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In order to assist in identifying seismic phases, there is an option for displaying the theoretical arrival times of several global (using IASP91) and regional phases while picking phases (using ttlocal based on layered model in STATION0.HYP). The steps to do so are the following:
| 1 | Before entering MULPLT from EEV, the theoretical travel times can be calculated for the current event. This assumes that the origin time and hypocenter is given in the header line or a subsequent type one line. If not, enter manually (from e.g. PDE) or use the EEV command INPUTEPI or INPUTONE. Then proceed to calculate the theoretical arrival times using EEV command IASP with the IASPEI91 traveltime tables (for more details, see section 30.5). The same command is also available inside MULPLT in multitrace mode. All arrival times (or a subset, see 30.5) for all stations in current S-file will now be calculated with program IASP if call from EEV. If call from MULPLT times will be calculated for all stations in waveform file(s). In both cases, the times are stored in file iasp.out (no importance for the user, just for information). See Figure 8.2 for an example in multitrace mode. Note that very many theoretical phases can be generated if the S-file has many stations. MULPLT will stop if more phases are used than the dimensions are set up for (include file seidim.inc), and you must use fewer phases (a warning is given when 500 phases are generated) or set up SEISAN with larger dimensions, see section 3. Theoretical local crustal phases for the current model can be calculated with program WKBJ and displayed, see section 29. Theoretical phases can also be calculated when using the location option, see next section. |
| 2 | When a trace is displayed on the screen, all theoretical phases inside the time window will be shown. To distinguish the theoretical phases, they are prefixed with a y and displayed below the trace (normal phases have I, E or blank and are displayed above the trace). Position cursor where you see a phase which you think corresponds to a theoretical phase and press y. The nearest theoretical phase will now be placed at that position with a prefix E. Only theoretical phases selected in this way will be written in the S-file. Note that the phase names can be up to 8 characters long, see Appendix 1 for the definition of long phase names when using old Nordic format. |
If the phases fit badly, start looking at the P-phase. If that does not fit the theoretical P-phase, change the origin time in the S-file so that the P-arrival fits, and recalculate the theoretical phases.
PROBLEM: In multitrace mode, only one theoretical phase can be picked. Replot must be made before picking the next.
If several phases have been read and saved in the S-file, the event can, in multitrace mode, be located with command l (Locat), just as in EEV. The screen is cleared and the usual location rolls over the screen. When the location is finished, the plot will reappear and the calculated travel times will be displayed as synthetic phases (see previous section). In this way it is possible to immediately visualize the differences between the read and calculated phases. The output files are hyp.out and print.out as usual.