SAC Tutorial Three

3.1 Requesting Seismic Data

An excellent resource for obtaining earthquake data is the Wilber webpage provided by IRIS. From this page you can choose datasets for a large variety of significant earthquakes (typically those greater than magnitude 5.0). The seismograms for all of the seismometers that record the event are grouped together and you can choose which observations you'd like to access quickly. The events are grouped by date, divided into 4 quarters of each year.

For this example, select the third quarter of 2006 [Q3 2006] in the window on the left hand side. It will bring up a map showing all the earthquakes with datasets for those 3 months. Now click on the red dot sitting right on top of Florida, to look at the waveforms for an unusually large earthquake in the Gulf of Mexico near the Florida coastline.

When the list of data comes up, click on this one:

2006/09/10 14:56:07.8 SPYDER® 5.8 26.34 -86.57 10.00 GULF OF MEXICO

The next page that comes up shows a list of seismic station networks. Select:

US United States National Seismic Network (USNSN)

And then click Proceed.

The next page will show all of the stations in this data group. We only want to select 3 stations: ACSO, ISCO, and HLID, so you need to unselect all the other stations. Then on the upper left, make sure to select channels BHE, BHN, and BHZ using shift-click.

Then down at the bottom of the page, make sure the data format is SEED, the time window shows 2 minutes before the P, and 20 minutes after the P. Then fill in your username (use your login name from UNIX), "florida06" for the label, your email address, and select "Notify me by email when complete". Click Process Request, and then it should take about 1 minute for the request to become available for you to download.

3.2 Retrieving Seismic Data

Once the webpage tells you the Request Completed, go to your UNIX window and start an ftp session to get your data:

[username@linux ~] mkdir sacdata
[username@linux ~] cd sacdata
[username@linux ~] ftp ftp.iris.washington.edu

You can just enter anonymous for the username and your email address for the password. Then you will need to go to the directory they provide you on the webpage. For example (mikeb should be replaced with your username though):

ftp> cd /pub/userdata/mikeb/florida06
ftp> ls

Listing the files should show a florida06.seed file. Then we use the get command to retreive your data:

ftp> get florida06.seed
ftp> bye

[username@linux ~] ls

3.3 Unpacking Seismic Data

Once you exit, ls should help you see that your SEED file is back on the UNIX machine. SEED stands for Standard Exchange of Earthquake Data. You can upack individual SAC data files from the SEED archive using the rdseed command. The -d option tells rdseed you want to dump the SAC data files, and the -f option tells rdseed what the SEED file name is.

[username@linux ~] rdseed -d -f florida06.seed
[username@linux ~] ls

Now you should see several SAC files that we can read into SAC. Each data file contains a single data set. For seismic data this means a single data component recorded at a single seismic station.

[username@linux ~] sac

SAC> r *.SAC
SAC> p1

Exercise 3.1

Describe what the data looks like in a file called data.txt. Be sure to note the similarities and differences between the data files.

3.4 The SAC Header

Each data file also contains a header record which describes the contents of that file. Certain header entries are always present (e.g., the number of data points, the file type.) Others are always present for certain file types (e.g., sampling interval, begin time, etc. for evenly spaced time series files.) Other header variables provide information needed by a particular operation (e.g., seismic component orientation used by the ROTATE command.) Still others are not used by SAC at all. They are simply informational. SAC data files, including all header information, can be read and written by user created programs as well as by SAC. See the SAC User's Manual for details. The LISTHDR command displays the contents of the headers for the data files currently in memory. You can examine the headers from the seismograms we just downloaded:

SAC> LISTHDR

If a particular header variable does not have a value for a particular file, then that variable is said to be ``undefined'' for that file. The LISTHDR command does not list undefined header variables, usless it is invoked with the INC or INCLUSIVE option (which includes undefined header variables).

Each header variable is described in the Users Manual. The most important ones are also listed below:

Summary

brudzimr@muohio.edu, 19th June 2007