Craig Messerman
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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<<<< GWAVASIG >>>>
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
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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
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
irised mailing list<wellsnevada.sac>_______________________________________________
irised<at>iris.washington.edu
http://www.iris.washington.edu/mailman/listinfo/irised
Attachments
-
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
-
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,
John Taber
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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1200 New York Ave., NW, Suite 800 Email: taber<at>iris.edu
Washington, DC 20005 www.iris.edu
Attachments
-
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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John Taber
Jerry Cook
Jeff Barker - Binghamton University
