# An Interactive System Based on the IASP91 Earth Model for Earthquake Data Processing
## 4 Case Application of the Earthquake Data Processing System
### Seismic Phase Identification and Traveltime Picking  
Run Processing\(\rightarrow\)Load Events to load the seismic event information, then set the possible phases, run Processing\(\rightarrow\)Parameterizing Stations to calculate the epicenter distance of each station from the epicenter and perform ray tracing to calculate the theoretical traveltime for a given phase, the results of which are shown in Figure 8.  
Figure 6: Data import (unfiltered). Figure 7: Seismic data after removal of bad traces and filtering. As can be seen from Figure 8, the observed P-wave phases are very close to the theoretical traveltimes, which prove that the phases are correctly identified. Of course, we can also calculate the theoretical traveltimes for the subsequent phases and the results are shown in Figure 9.  
Figure 8: Seismic data and theoretical traveltimes (band-pass filtered). Figure 9: Multiple phases of this earthquake and their theoretical traveltimes (band-pass filtered). Once the seismic phase has been correctly identified, we can start picking the measured phase traveltime. (1) Wavelet selectionGenerally speaking, the shorter the time period, the better the waveform characteristics can be identified. Therefore, when picking the seismic phase, the length of the display should not exceed 300 s. When picking the seismograms, select Picking in the Settings to select a recorder with a good signal-to-noise ratio, then select Pick Wavelet in the Settings to select the wavelet (the most distinctive part of the seismogram). Use Display\(\rightarrow\)Display Seismograms for wavelet selection (Figure 10). Always refresh the screen by running Display\(\rightarrow\)Display Seismograms before wavelets are selected. In addition, to ensure the accuracy of the seismogram picking, use as short a wavelet as possible to prevent the wavelet from including another seismogram and affecting the accuracy of the traveltime picking. The length of the wavelet should not exceed 30 s.  
(2) Automatic Arrival-Picking by using the cross-correlation technique  
In the Settings, the Pick Wavelet is unchecked and then Auto Picking is selected, and the start position of the cross-correlation is given in the Settings by setting Tstart according to the distance of most of the phases from the theoretical walk time, where when Tstart < 0, the cross-correlation starts at Tstart on the left side of the theoretical walk time, and vice versa: when Tstart > 0, the cross-correlation starts at Tstart on the right side of the theoretical walk. After setting the parameters, run Functions\(\rightarrow\)Phase correlation to correlate the seismic waveforms of other stations with the selected wavelet. If individual stations have poor correlation due to a relatively low signal-to-noise ratios, the system can be used to determine the optimum arrival time by setting a range of inter-correlation time periods between the wavelet and other waveforms after setting Auto picking in the setup options and then running Function\(\rightarrow\)Phase correlation until satisfied (Figure 11). If the selection is simply undertaken manually, different people picking will generate different errors that will affect the final result. During the picking of wavelets and subsequent interdependent picking, the waveform is enlarged and the length of time displayed is set in the window to  
Figure 10: Wavelet selection (band-pass filtered). shorten the time taken for the automatic picking to begin, which takes only a few seconds after running Auto Picking, which is very fast. After confirming that the correlation results meet the requirements, the correlation effect is plotted by running Functions\(\rightarrow\)Plot correlation (Figure 12) and the accuracy of the seismic phase picking is further evaluated based on the correlation coefficients. The time period for displaying the seismic data is set in the Settings by setting Tstart and Tlength to correspond to the time of the theoretical traveltime as the start of the data period.