Dear Chris (and list),
See http://neic.usgs.gov/neis/general/richter.html for an explanation of
Richter magnitude, the important sentence being "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." Therefore, from 7.0 to
9.0 represents a 100-fold increase in amplitude and almost a 1000-fold
increase in energy.
More importantly, please see David Alexander's words below which he wrote
following the Bam earthquake. When it comes to education and awareness for
disaster and risk management and vulnerability reduction, that means
education and awareness for me too!
Ilan
Earthquakes have at least four magnitudes, all of which may be different
within a range of up to about 0.5 units. The scales depend on which waves
are being measured. The Richter scale (whose real name is local magnitude)
is not used as it is unreliable and has long been superseded by other scales
(body wave magnitude, surface wave magnitude, moment magnitude, etc.).
Therefore the reports that talk about Richter magnitude (and they all do!)
are misleading. Like most other seismic parameters, magnitude measurements
are not absolutely accurate and actually mean very little in terms of the
size of the earthquake at the surface and its power to do damage. They are a
poor surrogate for a measure of energy release. Hypocentral depth is
important, as only shallow-focus earthquakes (<30-40 km) cause major
disasters. Bracketed duration is vital, as a building weakened in a
15-second earthquake may collapse in a 30-second one. Maximum acceleration
and mix of wave velocities are equally important. However, as all of this is
the result of energy released from hundreds of square kilometres of buried,
inclined fault plane, single, pin-point measures tell us very little. (by
David Alexander)
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