Thread: wells nevada record, station SHMT

Started: 2008-02-26 22:28:46
Last activity: 2008-02-27 07:10:59
Topics: IRIS EPO
Craig Messerman
2008-02-26 22:28:46
Hi All,
I'm attaching our record of the Wells quake. My students are analyzing it this week, but the arrival interpretation is difficult. The first motion seems to be the low amplitude waves that arrive at 14:19:18, but then a very distinct and stronger phase arrives just 21 seconds later, too soon to be an S. Could be PP? If that's the case then the S arrival seems buried between that phase and the surface arrivals. We haven't had the chance until now to work with such a close and recent event, so if anyone could take a look and advise, we'd appreciate it.

Craig Messerman
Sentinel High School, Missoula, Montana


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Attachments
  • John Taber
    2008-02-27 01:43:48
    Craig,

    Picking the phases for a closer event can be difficult if you only
    have a single station, so I've attached a plot made by Chuck Ammon
    that shows hundreds of stations from USArray. At your distance I
    think your first arrival is a refraction from the Moho (the base of
    the crust) and the second arrival is the direct wave traveling
    through the crust. The S wave would then be around 14:21:10.

    John Taber


    On Feb 26, 2008, at 4:28 PM, Craig Messerman wrote:

    Hi All,
    I'm attaching our record of the Wells quake. My students are
    analyzing it this week, but the arrival interpretation is
    difficult. The first motion seems to be the low amplitude waves
    that arrive at 14:19:18, but then a very distinct and stronger
    phase arrives just 21 seconds later, too soon to be an S. Could be
    PP? If that's the case then the S arrival seems buried between that
    phase and the surface arrivals. We haven't had the chance until now
    to work with such a close and recent event, so if anyone could take
    a look and advise, we'd appreciate it.

    Craig Messerman
    Sentinel High School, Missoula, Montana


    Notice: This e-mail may contain confidential information belonging
    to the sender which is privileged. The information is intended
    only for the use of the individual or entity named above. If you
    are not the intended recipient, you are hereby notified that any
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    reliance on the contents of this information is strictly
    prohibited. If you have received this transmission in error,
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    Attachments
  • Jerry Cook
    2008-02-27 02:13:26
    Hi Craig,
    I have had the same question for years! Many times on near-by quakes there is a very low amplitude recording for a few seconds followed by a larger amplitude phase. The "S" wave seems clear to me but, which "P" wave should be used for the equation (S-P)8? This always is confusing. Why are there two "P" waves and which should be used in the equation? This clearly is evident in your recording and mine also for the same event. See Attachment.
    J. Bob

    ________________________________

    From: irised-bounces<at>iris.washington.edu on behalf of Craig Messerman
    Sent: Tue 2/26/2008 2:28 PM
    To: irised<at>iris.washington.edu
    Subject: [irised] wells nevada record, station SHMT



    Hi All,
    I'm attaching our record of the Wells quake. My students are analyzing it this week, but the arrival interpretation is difficult. The first motion seems to be the low amplitude waves that arrive at 14:19:18, but then a very distinct and stronger phase arrives just 21 seconds later, too soon to be an S. Could be PP? If that's the case then the S arrival seems buried between that phase and the surface arrivals. We haven't had the chance until now to work with such a close and recent event, so if anyone could take a look and advise, we'd appreciate it.

    Craig Messerman
    Sentinel High School, Missoula, Montana


    Notice: This e-mail may contain confidential information belonging to the sender which is privileged. The information is intended only for the use of the individual or entity named above. If you are not the intended recipient, you are hereby notified that any disclosure, copying, distribution or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this transmission in error, delete it without copying it and immediately notify the sender by reply e-mail. Thank you.
    <<<< GWAVASIG >>>>


    Attachments
    • John Taber
      2008-02-27 06:04:39
      Jerry and Craig,

      I tried sending this message earlier today but I used too big a file
      and it bounced so here is smaller one:

      Craig,

      Picking the phases for a closer event can be difficult if you only
      have a single station, so I've attached a plot made by Chuck Ammon
      that shows hundreds of stations from USArray. At your distance I
      think your first arrival is a refraction from the Moho (the base of
      the crust) and the second arrival is the direct wave traveling
      through the crust. The S wave would then be around 14:21:10.

      On the plot below, the Moho arrival starts to arrive before the
      direct wave at around 200 km distance (if you draw a line
      connecting the first arrivals you may see a change in slope at that
      distance). The activity "How shallow earth structure is determined"
      at http://www.iris.edu/joomla/index.php?
      option=com_docman&task=cat_view&gid=24&Itemid=46 provides more
      background on interpreting refracted waves.


