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Subject: Love waves from Mw 8.0
From: sean@...........
Date: Sun, 26 Nov 2000 23:07:05 -0600 (CST)

An interesting observation in the data from the Mw 8.0 quake in
P.N.G. near Rabaul on November 16:

For those of you operating long period horizontal seismometers,
with digital recording, so you can see the time series in a relatively
compressed form (on a drum or helical record the waves from successive
rotations are seriously scrambled, especially with the Mw 7.7
second event.)

The event generated a very impressive set of Love waves that are
seen in the data of a horizontal sensor whose sensitive axis is normal 
to a line drawn toward the source. This is an SH type surface wave whose
group velocity equals its phase velocity so it does not disperse
or spread out as it travels over the surface of the earth. That is,
repeated occurrences of it are very similar, and are labeled G1, G2,
G3, etc, for repeat appearances traveling in the same direction.

For this large quake, the mechanism was such that a large Love
wave was generated propagating out both directions. From my station,
very little Raleigh wave is evident on the vertical record. The first 
Love wave arrival at CCM is at 0542 Z, 48 minutes after the origin time 
of 0454.  This would be the direct wave from the event. It has a 
period of about 80 seconds and a P-P amplitude of 320 microns/second
on the North-South record, and about 110 microns/sec on the EW record.
(meaning that the displacement here at St. Louis was about 4 mm ).
It has traveled about 12 500 km, which is consistent with a velocity
of 4.36 km/second for the shear wave at the surface.

The wave that went the "long way" around the earth arrived at 0639z,
or 57 minutes after the direct wave, and 105 minutes after the quake.
Its amplitude is about 150 microns/second.

Then the second occurrence of the direct wave arrives at 0813z, having
gone around the earth once in 151 minutes, implying that the circumference
of the earth is 39 500 km: approximately, as are my time values. Then the 
second arrival of the "long way" wave arrives at 0909z, again 57 minutes 
after the second direct wave. A these are clearly distinct (and at the 
wrong time) to be associated with the Mw 7.7, which has a very weak Love wave.


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Larry Cochrane <cochrane@..............>