Lesson 5: Local and regional event seismograms

For this exercise we will be working with data from the Tien Shan experiment from 1999 and 2000. Things you should learn from this exercise are:

Understand the first order characteristics of how seismograms change from 0 to distances of 10 to 15 degrees. This is commonly called local to regional distances.
Understand what the phases Pg, Pn, Sn, and Lg are and what they commonly look like on real data.
Be able to disciminate deep events from shallow events at regional distances.

Step 1: copying database files

Create a new directory somewhere under your home directory to hold the data for this exercise.
Make the new directory the current directory by using the cd command
Click here to download a copy of the data. Put these data in your working directory
Unpack the tar file using: zcat lesson5.tar.gz | tar xvf -
Verify everything is intact by running dbe or dbpick against the tsr database that should now be present in your working directory.

Step 2: start up dbloc2

For this exercise we want to use a GUI front end to an analyst system called dbloc2. Assuming you are still in the working directory you copied these files to, you just type:

dbloc2 tsr

which should bring up a control panel that looks something like this:

For this exercise we will only be using four buttons on this GUI:

On the bottom row: Next
On the bottom panel near the right side: "Show Waveforms"
Adjacent to summon is a pull down menu labeled above as "Vertical".
A pulldown method on the center panel with the buttons labeled as orid.

Step 3: Basic navigation

Assuming everything works (sometimes the first attempt to start dbloc2 can fail, so if it does try again) do the following:

Push the "Arrival Map" button (near the center of the window). This brings up a useful map showing where the stations are located geographically relative to the event location.
Push the "Show Waveforms" button.
View the data with a 1Hz highpass filter or you won't see much with most of these events.
Position the pointer over the "Vertical" button and hold down the left mouse button. A pull down menu appears. Select "All". This should bring up all the data for the first event. You should be aware that the data are brought up in order of increasing distance and have been time shifted to line up all arrivals on the theoretical arrival time of the P phase (rec and pal commands of dbpick).
Position your cursor over the button below "orid" on the left side of the center panel, push and hold the left mouse button, and then select "Show residuals" for this origin estimate.
Push "Show Waveforms" and explain what you see.
Push the "Next" button followed by "Show Waveforms" to get to the next event in the database.

Step 4: study a particular event

The final objective is to browse through this data set and see for yourself how seismograms develop at increasing distance from the epicenter of an earthquake. Make use of the "Arrival Map" and the tabulated event location information in the control panel to make sure you understand which event's waveforms you are viewing. Try to complete the following for discussion in next week's class.

Find a shallow event that was recorded out to a distance of at least 500 km. Use the dbpick print function to make hard copies for class discussion of seismograms from three distance ranges (note the dbloc2 panel shows the distance to each station in degrees): (1) as close as possible, but definitely less than 1 degree, (2) 1-3 degrees, and (3) greater than 4 degrees. This should be printed as three pages with all one of the three components observed at all station on each sheet of paper. Please mark the epicentral distance on each sheet. It would also be useful to show the map location displayed in the "Arrival Map" window also.
Find a deep event from the Hindu Kush region. The key parameter to identify is the estimated event depth. There are numerous intermediate depth events from this region. Find a station at a comparable distance to the one's you picked for (2) and (3) above (number (2), the "close" case, is impossible with these data because of the location of the sources relative to the earthquakes) and make hard copies as you did before. Come prepared to discuss how the two sets of seismograms differ. There is at least one Hindu Kush event that was large enough to be visible all the way out to stations in northern Kazakhstan (AKTK, KURK, VOS, ZRNK, BRVK, CHKZ) at distances over 15 degrees. Bring printouts of some of these stations for further discussion in class next week.