How to read the Canadian National Seismic Network Hourly Plots.

Neither the New England Seismic Network, nor the U.S National Seismic Network offers rapid access to seismic data for Maine. But the Canadian National Seismic Network (CNSN) does. Their seismic data is automatically posted to the internet every hour. Because we are such close neighbors, most Maine earthquakes are registered on the Canadian seismographs.

Each one of the hourly plots shows the simultaneous records from dozens of seismic stations across Quebec and eastern Ontario. Each seismic station is identified by a three-letter or four-letter code down the left side of the plot. For example, the seismic station at Montreal is identified by the letters "MNT." The horizontal line plotted next to the letter code is the record from that station. Earthquakes in Maine are detected most strongly by the closer stations in southern Quebec Province, many of which have a letter code ending in "Q" (like LMQ or GSQ). Small Maine earthquakes are not likely to show well on distant stations such as EEO in eastern Ontario.

If there has been an earthquake in Maine, all of the Quebec stations should show a strong signal within a minute or so of each other, with arrival time depending on how far they are from the earthquake epicenter. The initial seismic wave travels through solid rock at about 200 miles a minute. If a signal shows only on a single station, it is probably not an earthquake but some sort of local disturbance like a quarry blast, or a frost quake or other "noise" at the ground surface near that particular station. And of course, an instrument does malfunction occasionally.

The plots are labeled from left to right according to 24-hour Universal Time (UT), which is five hours ahead of Eastern Standard Time, and four hours ahead of Eastern Daylight Time.

To see an example of how a Maine earthquake is recorded by the Canadian seismic network stations, enter January 3, 2000, hour 21 (UT) on their hourly plot archive. On this plot you can see that all 17 stations in Quebec and eastern Ontario clearly show a regional event at a time of about 21:07. We now know that this was due to the magnitude 3.4 earthquake that occurred in Leeds, Maine at 4:05:50 p.m. EST (21:05:50 UTC).

It takes time for seismologists to evaluate the signals and to calculate the magnitude and location of an earthquake. The usefulness of the hourly plots is just to show at a glance that there has been a seismic "event" somewhere in the region. Further details are calculated by seismologists and posted on the appropriate web sites as they become available.

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Last updated on October 6, 2005

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