Earthquakes in Maine and California in an Otherwise Quiet Week


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Maine Earthquake: Magnitude 4.0

While a quick look at a map of historic seismicity shows that earthquakes of significant magnitudes in California are common, the far side of the continent is distant from active seismic boundaries and generally considered tectonically stable. The M4.0 which occurred in the state of Maine was what may have been one of the largest earthquakes in its history: the USGS earthquake listing for the state shows only one larger, an M5.1 in 1904.

Seismic activity in Maine since 1990 shows only small earthquakes – Image credit: USGS

Earthquakes away from plate boundaries are not uncommon, although they tend to be much smaller. Though they are far from present-day plate boundaries, these outlier quakes nevertheless often have their roots in large scale tectonic process. As the USGS observes in its commentary on earthquake activity in New England “Most of New England’s … bedrock was assembled as continents collided to form a supercontinent 500-300 million years ago…. The rest of the bedrock formed when the supercontinent rifted apart 200 million years ago.”

Such activity millions of years ago leaves faults within the bedrock, many of them deeply buried and, unlike other major tectonic zones such as the San Andreas Fault, largely unknown and unexplored. It is movement along these faults which causes the tremors which sometimes occur in the eastern part of the United States.

California, Maine and Predicting Earthquakes

In a week when six seismologists have been jailed in Italy for giving false reassurances about the possibility of an earthquake (the M6.3 in 2009, which followed, cost almost 300 lives) it’s worth using the week’s earthquakes to consider the position of earthquake prediction. The week’s earthquakes overall follow a broadly expected pattern. On that basis, it’s certainly possibly to predict where earthquakes will occur: we can be reasonably certain, for example, that the San Andreas Fault Zone will continue to experience earthquakes of varying sizes.

Yet, although seismologists continue to work towards the accurate prediction of earthquakes, it remains the case that the ‘when’ and ‘how big’ remain almost impossible to predict. In general terms, it is only the ‘where’ that is predictable. And, as the Maine earthquake shows, even then, the earth continues to produce the unexpected.


USGS. Earthquake Facts and Statistics: Frequency of Occurrence of Earthquakes. Accessed October 23, 2012.

USGS. M4.0 – 6km NNE of Waterboro, Maine. Accessed October 23, 2012.

USGS. Real time earthquake mapAccessed October 23, 2012.

USGS. Tectonic Summary: San Andreas Fault.  Accessed October 23, 2012.

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