Hi Jeff -

Some thought about seismic waves which may affect what you do or do not =
receive.

One very important point is the receiver's sensitivity, and another is =
the receiver's pass band, in other words what is the absolute =
sensitivity, and what is the frequency range of waves that it can =
detect.  You said that your instrument was "orientated basically =
north-south".  In seismology it is important that the horizontal =
channels be aligned as near to cardinal directions as possible ie. =
north/south or east/west.  The problem is how the waves arrive at your =
station.  You mentioned that you got a good signal on the M 7.2 Baja =
California, but  very weak signal on the M 7.2 in Vanuatu.

Basically there are two types of seismic waves, body and surface.  The =
body waves travel through the core of the earth, and their arrival times =
are usually within 1 or 2 seconds of the travel charts.  They have an =
amplitude decay rate of 1/ R^2.  Surface waves travel in the upper crust =
of the earth and are also known as dispersive waves.  Different =
frequencies of waves travel at different depths.  The higher the =
frequency, the closer to the surface they are.  They have an amplitude =
decay rate of 1/ Sqrt R.  NOTE R =3D Distance, Km.

The frequencies that your receiver can handle is also important.  While =
surface waves could exist at 10 seconds and higher, they are normally a =
much lower frequency.  This would put you on the short side of surface =
waves.  However, Bob McClure, a member of the PSN group has developed =
software to increase the period  of instruments.  Here is the link to =
his web site:  http://sites.google.com/site/bobmcclure90/inversefilter

Additional information can be found on the late John Lahr's web site.  =
Here is the link:  http://jclahr.com/science/psn/

When you look at individual seismic waves, they are not uniform in =
amplitude as you measure around the compass.  Basically you can divide =
the waves into 4 quadrants of 90 degrees each.  The peak P waves =
correspond to null for the S waves.  The peak S waves correspond to null =
for the P waves.  What you see at your station is dependent upon the =
focal mechanism of the event and position of the wave as it departs the =
event.

There are two basic surface waves, Love and Rayleigh.  The Love wave is =
a transverse wave, vibrating 90 degrees to the direction of travel.  Its =
wave is known as a SH (S type wave Horizontally) wave.  The Rayleigh =
wave which is a little slower is a radial wave and its motion is a =
combination of a P wave and SV(S type wave Vertically) wave.  What your =
receiver sees is dependent upon the ray path of the arriving wave at the =
station.

Here is the link for the USGS and Global CMT focal mechanisms: =20

         =
http://earthquake.usgs.gov/earthquakes/recenteqsww/Quakes/us2010wubj.php

Several factors could have contributed to you not receiving a strong =
signal on the M 7.2 event Vanuatu.  Among them are the basic sensitivity =
on your receiver, focal mechanism of event did not transmit strong waves =
on the ray path (great circle route to your station), receiver =
orientation did not match peak amplitude of surface waves, background =
noise at your station.  I am certain that there are many more, but these =
are some of the more important ones that I can think of now.

You mentioned your ability to receive the M 7.2 event from Baja =
California.  You were almost due north of that event and with a =
north/south orientation, you would have received strong Rayleigh waves, =
but almost no Love waves.  In the M 7.2 event at Vanuatu, you were =
aligned more for the Love waves than the Rayleigh waves.

Hopefully this will help you understand what you are receiving.

Cheers

Bob Hancock
Three Points, AZ

On May 29, 2010, at 10:20 PM, jzambory@......... wrote:

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