western quebec and southern ontario - Department of Geology

waveform (Pn, Pg, Sn, Sg, PmP, SmS, pPg, pPmP. sPg, sPmP, etc.) and polarity of firstarrival times of P-waves and to manually assign weights for the picks. The waveformdata in SEED format for each event were initially obtained with duration of 600 secondsand various sample rates (40, 50 or 100 samples-per-second (sps)). These files wereconverted into SAC files using rdseed program (IRIS) and subsequently to a specialUWO format used in the PLTSEK program. Data were analysed using a sample rate of50 sps. The two data sets for each event (one from the Canadian and one from the USstations) were combined to form one UWO file for each event. These files were then usedin PLTSEK for visualization and analysis of the waveforms and for picking the arrivaltimes (Pn, Pg, Sn, and Sg, etc.) and the polarities of P-waves (Pg, Pn).The weights of the arrival times were assigned manually based on the quality offirst arrival: sharp and clear or unclear. Numbers 0, 1, 2, 3, 4 (0 for best and 4 for poorarrivals) were assigned corresponding to the weighting scheme in the HYPOCENTERprogram. The accurate arrival time is important for finding the exact location of theevent. The smallest uncertainties for P- and S arrival picks for all stations are 0.02 sec,but in many cases they are up to 0.06 sec because of the presence of noise. Usually the P-arrivals (Pg or Pn) can be picked with better accuracy than the subsequent arrival (Sn, Sg,etc.) because they have to be distinguished only from the background ambient noise. Thesecondary arrivals come at the coda of the P-waves and it is much more difficult todistinguish because of the presence of P-coda and lower signal to noise ratio. Figure 3.6shows examples of the arrival time picks for P- and S-waves. On Figure 3.6a the signal tonoise ratio is large and P g and S g are clear. In cases where the signal to noise ratio issmall, picking the first arrival times is comparatively difficult (Fig. 3.6b, top left).33