Seismic Change

Geophysicist Ebel shakes up thinking on 17th century Northeast quakes by combing historical accounts

By Sandra Howe
Staff Writer

Prof. John Ebel (Geology and Geophysics) generally relies on modern technology when researching seismic activity in the Northeast, but for his most recent study he consulted centuries-old personal writings and correspondence - and found they often gave a more accurate picture than available scientific data.

As part of his research for "The Seventeenth Century Seismicity of Northeastern North America," Ebel investigated original sources of earthquake reports from that century, including diaries, letters and town records. These sources, Ebel said, indicate that earthquake reference books in common use today are filled with errors, containing entries for earthquakes which never occurred and omitting others that did take place.

Ebel, director of the Weston Observatory, said the study provides new information on the area's seismic activity and influences estimations of when, where and how strong future quakes might be. He hopes his findings will cause residents to take the threat of significant earthquakes in the Northeast seriously and prompt them to prepare for such events, as they do for fires or hurricanes.

"No matter what we do with modern data, we still need accurate historical information to get an overall sense of seismic activity over time," said Ebel, whose article was published in Seismological Research Letters . "The steady rate of the occurrences in the 17th century are important for me as a seismologist. This study is further evidence that earthquakes occur regularly in this area and I have every reason to believe they will continue to occur."

According to Ebel, at least nine earthquakes above magnitude 5 on the Richter scale, which can cause considerable damage, took place in the Northeast during the 17th century. Based on this evidence, Ebel predicts that during the next 100 years the Northeast will experience between five and 10 earthquakes greater than magnitude 5.
Prof. John Ebel (Geology and Geophysics) "No matter what we do with modern data, we still need accurate historical information to get an overall sense of seismic activity over time." (Photo by Lee Pellegrini)

This data has implications for cities and towns in the region, Ebel said, because it provides a new and higher standard for how severely an earthquake might affect the Northeast. Guided by this information, Ebel says, municipalities should strengthen their building codes and create programs to prepare residents for earthquake related emergencies.

Ebel embarked on the project when he began to suspect the information in seismological reference books was inaccurate or misleading. During that era, for example, British North America and French Canada used different calendars, which meant a discrepancy in dates. Consulting historical accounts of the period, many of which he found in O'Neill Library, he discovered most earthquakes reported to have taken place between 1638 and 1659 did not occur, and many details in the reference books concerning those which did are highly suspect.

While few of the original accounts are particularly dramatic, Ebel said the descriptions reveal some important information related to the intensity, time and location of certain quakes. For example, a 1663 earthquake which caused major landslides along the St. Lawrence River also was reported by a Rev. Danforth in Roxbury, who said it "shook men's houses and caused many to run out into the streets."

Ebel paid particular interest to an earthquake in 1638 which was felt by residents in New England and Canada, according to the first-hand accounts he read. Observers suggested it was a shock of "unusual significance," he said. "It seemed like a frightening event from the descriptions I read and I knew it had to be especially strong to be felt so widely, but no one had ever dared give it a magnitude before because they didn't have enough evidence."

His findings indicated the temblor had a magnitude 6.5 - close to the size of the one that hit Los Angeles in 1994 - with an epicenter somewhere in central New Hampshire, rather than Quebec or Cape Ann as was previously thought. This earthquake was more severe than the 1755 Cape Ann earthquake, he said, which at magnitude 6 was long thought to be the region's strongest.

"The largest earthquake ever known in the area gives us an idea of what to prepare for," Ebel said. "If we allow for at least a 6.5 magnitude in future calculations for seismic activity in New England, this means it is very possible we would see an earthquake which could do significant damage - unless, of course, we take preventive measures."

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