B.C. earthquake magnitude differs, depending on the source

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      U.S. and Canadian authorities are in constant contact with one another on numerous issues.

      So why can't they get their stories straight on something as important as the magnitude of an earthquake?

      This morning, the U.S. Geological Survey reported that an earthquake measuring 5.7 on the Richter scale was centred 108 kilometres south of Port Hardy.

      It occurred 10 kilometres below the surface.

      Meanwhile, the Natural Resources Canada website has put the magnitude at 5.5 on the Richter scale.

      The measurement was developed in 1935 by California Institute of Technology researcher Charles F. Richter, according to the U.S. Geological Survey website. Here's a short explanation:

      The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes. On the Richter Scale, magnitude is expressed in whole numbers and decimal fractions.

      For example, a magnitude 5.3 might be computed for a moderate earthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude; as an estimate of energy, each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

      As you can see from the excerpt above, a difference of 0.2 on the Richter scale amounts to a significant variation in the amount of energy released.

      Following a massive earthquake last October in Haida Gwaii, the Canadian government and the U.S. Geological Survey also released contradictory numbers over the magnitude.

      I'm not a seismologist—and perhaps it's a tricky business getting the number right immediately after the event. But for journalists and for those paying attention to the news, it's awfully confusing when media outlets are reporting contradictory numbers, depending on which source they're relying on for information.



      ten fold increase

      Aug 4, 2013 at 9:50am

      So look at that precision in terms of an 8.3 earthquake, which is 100 times more powerful than 6.3, so that whole 6.3 earth quake, added to an 8.3 quake would make 8.363, or 8.4. See the math now? 8.3 + 6.3 = 8.4? So, to me, 5.5, 5.7, 10km deep, 500km downrange? Close enough.

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      Aug 4, 2013 at 10:14am

      The media could easily report the range of estimates (5.5 to 5.7) and deal with the final number later like scientists do. I really don't buy the notion that the public will be overly concerned with what is an insignificant difference in terms of impact (property damage, injury, tsunami).

      According to Natural Resources Canada:

      "While each earthquake releases a unique amount of energy, the magnitude values reported by different seismological observatories for an event may vary. Depending on the size, nature, and location of an earthquake, seismologists may use several different methods and even different magnitude scales to estimate magnitude.

      "The uncertainty in an estimate of the magnitude is about plus or minus 0.3 units, and seismologists often revise magnitude estimates as they obtain and analyze additional data. It may be several days before different organizations come to a consensus on what is the best overall magnitude estimate."

      Stay calm, media person.

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      Rafael Abreu París

      Aug 4, 2013 at 7:52pm

      Now I stumble across this gem because I'm doing the news search for quake effects that I later pass on to my colleagues.

      Charlie... Chill, ok? There is nothing wrong with the magnitude values presented by the Canadian colleagues at the PGC nor my group in the USGS. And there is no discrepancy in the values either. Both the 5.7 and the 5.5 are correct.

      The PGC used the Local Magnitude (ML) computation to come up with their 5.5 ML. We used the Body Wave computation (Mb) to come up with the 5.7 value. And while we do have seismic stations in common; both groups operate with very different data sets, parameters and protocols.

      So, different methodology and protocols can yield a 0.2 difference in the magnitude computation.

      If you were a bit more informed, you would know that difference in magnitude is pretty much irrelevant when we move to Intensity, the value that measures ground shaking. In terms of ground shaking a M5.7 is pretty much the same as a M5.5; barring local geology effects.

      Incidentally, the NEIC reviewed their magnitude, according to their rsponse protocol. Our official magnitude for the event is M5.3 using the Body Wave Moment Tensor method.

      Freedom of speech is not an open license to cast biased uninformed opinions over the work of others, Charlie. Some of us are offended by such nonsense.

      For the next one... Why don't you ask about what you see and don't understand? My colleagues and I will be more than happy to inform and educate whoever wishes so.

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      John Vidale

      Aug 5, 2013 at 8:14am

      As Rafael said, small differences in magnitude estimated different ways from different sets of seismometers are expected, and do not represent an error.

      Since you raise the issue, the Mb, ML, and Mw magnitude estimates use energy in different frequency passbands ranging from 1 to 200 seconds period, variously on local stations or global stations, and primarily measure seismic wave types P waves, S waves, or surface waves.

      As time passes, more information becomes available, more review can improve the data selection, and more sophisticated and accurate models of the geometry and timing of the earthquake(s) can be used.

      Nothing out of the ordinary or amiss here.

      Ben Sili

      Aug 11, 2013 at 9:05pm

      Amazingly the proper explanations kindly provided here by Rafael and John managed to get thumbed down by some readers!!!

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