Thread: Wood-Anderson Response

On 03/20/2013 05:42 PM, Blaine Bockholt (bbckholt) wrote:

I am working on a local magnitude project and I am having a

hard time with the WA correction. I am using the evalresp

option and the response file which I downloaded from the IRIS website.

My code is as follows:

r ASTN*
trans from evalresp fname RESP.ASTN.ET to wa

I have a few questions: What are the units of the Wood-Anderson

correction? I am getting conflicting answers. Also, I know there

is a constant which is important, but is that constant found/calculated

when I use the evalresp option or do I need to find/calculate it myself?

Thanks for your help.

Blaine


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Dear Blaine,

Thank you for this pertinent query.

The "evalresp" response file contains (or should contain)
everything needed to specify the instrument from which
you are "transfer"ing, including the "constant", if the
SEED file from which it was derived has correct headers.

There may now be conflict because the latest IASPEI guidance
on local magnitudes (link below) recommends that new values
determined by Uhrhammer and Collins (1990, BSSA 80, 702-716)
be used rather than the Wood-Anderson seismometer parameters
hard-coded into SAC (subroutine src/icm/wa.c).

The parameters that change are the magnification factor
("const_" in wa.c) from 2800 to 2080 and the damping
factor ("h" in wa.c) from 0.8 to 0.7.

IASPEI magnitude recommendations are at: http://www.iaspei.org/commissions/CSOI.html#wgmm
Go to link "Summary of Magnitude Working Group Recommendations...2011 (pdf 211 kb)"

See also p. 24 of Chapter 3 of IASPEI New Manual of Seismological Observatory Practice
http://ebooks.gfz-potsdam.de/pubman/item/escidoc:4015:7

SAC pole-zero files (in radians) containing old and new Wood-Anderson responses
follow (can be used with "transfer to polezero subtype <filename>").
The first one when convolved with a spike gives almost identical
output to "transfer to wa".

POLES 2
-6.28318 4.71239
-6.28318 -4.71239
ZEROS 2
CONSTANT 2800.0

POLES 2
-5.49779 5.60886
-5.49779 -5.60886
ZEROS 2
CONSTANT 2080.0


Yours sincerely,
Sheila Peacock,
Blacknest.