'A very Terrible Earthquake'

From the Diary of Ebenezer Parkman (1703 – 1782), published by the American Antiquarian Society, 1974.  Ebenezer Parkman was the second Town Minister of Westborough from 1724 to his death in 1782, as well as the Chief Magistrate and Town Instructor.

October 29th, 1727 (Gregorian calendar):  After 10 o’Clock at night the Sky was clear, the air cold, there was a very terrible Earthquake which lasted Shaking Extremely about a minute and a half, - a trembling continued for a Considerable Longer.  And within 65 minutes 5 more Rumblings and quiverings might be perceiv’d, Especially the last of those 5. But yet this was not Like the First of all.  In about 18 minutes more a Seventh, and near Two o’Clock an 8th, and between 5 and 6 in the morning (perhaps 35 minutes after 5) there was a Ninth.  The first of all these, if not all the rest were heard (I am ready to think) all over New England.

October 31st, 1727 (Gregorian calendar): In the night Sometime (the weather being Cloudy if not Rain) there was heard by Diverse persons another Such noise.

November 18th, 1755 (Julian calendar): This Morning about a Quarter past 4 We were all wak’d up by a very Terrible Earthquake.  The shock Seems to me to be as great and to last as long as the great Earthquake, October 29th, 1727, but the manner of Shaking I think is different – That more horizontal, this partly Vertical. My Children rose and gather’d into my Chamber, where we gave Thanks to God for our Preservation, and begg’d His Mercy towards us. We heard another shock at 28 minutes after 5.

I gave a talk last November on behalf of the Westborough Community Land Trust on Earthquakes in New England and the potential of earthquakes in Westborough. It started me to thinking whether any of the historic large earthquakes in the region had been felt here. As Reverend Parkman’s diary entries show, the answer to that one is yes.

Contrary to what most of us think based on our limited time on Earth, New England is subject to earthquakes – and big ones, although their frequency is many times smaller than in places such as California and elsewhere on the Pacific Rim, where the Earth’s tectonic plates are grinding past one another or being shoved one under the other or, as is the case in the middle of the Atlantic ocean, being pulled apart from each other.

Although New England is situated in what geologist’s term an Intraplate zone, this does not mean the area is not subject to seismic stress.  New England is being squeezed as the Pacific Plate puts eastward pressure on North America and the Mid Atlantic Spreading Center puts westward pressure on the region.

The 1727 earthquake was located near present day Newburyport and is estimated to have had magnitude of about 5.6 (equivalent to 3.8 kilotons of TNT).  It caused soil liquefaction in that area, resulting geysers of water and sand called sand blows, in addition to knocking down chimneys. Damage in Westborough is not known definitively, but modern seismic studies suggest that 55 miles from the epicenter, the residents would have experienced what is described as “rather strong” shaking, equivalent to a large train passing very close to a house.  Windows and dishes breaking, perhaps church bells ringing.

The 1755 earthquake was located off the coast of New England, near Cape Ann, roughly 65 miles from Westborough.  Its estimated magnitude was 6.2 (equivalent to 34 kilotons of TNT, twice the size of the Hiroshima A-bomb).  Again, not a lot of definitive data regarding what happened in Westborough, other than the diary entries, but modern analyses indicate that people would have experienced strong shaking, with people being frightened, walking unsteadily, with heavy furniture moving or being overturned.  Note that in Boston, people saw the ground move in waves and there was considerable damage to masonry buildings and chimneys, as the city was much closer to the epicenter. However, damage to buildings was reported as far away as New Haven and Springfield. Parkman’s diary entries square with these observations, as he said the shaking was more vertical than horizontal.

What is also interesting is the duration of shaking – about 1.5 minutes.  Compare these quakes to last year’s Mineral earthquake in Virginia.  The shaking there lasted only 30 seconds.

In 1727, most of the houses would have been classic colonials - made of wood with chimneys made of stone or brick.  At the time of the 1727 earthquake, Parkman’s residence would have located on land now occupied by the Westborough State Hospital – a hill made of very dense and stable glacial till.  By 1755, Parkman’s house would have been located near present downtown Westborough but Parkman does not discuss any damage.  I can only surmise that the flexible wooden structures that comprised most buildings in the area withstood the shaking better than in more built-up areas like Boston.

Big deal you might say – these occurred between 250 and 300 years ago.  A geologist would say that a couple of centuries is far less than the blink of an eye in geologic terms, given that the earth is 4.5 Billion years old. The point is – New England is no stranger to strong damaging earthquakes.

Most of the active faults in the region are old northwest – southeast trending faults left over from 225 million years ago, when New England separated from Europe and Africa as the continents rifted apart  and the Atlantic Ocean was formed.

Westborough is probably underlain by many faults – the most significant of which is the Bloody Bluff fault, a regional fault that bisects the town roughly along Main Street. Fortunately, it is inactive and has not moved for over 200 million years. However, we don’t know where most of the faults are, because they are buried under a thick layer of glacial deposits.

Are there northwest – southeast trending faults in Westborough?  If you look at geologic map of Westborough – the answer is yes, although the mapped extent of the fault is pretty small – about a quarter of a mile.  Based on the shape of the landforms in this area of town, the fault is probably much longer – perhaps a few miles, but the geologic map is limited to only showing the fault where it can be identified by rock outcrops and photographic data. Where exactly is this fault?  In the field behind my house, near .  Is it anything to worry about?  Probably not.

The nearest area with any significant seismic activity is in the vicinity of Littleton about 30 miles north of Westborough. The Native American word “Nashoba” roughly translates to “hill that shakes”. Seismologists think the consistent small earthquakes recorded in this area are aftershocks of a very large earthquake (magnitude 6.4 +) that occurred sometime between 500 and 900 years ago.  If another earthquake of that size happened today near Littleton – it would definitely ruin our day here in Westborough.

In Westborough, I would bet that the Fire Station would be compromised.  I would imagine that some old unreinforced masonry buildings, perhaps the old ones around the rotary, might lose their brick facades.  Older church steeples would also be suspect. Large parts of downtown Westborough are underlain by water-saturated sand and gravel.  It is not out of the realm of possibility that some areas could see liquefaction, which could cause foundations to shift and significantly damage roads.

Can this really happen here?  Yes it can and someday it will. Seismologists cannot predict when – but about 70% of earthquakes occur where they have occurred before – so Cape Ann, Littleton MA, and Newburyport MA are all candidates.  Another location is Concord NH, the location of a very large quake in 1638, estimated at more than a magnitude 6.

The chance of it happening in your lifetime or mine is pretty small – but the likelihood is that a large earthquake will occur in one of these locations. Nature being what it is we will most likely be surprised by when and where the next big earthquake will occur in New England.