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RE: The Hector Mine Earthquake, 10/16/1999

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If my calculations are right, 2 hours would work out to a wave velocity of
about 1390 feet/second which is consistent with common near surface shear
wave velocities.

On a similar vein, trinet.org shows response spectra for three different
periods, 0.3 seconds, 1 second and 3.0 seconds for the Hector Mine EQ.  For
the 3.0 second spectra, values located along the coast in the LA Basin (I
assume where the ground is soft) are higher than intervening areas.  This is
also true of the north end of the Salton Sea.  This is similar to the
amplification (basin effect) of soft soil deposits in the Loma Prieta
earthquake (eg., the area under the Cypress Freeway which collapsed).  While
the longer periods don't affect many buildings, they are of interest to
those involved with large storage tanks (sloshing modes) and long span
bridges.

Bill Cain, SE
Oakland, CA


	-----Original Message-----
	From:	Charley Hamilton [SMTP:chamilto(--nospam--at)eng.uci.edu]
	Sent:	Tuesday, October 19, 1999 9:00 AM
	To:	seaint(--nospam--at)seaint.org
	Subject:	Re: The Hector Mine Earthquake, 10/16/1999

	Greg -

	The 1964 Alaskan earthquake caused a seiche in Lake Pontchartrain.
Same 
	basic phenomenon where very low frequency seismic energy is captured
by a 
	"liquid storage tank" (in that case, the lake).  I forget the
magnitude of 
	the seiche, though.  One thing to check out is what frequency range
your 
	instrumentation records.  I suspect the seiche should come from very
low 
	frequency levels & some accelerometers do not record below about 0.5
Hz.  
	Not sure about the time frame, though.  2 hours could be about right
since 
	the propagation rate is relatively slow (I think) for most long
period waves.
	(in comparison with the high freq stuff).  

	Charley

	-- 
	Charles Hamilton, EIT                   Graduate Student Researcher

	Department of Civil and                 Phone: 949.824.8694
	    Environmental Engineering           FAX:   949.824.2117 
	University of California, Irvine        Email: chamilto(--nospam--at)eng.uci.edu