[SAC-HELP] units of EVDP in SAC

[Philip Crotwell]

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
>> Beach Hall - U2045
>> 354, Mansfield Rd;
>> Storrs, CT 06269
>> Tel :  860 486 0475 (Office)
>>           860 486 3914 (Lab - P405)
>> Home page: http://sites.google.com/site/janukaattanayake/
>> Research group: http://www.phys.uconn.edu/research/geophysics/index.html
>>
>>
>>
>>
>>
>>
>> ________________________________
>> 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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>>
>>
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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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