[irised] wells nevada record, station SHMT

[John Taber]

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
>
>
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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


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