Reading seismograms are like reading an ancient foreign language, so it will take some time to interpret the signals that may look like hieroglyphics when you first look at them. In this analogy, the "tablets" are the seismograms, which comes in 3 different types: up-down (Z component), north-south (N component), and east-west (E component). These are the 3 components that record the complete motion the surface of the earth experiences at any moment during the day. We then look at one of these seismograms (tablets) to see if there are any signals (words) that we can interpret. This is the first issue with reading "seismish", just because you have a seismogram, doesn't mean there are any significant signals on it. Just like an archeologist digging in the dirt, you're looking for an important find that has a recognizable look to it. As you might expect, it takes some time to recognize the different signals, but we will try to walk you through the main types you should look for.
Use data we retrieved during Exercise 3. Login to the department server using the VNC program, open a command line window, and then go into your sacdata directory so that you are in the same location as the SAC files extracted during Exercise 3. Since this data came from a particular earthquake, we should expect to see various seismic waves recorded in this data. In essence, someone has already identified this as the right time to look at the seismogram, whereas the data may lack any identifiable signals for many hours leading up to this event.
To examine the P and S waves, we will use sac. First run the sac program from the linux command prompt, and then read in the data.
[username@linux ~] cd sac/sacdata
[username@veracruz sacdata] sac
SAC> r *.SAC
SAC> p1
What do you see? Hopefully several bumps that correspond to different arrivals of seismic waves. You should see groups of 3 seismograms that have similar looking signals because these are the 3 components of each station. As we've talked about in class, the P waves travel faster so they should appear first, and the S waves travel slower so they will arrive afterwards. You may also notice surface waves on the seismograms, but we will not be focusing on them in this activity.
For this activity, I'd like you to mark the P arrival times and the S wave arrival times on these seismograms and store that information in the data file. We can mark the arrivals using the ppk command.
SAC> ppk
We used it before to just plot data, using the X key to zoom into a particular area of the seismogram where the cursur is positioned (hit X once for the beginning of the zoom window, hit X again for the end of the zoom window). In this case we will also use the T key to mark a time on the seismogram where the cursor is positioned, followed by typing a number for which arrival it is (1 is typically used for the P wave, 2 is typically used for the S wave). So you should use the ppk command to zoom in around the arrivals using the X key and mark the onset time of the P and S waves with T1 and T2 respectively. The key thing to keep in mind when marking the arrival time is that you should be marking the time when the arrival begins. So in some cases the S wave may consist of several peaks and troughs on the seismogram, but we only want to mark the first motion away from the background noise level. Once you have marked the times on a few seismograms, you will likely need to use the Q key to stop the ppk command. When you have marked the P and S arrival times on all of the seismograms, you will want to save the results in your data files with:
SAC> write over
Did you find certain seismograms were easier to mark the P and S wave arrival times. Why? How do the P and S waves differ from each other on a given seismogram? How might you use that information to help you identify which is wave is which on a future seismogram? Which station is closest to the earthquake and which station is furthest? Record your answers in a file called bodywaves.txt.
Tn |
PPK key code that marks a time and gives it a number n. |
brudzimr@muohio.edu, 19th June 2007