Lesson 2: Using theoretical times to identify teleseismic phases
This exercise will use the same data set we used in lesson
1. That is, the set of large teleseismic events recorded on the Anza
network, which you worked with before.
Proceed as follows:
The default dbpick filters are not very good for this excercise.
Click here to download a file called
.dbpickrc. Use the File->save as menu item on your browser
to place this file in the directory where you put the lesson 1 dataset.
Save the file with the name ".dbpickrc".
Start up dbpick on your copy of the database. There are a number
of ways to do this but for this exercise it is cleanest to do do this.
repeatedly. This is a useful was to scan through the
data quickly. Note the association display noting event location,
distance, and azimuth information that gets spewed out in the dbpick command
Zoom in/out to define a time window from just before P to a few minutes
after the S arrival.
Type: sp basic
You should now see a set of theoretical arrival times for a set of "basic"
teleseismic phases. They will be shown on the display with blue colored
lines with phase labels.
Switch between channel groupings to see which phases are seen as coherent
arrivals across the entire array. That is type (without the quotes):
"sc *:BHZ" to look at P phases and "sc *:BHN" and "sc *:BHE" to look
at S phases. Make a mental or written note on which phases you think
are actually observed on which channels. Experiment with the filters
on the Filter menu. Many phases are only seen well at long
periods while a few are more "broadband". (Explore this yourself
and report your results next time. Consider why this is true and
come prepared to discuss it.)
You might want to be more specific and type things like: sp ScS.
This would bring up the theoretical time only for ScS.
When you have a phase you think is real you can pick it. A feature
of dbpick, however, is that unless the phase is a simple phase name like
P or S, you need to force the pick to be called something else. e.g.
if you want to pick ScS type: ph ScS. Then pick
the arrivals as you did last week for picking P. When you point and
click the flag will appear with ScS instead of the default of U.
Use snapshot to make a hard copy of your results to bring to next weeks
class for discussion.
Draw a ray diagram showing what each of the phases you interpreted represent.
This will help in the discussion to remind people what those obscure names
Are there any other interesting features you see in the event you analyze
and/or one of the phases you picked?