PSN-L Message - Subject: Re: Seismometer Siting

Yes, it does help. I just bought this book at your previous = recommendation,=20 and I haven't got to page 57 yet. As you had said, it is rather = math heavy=20 and I am a slow reader too. I had ran across that "transverse" = term=20 farther into the Introduction. It too had raised my = eyebrows.

I had thought that maybe the seismometer was placed obliquely, but = that=20 didn't make sense either....so, I asked the group.

Thanks again,

Jerry

----- Original Message -----

From:=20 Bob = Hancock=20

To: psn-l@..............

Sent: Sunday, September 27, = 2009 2:46=20 PM

Subject: Re: Seismometer = Siting

Jerry -

They do not give a reason for that particular bit of research. = It=20 would interesting to know what the project was. As you = previously said,=20 seismometers are normally orientated, vertical, north/south and=20 east/west.

S waves are transverse waves which oscillate 90 degrees to the = ray path.=20 There are two components to a S wave, SH (S wave horizontal = component),=20 and SV (S wave vertical component).

When seismologists view a seismogram, they usually rotate the = data first,=20 so instead of looking at ZNE data, they add two additional channels, = for the=20 following ZNERT. The R stands for radial and T stands for=20 transverse. There is a option in the SAC program which = allow you to=20 align the two horizontal channels mathematically with the radial and=20 transverse components, Check page 57, right side column for the = two=20 complete paragraphs. I have copied the two paragraphs below for = the=20 benefit of those who do not have access to the book.

"...In real applications, we often define the z axis as the = vertical=20 direction and orient the x-z plane along the great circle connecting a = seismic=20 source and a receiver. Plane waves traveling on the direct path = between=20 the source and the receiver thus propagate in the x-z plane. The = shear=20 wave polarization directions are defined as SV, for shear waves with=20 displacement in the vertical (x-z) plane, and SH, for horizontally = polarized=20 shear waves with displacement in the y direction, parallel to the = earth's=20 surface. Both have displacements perpendicular to the = propagation=20 direction and the other polarization (Fig. 2.4-4, overleaf). = Although we=20 could choose any two orthogonal polarizations in the plane of the = shear wave=20 displacements, using SV and SH is particularly convenient. We = will see=20 that P and SV waves are coupled with each other when they interact = with=20 horizontal boundaries, whereas SH waves remain separate.

Seismometers record horizontal motions in the north-south and = east-west=20 directions, which rarely correspond exactly to the SH and SV = polarizations.=20 As a result, data from the horizontal components of seismometers = are=20 often rotated. The = direction=20 connecting the source and the receiver, corresponding to SV = displacements, is=20 called the radial direction, so a = seismogram=20 rotated to this direction is called the radial component. = Similarly,=20 the orthogonal direction corresponding to SH displacements is called = transverse direction, so = a=20 seismogram rotated to this direction is called the transverse = component...."

Following is a mathematical explanation of rotation from = Professor George=20 Zandt, University of Arizona:

"...Rotation, or more precisely, rotation of seismograms from the = ZNE=20 coordinates to ZRT coordinates is relatively straightforward. The = choice of=20 recording the NS and EW components of ground motion is rather = arbitrary,=20 right? You just need to record two orthogonal (at rt angles) = directions, could=20 be NE and SE, but by convention sesimologists always use N and E. If = your=20 events are coming from N or S or E or W, then the data is called = "naturally=20 rotated" and you can clearly see the separation of Love and Rayleigh, = for=20 example. But for any other direction, its not naturally rotated. So, = you can=20 rotate the data yourself to the radial and transverse directions.=20 Mathematically, this is quite simple and involves multiplying the data = by=20 cosine of the difference in angles. There is a command in SAC that = does=20 this...."

Hope this helps you a little.

Bob Hancock

On Sep 27, 2009, at 12:11 PM, GPayton wrote:

Page 3 of Introduction, left column at bottom. = Referencing=20 Figure 1.1-4.....

Jerry

----- Original Message = ----- From:=20 "Bob Hancock" <icarus@.........>
To: = <psn-l@..............>
Sent= :=20 Sunday, September 27, 2009 2:02 PM
Subject: Re: Seismometer=20 Siting

Jerry -

I have the book - what page and where on = the page=20 did you read this.

Bob Hancock

On Sep 27, 2009, at 11:14 AM, GPayton=20 wrote:

Over and over, I have read that normally = a=20 seismometer is sited for N-S, E-W, and Vertical=20 sensing.

However, I just read a paragraph in An=20 Introduction to Seismology, Earthquakes and Earth Structure = about a=20 seismometer in Hawaii that "was oriented to receive S = phase=20 signals." How is that = accomplished?

I could venture a guess, but probably = would be=20 wrong.

Thanks,.

Jerry = Payton

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