      John Taber


      On Feb 26, 2008, at 8:13 PM, Jerry Cook wrote:

      Hi Craig,
      I have had the same question for years! Many times on near-by
      quakes there is a very low amplitude recording for a few seconds
      followed by a larger amplitude phase. The "S" wave seems clear to
      me but, which "P" wave should be used for the equation (S-P)8? This
      always is confusing. Why are there two "P" waves and which should
      be used in the equation? This clearly is evident in your recording
      and mine also for the same event. See Attachment.
      J. Bob

      From: irised-bounces<at>iris.washington.edu on behalf of Craig Messerman
      Sent: Tue 2/26/2008 2:28 PM
      To: irised<at>iris.washington.edu
      Subject: [irised] wells nevada record, station SHMT

      Hi All,
      I'm attaching our record of the Wells quake. My students are
      analyzing it this week, but the arrival interpretation is
      difficult. The first motion seems to be the low amplitude waves
      that arrive at 14:19:18, but then a very distinct and stronger
      phase arrives just 21 seconds later, too soon to be an S. Could be
      PP? If that's the case then the S arrival seems buried between that
      phase and the surface arrivals. We haven't had the chance until now
      to work with such a close and recent event, so if anyone could take
      a look and advise, we'd appreciate it.

      Craig Messerman
      Sentinel High School, Missoula, Montana


      Notice: This e-mail may contain confidential information belonging
      to the sender which is privileged. The information is intended
      only for the use of the individual or entity named above. If you
      are not the intended recipient, you are hereby notified that any
      disclosure, copying, distribution or the taking of any action in
      reliance on the contents of this information is strictly
      prohibited. If you have received this transmission in error,
      delete it without copying it and immediately notify the sender by
      reply e-mail. Thank you.
      <<<< GWAVASIG >>>>

      <0802211413PCAZ.sac>_______________________________________________
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      irised<at>iris.washington.edu
      http://www.iris.washington.edu/mailman/listinfo/irised



      John Taber
      E&O Program Manager Tel: 202-682-2220
      IRIS Fax: 202-682-2444
      1200 New York Ave., NW, Suite 800 Email: taber<at>iris.edu
      Washington, DC 20005 www.iris.edu



      Attachments
      • Jeff Barker - Binghamton University
        2008-02-27 07:10:59
        Hi all,

        Without making measurements of your particular seismograms, I think
        John Taber has it right. At regional distances, beyond say 150 km
        out to several hundred km, you are likely to see a small arrival, Pn,
        which is the head wave or critical refraction from the top of the
        Mantle arriving before the larger Pg, which is a combination of the
        direct wave and multiple reflections within the crust. There are
        comparable S phases but you are unlikely to see Sn on a
        vertical-component seismometer.

        John Lahr's website has a link to a USGS site where you can calculate
        arrival times of many phases (waves) at your station from recent
        earthquakes. The URL is
        http://neic.usgs.gov/neis/travel_times/artim.html
        Set the maximum distance to something like 10 degrees and choose an
        appropriate minimum magnitude (5.0 is good). Be sure to click the
        radio button for "All branches". You get lots of predicted arrival
        times, but look for Pn, Pg and Sg. For Jerry's station the arrival
        times from the Wells earthquake are 48, 57 and 98 seconds after the
        earthquake's origin time.

        On my website is an animation that shows Pn and Pg propagating in a
        model of the crust. It's the second animation on the following page:
        http://bingweb.binghamton.edu/~jbarker/animations.html
        Pn is the "head wave", a weak, linear wave seen arriving first at the
        surface for distances beyond about 150 km (for this model).

        By the way, it was exactly this sort of data that Mohorovicic used to
        interpret that there was a difference between the Crust and the
        Mantle. That's why we call this boundary "the Moho".

        Jeff Barker

        P.S. - Craig, your SAC file didn't include the station lat and lon, so
        I couldn't look it up. I did notice that the station name is defined
        as SUNYB. This must be the default from AmaSeis, because that stands
        for SUNY Binghamton.

        --
        ----------------------------------------------------------------
        Jeffrey S. Barker
        Assoc. Prof. of Geophysics      jbarker<at>binghamton.edu
        Dept. of Geological Sciences    geology.binghamton.edu/~barker
        SUNY Binghamton                 (607) 777-2522
        Binghamton, NY  13902-6000      FAX (607) 777-2288


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