December 29, 2006

Measuring Earthquakes

This last quake was 7.2 by Japanese standards and 6.7 according to the scientists here in Taiwan. What gives? The Taipei Times continued to not answer this question this morning, and Will at the Early Bird Diner really wants to know.

I found out a few things, but nothing that resembles a complete answer to the question What's the root cause of this discrepancy?

First off, the Richter Scale was rendered obsolete in 1979 when Tom Hanks (the seismologist, I presume) and Hiroo Kanamori came up with the Moment Magnitude Scale.

Here's a short explanation from the USGS:

Q: What is moment magnitude?
A: Moment is a physical quantity proportional to the slip on the fault times the area of the fault surface that slips; it is related to the total energy released in the EQ. The moment can be estimated from seismograms (and also from geodetic measurements). The moment is then converted into a number similar to other earthquake magnitudes by a standard formula. The results is called the moment magnitude. The moment magnitude provides an estimate of earthquake size that is valid over the complete range of magnitudes, a characteristic that was lacking in other magnitude scales.

I may be missing something, but the Taiwan Weather Bureau is listing the quake as ML6.7, this is Local Magnitude, which according to Wikipedia is the same as the Richter Scale. It looks like the boys in Denver are using the moment magnitude calculations. So, "MagnitudeX" and "Y on the Richter Scale" are meant to be close, which now counts in horse-shoes, hand-grenades, nuclear war, and now, earthquakes.

The USGS also has an interesting summary of the quake.

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