How to read the reflection phases of Moho

PmP and SmS are secondary phases that are reflected from Moho discontinuity. Moho reflected phases contain information from the entire crust, thus their travel time and waveforms contain significant information about crustal structure. We used seismogram data associated to earthquakes with magnitude larger than 3.0 and focal depth shallower than 35 km occurred during 1996-2017 in NW Iran. The seismographs of each earthquake are investigated separately. After removing mean and trend, we examined appropriate band pass filters. In this study, the response of sensor in our interest frequency (1-10HZ) is similar; thus, we did not remove instrument response.   Particle motion of PmP reflected phase is similar to P direct phase as compressional waves and particle motion of SmS and S phases show that these phases get to stations as shear waves. If the travel time difference of picked Moho reflected and direct phases is less than 0.75 sec, picked reflected phase is reliable. We found that Butterworth band pass filter with corner frequencies of 0.5 and 4 HZ is appropriate for providing clear PmP reflected phases and the corner frequencies of 0.2 and 4 HZ are appropriate for SmS reflected phases. In general, the amplitude of the reflection phase could be shorter than the direct one due to large attenuation along the long ray path and missing some energy at the reflection boundary, but in some samples, amplitude of Moho reflected phase is larger than the direct phase, due to increment of Q values in lower crust with respect to upper crust and source of the earthquake. In this research, we often investigated the cases that amplitude of the reflection phases is larger than direct ones. We picked P and PmP from vertical component and considered the transverse and radial components for picking SmS reflected phase. S phase is read from the component where the SmS phase is more clear and noticeable. However, we can read S phase from each of the two horizontal components but in general, to avoid each time delays likely generated by anisotropy, it was read from the identical components used for picking SmS phase. Consistent with our research, the slope of the curve of difference between reflected and direct phases increases with focal depth and epicenteral distance, but for focal depth greater than Conrad discontinuity and epicenteral distance more than 80 km, the rate of increment of slope increases. Regarding this, a total of 216 PmP, 310 first P, 254 S and 161 SmS phases were picked from seismograms in the local earthquakes of NW Iran.