[SAC-HELP] units of EVDP in SAC

[Renate Hartog]

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?
>
> *_______________*
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> *Earthquake Seismology & Economics
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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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