18.15 P_ALIGN: Time shifting waveform data to align on P or S-phase arrival times.

If one wishes to compare signals (align in time) from different earthquakes observed at the same station, the P_ALIGN program can be used. The program works by time shifting the waveform header times to a common time and then putting all the new waveform file names into an S-file. First use e.g. the SELECT program to extract information of earthquakes in a defined area that have been observed by a given station, and use GET_WAV to copy the waveform files to your working directory. Then execute P_ALIGN. The input is the Nordic file (e.g. select.out) and the station name for data to be compared. The output is:

• Waveform files with time shifted headers, all have the same time, but station names are labeled STA01, STA02 etc in the same order as given in input file. Only first 3 letters of station code is used.
• tsd.out : A file in Nordic format with the new waveform file names of the time shifted data. This file can be split and then used with EEV for plotting all traces. It can be split into a local data base or any other data base. Or copy tsd.out to a file e.g. named 27-1200-00L.S207011 and then start EEV.

The waveforms in the output file tsd.out can also be plotted with the command mulplt -sfile tsd.out.

The arrival time of the selected phase in the new waveform files is the pseudo date and time: 2070-11-27 12:00.

Note: The station name is renamed in the output. If there are more than one P-phase observation for a single event (e.g. Pn and Pg, or P read on two different channels) it is the first P that is time shifted. The program can only time shift 99 waveform files and they have to be in SEISAN format. The waveform file must be present in the working directory.

Figure 18.2 show an example.

Figure 18.2: Example of aligning traces from 5 events for the same station. Note that the alignment is critically dependent on the original P-picks.