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Written by:
4/25/2011 12:52 PM 

King County OEM’s Pascal Schuback had a neat 138 character tweet today that took me well over an hour to digest:

Know the difference 2B ready RT @USGS: How much bigger is a 8.7 2A 5.8? Click "Try It Yourself" Calculator at top of http://go.usa.gov/baq

As they note on their site, http://earthquake.usgs.gov/learn/topics/how_much_bigger.php:

The magnitude scale is really comparing amplitudes of waves on a seismogram, not the STRENGTH (energy) of the quakes. So, a magnitude 8.7 is 794 times bigger than a 5.8 quake as measured on seismograms, but the 8.7 quake is about 23,000 times STRONGER than the 5.8! Since it is really the energy or strength that knocks down buildings, this is really the more important comparison. This means that it would take about 23,000 quakes of magnitude 5.8 to equal the energy released by one magnitude 8.7 event.

The thing they don’t really mention is that these numbers discuss the total energy released by a quake – NOT the intensity of shaking on the surface as experienced by humans and buildings – which is known as the (Modified) Mercalli intensity scale. A deep quake (Nisqually was 6.8 on the moment magnitude scale but 32 miles deep) has a much less serious impact than a shallow quake – as was modeled by the recent Tacoma Fault scenario (7.1 at 9 miles deep). Shallow quakes are much more likely to have damaging aftershocks too boot. Local soils can further alter conditions felt on the surface, as the caption of the Tacoma Fault map indicates.

Probably even more useful is the peak acceleration – which is used by the USGS for their http://earthquake.usgs.gov/hazards/. Their documentation for this goes into hopeless detail on how best to measure the impact of quakes across the US.

How likely are any of these? That’s a whole other convoluted topic, but the simple answer is to go to http://earthquake.usgs.gov/hazards/apps/ and map it yourself. Most anyway you look at this though: Vashon needs to be ready for a big one!

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