic zones, to determine earthquake hazard in the eastern U.S., Pratt said. But much of the work to date using PBRs to determine earthquake hazard comes from countries such as New Zealand, Australia and the western United States.

“I think the main reason is that this area is just beginning to develop. The limited work to date has focused on the places with the greatest hazards and risks,” McPhillips explained. “Another likely reason is the early focus on precarious rocks created by weathering and erosion of core rocks in the bedrock. This process does not form many precarious rocks in the Northeast, but it turns out that other processes do.”

All of the PBRs studied by McPhillips and Pratt are erratics – chunks of rock picked up and left behind by glaciers. The boulders balance on rock pedestals that have been gouged and grooved by the glaciers' movements. The last layers of ice retreated from the region between 15,000 and 13,000 years ago, suggesting that the PBRs are about that age.

The rugged, heavily forested landscape made it difficult to identify sensitive PBRs in the region, so “we needed local knowledge,” says McPhillips.

“The project really got going when Tom found these hobby books with documentation of interesting stones. [New York hiking guide] Russell Dunn was particularly helpful in New York and Jan and Christy Butler in Vermont have produced an excellent guide there. Climbers have also picked up some useful information on forums such as Mountain Project.”

The researchers quantified the fragility of five PBRs after studying them with ground-based lidar, field observations, and seismic measurements. After defining the boulders, pedestals, and contact points between the two, McPhillips and Pratt were able to calculate the probability of the boulder falling from its pedestal as a function of the maximum ground acceleration and the ratio between maximum ground speed and maximum ground acceleration.

The findings should help better define the earthquake hazard in this part of the eastern U.S., the researchers said. Northern New York and northwestern Vermont are at increased risk of earthquakes compared to most other parts of the eastern U.S. due to high historical earthquake rates.

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

McPhillips was pleased that their data agreed with the USGS earthquake hazard model, particularly because “we were able to calculate constraints on earthquake intensity over time intervals that were several orders of magnitude longer than any previously available data.”

“I look forward to finding more PBRs – we can probably do better than the five we start with in this article,” he added.