Thread: units of EVDP in SAC

This exchange is exactly what I had hoped would happen. Thank you all.

Attached is today's draft of the traveltime help file a a PDF. You can see my current thoughts. A couple of comments:

1. Like Philip in his taup_setsac, I am allowing for either km or m for evdp. We differ in what the default is. If the input file has evdp in m -- and one knows it -- one can either change evdp to km using ch or evaluate before going into sss or one can use the depth-m command-line option (see second example).

2. Note that we have a verbose option, which prints out phase times while still in sss. It also prints out the depth used.

3. As seen in the second example, if one has not turned on the verbose option, the depth used is still printed out.

4. If one does not change the evdp from meters to km, error messages will be produced that tells you the depth is too great.

Arthur

On Wed, 20 Jul 2011, Philip Crotwell wrote:

This issue caused me quite a few headaches with TauP_SetSac.
Ultimately I decided the best course of action was to do what the sac
manual says, and so evdp is in meters. Of course a huge percentage of
seismologists put the value in km. My personal bias is that if the sac
spec says it is in meters, then any sac commands that use evdp should
do so in meters, perhaps with a warning if the value looks like it
might be kilometer, ie >1000. To change the meaning of a header at
this point in sac's life seems like asking for more trouble and
confusion.

The best answer to this is to have the units in the header. But, one
of the strengths of the sac file format is the simple header. One of
the weaknesses of the sac file format is the simple header and so I
doubt there is any place to put it.

Philip

On Wed, Jul 20, 2011 at 12:53 PM, Renate Hartog <jrhartog<at>uw.edu> wrote:

We (Pacific Northwest) sometime have small local earthquakes for which our
locator gives a depth < 1km. Explosions certainly can have depth < 1km. So,
George's scheme could potentially go wrong for those. However, it might be a
non-issue for him because the DMC doesn't keep track of small, local
earthquakes, i.e. there won't be any event information in the SEED file.
Larger ones probably always nucleate deeper than 1km.

The easiest (least change) solution is to leave EVDP in m and provide an
option to traveltime like Januka suggested. If the EVDP is a float field, it
could be in km, as you could specify a shallow depth as, e.g., 0.255 km.
Another thought is that with the advent of relative location algorithms,
depth differences on the 10-s of m scale might be resolvable for clustered
earthquakes and for that reason leaving the units stay "m" might be worth
it.

-Renate
Pacific Northwest Seismic Network
Seattle, WA

On Wed, Jul 20, 2011 at 6:41 AM, Januka Attanayake <jattanayake<at>yahoo.com>
wrote:

Isn't EVDP for nuclear explosions is < 0.5 km ? I am not sure if there are
any records of them in the IRIS DMS though. I suspect this is the reason why
meters was used as the default unit to begin with?

_______________
Serenity isn't freedom from the storm, but peace within the storm
Januka Attanayake
Earthquake Seismology & Economics
The University of Connecticut
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354, Mansfield Rd;
Storrs, CT 06269
Tel : 860 486 0475 (Office)
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________________________________
From: Arthur Snoke <snoke<at>vt.edu>
To: SAC-help Listserv <sac-help<at>iris.washington.edu>
Sent: Wednesday, July 20, 2011 7:05 AM
Subject: Re: [SAC-HELP] units of EVDP in SAC

Comments on the two responses I got so far:

1) Because traveltime did not work in SAC/IRIS, in the Help file for
sss/traveltime
http://www.iris.edu/software/sac/commands/sss.com/traveltime.html we
recommend using taup_setsac. As with RDSEED, the default is meters for
EVDP.

2) I thought about some variant of George's macsac 1000 switch between
meters and kilometers, but not having worked with very shallow events (of
any kind) I was not sure where the threshold should be set. A 0 value is no
problem, but does one use focal depths of 0.5 km? Patly because of my
testing with traveltime, I am planning to changed the EVDP for fg/seismo to
15 km.

On Tue, 19 Jul 2011, Arthur Snoke wrote:

To my knowledge, since IRIS/SAC v100, EVDP (focal depth in the SAC
header) has not been used in any SAC command. Please let me know if there
are commands I have overlooked.

Because of historical reasons, EVDP (focal depth) in SAC has the units
according to the SAC manual. RDSEED converts EVDP from kilometers (which it
is in blockette 71 in SEED) to meters when writing SAC waveforms.

For v101.5, the units of EVDP will be of interest because we will (re)
introduce a version of the TRAVELTIME command, in which for waveforms with a
defined location and origin time one can calculate arrival times for
body-wave phases using the iaspei-tau tables for either ak135 or iasp91
velocity models. For this command, the units of EVDP matter.

I personally would like to evolve towards having kilometers being the
default unit for EVDP. As a first step, kilometers would be the default
choice for EVDP in TRAVELTIME.

The point of this note is a canvass of the community to find if there
are any problems with using KM rather than M for EVDP. Let me know if there
are other programs that read SAC files and use EVDP assuming it is in
meters.

Arthur

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--
Renate Hartog
Research Scientist
Pacific Northwest Seismic Network
Department of Earth and Space Sciences
University of Washington
Box 351310
Seattle, WA 98195

Tel. +1-206-685-7079 (office)


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