PSN-L Email List Message

Subject: Re: Samoa quake
From: Bob Hancock icarus@.........
Date: Tue, 29 Sep 2009 17:34:55 -0700

This a surface wave arrival time problem.  Larry's program WinQuake  
uses a generic algorithm to compute the arrival times of the Love and  
Rayleigh waves, and totally accurate on some ray paths, and has  
variations in time on other ray paths.  Neither of the two major  
programs used by seismologists, SAC and TAUP, compute surface wave  
arrival times.  These waves are normally identified visually.  I have  
seen some events where the WinQuake computed arrival times for the  
surface waves were early, and others where the WinQuake arrival times  
were late.  I have also seen the same thing with the USGS program for  
phase arrival time.  It is not unusual to see a discrepancy of 5  
minutes or more on the arrival times.  This is because there are no  
accurate models that cover both oceanic and continental crusts.  This  
event traveled across the pacific ocean (oceanic crust) and traveled  
halfway across the US (continental crust) to your station.  One  
reference that I used, ANATOMY OF SEISMOGRAMS by Ota Kulhanek,  
reported variations for surface waves from 1 km/s to 4.5 km/s.  In  
addition, these waves are dispersive and each wave packets contain  
waves of different frequencies.

Using the standard USGS Phase arrival time program - see link:

I looked at the computed arrival times of the surface waves from the  
USGS program and compared them to the WinQuake computed surface wave  
arrival times.  There was a discrepancy of ~10 minute between the two  
programs for the arrival time of the Love wave and ~13 minutes for the  
arrival time of the Rayleigh waves.  The actual arrival times were in  

Larry's program does an excellent job of giving some you an idea where  
the surface waves are and then through experience you will learn how  
to identify them.  Again, the problem is the lack of an accurate model  
that can account for all the crust of the earth.  If you could look at  
this event at a distance of 90 degrees from the epicenter, and with  
stations arranged in a circle at that distance, you would still see  
different surface wave arrival times due to variations in the crust  
that each individual ray path had to travel through to reach the  

Bob Hancock

On Sep 29, 2009, at 2:41 PM, Thomas Dick wrote:

> Here in Evansville, IN some things don't fit on this one... E-W  
> shows a peak 6 min before arrival of LQ and LR seems disappointingly  
> small. Looks that way on local universities as well.
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