Hi Geoff -

You are correct that three channels can get distance with the S-P time =
and using charts, you an get magnitude if your equipment is properly =
calibrated, and you can determine direction, assuming you can get the =
initial P wave motion off of all three channels and one of the channels =
is on a null.  Depth is dependent upon receiving certain waves (pP-P), =
and if for some reason they are not visible you might not get depth.

As for a visualization of the fault plane, you need to reference the =
focal mechanism (beech ball) for the event.  There are three variables,  =
Strike, Dip, and Rake.  Following are some definitions I have found =
useful for me to understand the concepts:

Strike - the fault-trace direction in decimal degrees (0 to 360, =
relative to North), de=EF=AC=81ned so that the fault dips to the right =
side of the trace. That is, the fault always dips to the right when =
moving along the trace in the strike direction (from one point to the =
next). This means that the hanging-wall block is always to the right. =
This is important because rake (which gives the slip direction) is =
de=EF=AC=81ned as the movement of the hanging wall relative to the =
footwall. For a vertical, strike slip fault (for which "hanging wall" =
has no physical meaning) we still call the right-side block the hanging =
wall to distinguish between right lateral and left lateral motion.

Dip - the angle of the fault in decimal degrees (0 to 90, relative to =
horizontal).

Rake - the direction the hanging wall moves during rupture, measured =
relative to the fault strike (between -180 and 180 decimal degrees). =
Rake=3D0 means the hanging wall, or the right side of a vertical fault, =
moved in the strike direction (left lateral motion); Rake =3D +/-180 =
means the hanging wall moved in the opposite direction (right lateral =
motion). Rake>0 means the hanging wall moved up (thrust or reverse =
fault). Rake<0 means the hanging wall moved down (normal fault).

This information is listed on the USGS notifications that are contained =
in their list of recent events under "Scientific & Technical" and =
denoted by a colored beech ball.  They are listed under "Best Double =
Strike Couple."  There are two listings NP1 and NP2.  NP1 is assumed to =
be the axis that slipped and NP2 is the secondary axis.

To find focal mechanism of older events you need to reference the Global =
Centroid Moment Tensor catalog at the following address:=20
 http://www.globalcmt.org/CMTsearch.html

To translate these three numbers Strike, Dip, & Rake into a visual =
graphic, I use the following program written by Prof Charles Ammon at =
Penn State University:
http://eqseis.geosc.psu.edu/~cammon/Java/fmech.html

This is a Java based program and you will need the Java Run Time =
Environment on your computer to make it work.

An alternative program that I have also used that has lesser =
limitations, but does not require the Java Run Time Environment was =
written by Prof George Helffrich in the UK:
http://www1.gly.bris.ac.uk/~george/focmec.html

Cheers,

Bob Hancock
Three Points, AZ






On Mar 29, 2010, at 4:46 PM, Geoffrey wrote:

> Then i understand that with three channels you can
> with a single station get distance, magnitude and direction as
> well as focal depth ??
>=20
> For best results Id hazard a guess that
> all data must be taken in synchronous fashion
> meaning all three channels are sampled simultaneous at the very same =
rate.
>=20
> And, is there any program to
> take such data and recreate a three dimensional picture
> showing angles and the absolute magnitude.
> There really is motion only in one direction
> but it seems you must have three different
> channels ro recreate that absolute motion in
> whatever direction ???
>=20
> It seems to me that to recreate reality all known physical dimensions =
must be recorded
> simultaneously.
>=20
> You know what this says about everyday life ???.
> One rarely sees or senses reality.
> :-)
> Thanks for your reply.
> It opens my mind a bit.
>=20
> Best Regards,
> geoff
>=20
>=20
>=20
>=20
> ----- Original Message ----- From: "Bob Hancock" 
> To: 
> Sent: Sunday, March 28, 2010 10:37 PM
> Subject: Re: Un Regards to: 100328.213800.rb1.psn
>=20
>=20
> Hi Geoff -
>=20
> With the two horizontal channels,and the initial direction of the P =
wave on each channel, you can achieve a great circle course line, but =
you will not know which side of the receiving station that the event =
occurred.  To do that you also need the initial direction of the P wave =
on the vertical channel.
>=20
> Bob Hancock
> Three Points, AZ
>=20
>=20
> On Mar 28, 2010, at 10:22 PM, Geoffrey wrote:
>=20
>> Wouldn't it be possible to somehow derive a bearing
>> or angle of incidence somehow relating to the
>> differences between the N/S and the E/W signals
>> in this EVENT posted to PSN database ?
>>=20
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>=20
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> Public Seismic Network Mailing List (PSN-L)
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