THREE: WHAT IS MAGNITUDE ?


IIIc. MAGNITUDE
The Richter scale is one of several measures of the size or "magnitude" of an earthquake. Earthquakes have a property (common to many natural processes) that there are many small ones and few large ones, referred to as the Gutenberg-Richter Law.



Although "magnitude" is related to the amplitude (height) of a wave, we cannot communicate the concept of magnitude using sound by changing the volume of the sound, in part because our speakers do not have much dynamic range. So instead of volume, we explain the meaning of magnitude by the duration that the Earth is shaking after an earthquake. We use several earthquakes from the region of Sumatra to illustrate these differences.

Sumatra earthquakes



Northern Sumatra, Mw 7.2, 2002/11/2 1:26 GMT
with Mw 5 aftershock
12 hrs at 20 sps

40 k

56 k

64 k


SYNTHETIC calculation of waves from same source
4 k, synthetic calculation

8 k, synthetic calculation


Sumatra-Andaman Islands Great Earthquake, 2004/12/26
Mw 9.0, Depth 28 km.
the earthquake that produced the catastrophic tsunami.

The following movies (made by Vala Hjorleifsdottir, when she was at the Seismological Laboratory, CalTech) show the surface wave propagation near the source:
Note that the duration of the rupture is about 12 minutes (HUGE!).



and a movie of the surface waves propagating around the globe:


Sounds:
24 hrs of data, 20 sps

40 k

56 k



coming soon: Sumatra M9.0, three weeks of data... !!!!!




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