STP, Unix, SAC tutorial, Ge167 Winter 2014

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1 STP, Unix, SAC tutorial, Ge167 Winter 2014 Asaf Inbal 1 Downloading waveforms In this tutorial we ll learn how to download waveforms using a tool called STP (Seismic Transfer Program) and manipulate them with SAC (Seismic Analysis Code). We ll also write some basic shell scripts that will help us on our way to becoming seismologists/engineers. Throughout this tutorial, lines starting with # indicate comments. Let s start: We d like to look at waveforms from the M7.2 El Mayor-Cucapah earthquake, which was well recorded by the Southern California Seismic Network (SCSN). To download the waveforms using STP open a terminal (press the small TV on top left-hand side of your screen) and type the following commands: > STP now, let s have a look at the available commands within STP: > help or, for a more detailed description: > help command name To download waveforms for a specific event we need to know the event id number. Let s search the SCSN catalog for events that will match some search criteria. The EQ happened on April 4th, 2010, so we ll look for large events (magnitude 6.5 and above) that had occurred within 24 hours of midnight of 03/11/2011. The command is: > event -mag t0 2010/04/04 +24h and the output should be: uk 2010/04/04,22:40: w ts 2010/04/04,22:40: w le 2010/04/04,22:40: w uk 2010/04/04,22:41: l 0.5 with the event id as the first number on the left. The event id we will use is the one the 1

2 corresponds with the word le, which stands for local event. The next fields are the origin time, coordinates, magnitude, magnitude type, and location quality. Now that we have the event id we can download the waveforms. We would like to correct for the instrument gain: > gain on followed by: > trig The trig command will download the waveforms for stations in CA that were triggered by the EQ. While we wait for STP to finish, let s practice using Unix. Open another window (Shift+Ctrl+N) or tab (Shift+Ctrl+T). You can move between tabs by pressing Alt+num., where num. is the tab s number. To look at the contents of the directory: >ls -l You should see a new directory called Question: How do you know if this is a directory or a file? On Linux machines, directories names are usually printed in blue color. Another way is to find out is to look at the first letter of the first column of the output ( d for directory - for file). The rest of that row contains the permissions, which tells us who can do what with this file, the size and the date it was created. To look inside this new directory type: >ls which will list the directory contents. Note that the file names follow a specific format: eventid.network.station.channel.sac. According to the SCSN convention, channel names starting with BH or EH(???) indicate broad-band sensors. The third letter contains the component (vertical, east-west, north-south). If you want to change from the current directory to type: >cd To go back to the parent directory: >cd.. If you what to know the name of the current directory type: >pwd 2

3 On Unix based machines,.. and. are synonymous with parent and current directories. To count the files in type: >ls wc -l Here we are issuing two command in a row. The first command outputs the contents of We then use the symbol to pipe the to another command called wc (shorthand for word-count), with the -l flag signaling we are only interested in the number of lines. Issue this command twice and to see whether the waveform download is finished. By now STP should have finished downloading the files, so let s log out from our STP session by typing: >quit in the STP window. 2 Analyzing Waveforms and making nice figures Now we are going to look at some seismograms. Our main objective is to identify prominent phases (body waves, surface waves) associated with the EQ. First, let s have a look at a single station. SAC files are written in binary format, which contains a header followed by the actual data. The header contains always contain information about the length of the segment, the sampling rate, maximum and minimum values, the start time, and the offset with respect to that time. The header section will sometimes contain information about the station (name, location etc.) and the event. Since we used the trig command to download the data, the station and event data are written in the header section. Some of the commands in SAC will only work if event and station information are present. >r /file name #where file name is the name of a file in the directory #List the file s header. Pay attention to the sampling rate, begin times, length of segment, station and event information. >lh As you noticed in the former section, the EQ triggered hundreds of stations, of which only a subset should be used in this exercise. Working with a small number of stations shortens the processing time and makes the plots more readable. To choose this subset we are going to narrow down our search to the vertical sensors at the broad-band stations. This will leave us with (only) a few tens of stations. To be able to see the move-out we will sort the stations with respect to the hypocentral distance before we plot. We re going to write a short script that 3

