Researchers at BC’s Weston Observatory in Weston, Mass. track and study earthquakes. NNB yesterday asked Director John Ebel to tell us what he knows about the Canadian earthquake.

NNB: Can you tell us a little about how earthquakes in the Northeast differ from west coast earthquakes, For example, the difference between the earthquake process in plate interiors and those at plate boundaries.

JE: In most ways, earthquakes in California and northeastern North America are very similar. They give the same kinds of seismic waves, and they look similar on our seismograms. One difference is in the frequency of earthquakes. For example, the rate of earthquakes in California is about 100 times greater than the rate of earthquakes in the northeastern U.S. Western Quebec is only slightly more seismically active than New England. Thus, while damaging earthquakes can happen in California and the northeast, they are more frequent in California.

Another major difference in the earthquakes is that earthquakes in eastern North America are felt over a much larger area than earthquakes of the same magnitude in California. For example, today’s earthquake was felt as far west at Milwaukee, which is a distance of several hundred miles from the epicenter in Quebec. A similar sized earthquake in California would not be felt much beyond 100 miles from the epicenter. The reason for this is that the rock beneath California is hotter than rock at the same depth beneath eastern North America. Hot rock tends to absorb seismic wave energy more than does cold rock.

NNB: The Weston observatory website says that "a commonly accepted explanation for the cause of earthquakes in the Northeast is that “ancient zones of weakness” are being reactivated in the present-day stress field." Could you give your thoughts on that theory?

JE: We believe this because when we do detailed analyses of the seismic waves generated by earthquakes in the eastern U.S., the direction of the maximum pressure in the rock that triggered the earthquake is aligned close to the direction that the tectonic plate is moving today. In fact, an hour ago I saw a first analysis of today’s earthquakes that was consistent with that theory. Thus, the present-day stress field is being caused by the movement of the tectonic plates.

What we also know is that most earthquakes in eastern North America tend to occur where we have many old faults preserved. Those old faults formed hundreds of millions of years ago when eastern North America was at a plate boundary, much as the West Coast of North America is today. While we cannot yet identify individual faults that are seismically active today, the greatest earthquake activity in eastern North America tends to occur where there are faults that are 250 million years old or younger. These are the “pre-existing zones of weakness” that is being referred to in the quote above.

NNB: The Globe reported that in one building "chairs were moving across the floor, and the building felt like it was moving and was not structurally sound. Would that be possible from this earthquake? If so, why were there so few reports of similar shaking?

JE:I am not an engineer, so I have only an informed but layman’s knowledge of earthquake engineering. However, I know from having studied many earthquakes that apparently some buildings shake more than other nearby buildings in an earthquake. For example, in a study done a number of years ago by a student and myself, we learned that when earthquakes are felt in Boston, the Customs House Tower seem to shake more strongly than other, nearby buildings. I assume this has to do with (1) the way the particular buildings are constructed and (2) the soils or rock upon which the buildings have their foundations. Buildings on soft soils like river bottom sediments or landfill tend to shake more strongly than buildings with foundations on rock.

NM: Why were there so few reports of similar shaking?

I don’t know, but I suspect it has to do with the peculiarities in the way the one building the Globe reported on is constructed.

NNB: The Globe also reported the largest quake ever recorded in the Northeast hit Quebec in 1663, shattering chimneys in Boston, nearly 400 miles away. Might today’s quake be related – on the same fault line for example?

JE: Ironically, just last week I submitted a paper to a scientific journal (Journal of Geophysical Research) on a study I carried out on the 1663 earthquake. The 1663 earthquake appears to be associated with movements on faults that border the St. Lawrence River. Those faults formed over 500 million years ago when an ancient piece of continent rifted away from when is today the Canadian part of North America. Today’s earthquake was not centered on the St. Lawrence rift, but rather it was centered in a seismic zone within the Canadian part of the North American continent. Thus, today’s event was associated with a different zone of weakness than was the 1663 earthquake.