4 outputs every tenth station in a list sorted according to the hypocentral distance. Open another window/tab. Open a text file you will call script.sh. To do that from within the shell we re going to invoke Vi, a powerful text editor. > vi script.sh To write text to our new file you ll need to go into insert mode, so press i. Note that the word INSERT appeared in the bottom left-hand side of the screen. To exit insert mode press the escape button. In that file you ve just opened type in the following lines: # a script to sort stations. first switch to use bash: #!/bin/bash # run script until you reach END, output to file sta list.txt sac << END > sta list.txt # read broad-band, vertical channels r./ /.bhz. # sort according to distance sort dist descend # from now on print output echo # list header files, station names, station latitudes lh columns 2 KSTNM STLA quit END #find "FILE" in sta list.txt,pipe to awk, print every 10th line. This should be writtem as one line #of code grep FILE sta list.txt awk NR%10==0 {printf "%s ",$2} END {printf \n } In Vi, press the escape button to leave insert mode. Typing : will transfer you to normal mode, where you can save the file and quit by entering: :w followed by :q. The first few lines of code will actually run SAC, perform the sorting and write the output to a new file called sta list.txt. This new file contains some text we are not interested in, followed by the filenames, the station name, and the station latitude. Note that the lines containing the file names start with the word FILE. We ll use grep and awk, two unix tools for searching and performing numerical operations on ascii files. The awk syntax is a bit involved, so we ll leave that for another time. Before we can run the script we need to make the file executable by typing: > chmod +x We then run the script by using: 4

5 >./script.sh The output is the list of files in that correspond to stations sorted with respect to the hypocenter. Copy this line. You also created a file called sta list.txt. If you want to check out it contents you can open it with Vi or run: > more sta list.txt Now we are going to read the list of files you ve just created. In in your sac window run these commands (don t enter the comments): #read in the files. By default, each read command writes over any previously stored files. > r the list of stations you ve just copied #turn off quick dirty plot >qdp off #plot the seismograms. The figure appears in a new window. Can you identify P and S arrivals? >p1 #enter sss mode to compute the predicted travel times >sss >prs orient landscape #compute travel times. write to header >traveltime picks 0 #plot new figure with trave time >prs ttime on #save the plot >bd sgf >prs ttime on #convert to postscript file. >sgftops./f001.sgf 1.ps #quit >quit To view the figure run: > gv 1.ps & Seismometers at short hypocentral distances clip when the ground motion exceeds the instrument s dynamic gain. For this reason we mainly use accelerometers to measure strong ground motions near large earthquakes. Let s see how these two types of instruments compare: >sac #read a broad-band channel and a accelerometer 5

6 >r / CI.ERR.BHZ.sac / NP.5062.HLZ.sac >qdp off #plot the traces >p # now zoom in at a window containing large motions to see the clipped wiggles >xlim >p #zoom out >xlim off >q 3 Spectral analysis In this section we would like to see how the amplitude of different frequencies decays with distance from the source. To do that we ll plot the spectra recorded at these three stations: BK.JCC, CI.MLAC and CI.CAR. First, you ll need to read the three components of each station and pick the P-and S-wave arrivals. Once you do that, you should write the files to disk. Here s an example >r./ / ci.mlac.bh?.sac >chnhdr T0 150 T1 200 # write new files to disk and quit > w MLAC.BHE.sac MLAC.BHN.sac MLAC.BHZ.sac I usually use xlim xmin xmax to zoom in and xlim off to zoom out. You can also use plotpk command for picking the phases. Read the three channels, and run plotpk, and help plotpktable to see how to use the manual picker. At this point you should have 9 new files in you current directory, with the P and S arrivals stored in the header variables T0 and T1 in each file. Next, we are going to create time windows around the S-arrivals, take the fft and plot: >cut T # note the use of * as wild card >r *.BHE.sac >chnhdr B 0 #remove trend >rtr #remove mean >rmean #take fft 6

7 >fft #turn on logarithmic scales >xlog ; ylog >qdp off >color on inc >p2 7

Contents. Note: pay attention to where you are. Note: Plaintext version. Note: pay attention to where you are... 1 Note: Plaintext version...

Contents. Note: pay attention to where you are. Note: Plaintext version. Note: pay attention to where you are... 1 Note: Plaintext version... Contents Note: pay attention to where you are........................................... 1 Note: Plaintext version................................................... 1 Hello World of the Bash shell 2 Accessing