نوع مقاله : مقاله تحقیقی (پژوهشی)
نویسندگان
1 موسسه ژئوفیزیک دانششگاه تهران
2 استادیار گروه آموزشی زلزلهشناسی رئیس مرکز لرزهنگاری کشوری
3 دانشیار گروه آموزشی زلزلهشناسی
چکیده
کلیدواژهها
موضوعات
عنوان مقاله [English]
نویسندگان [English]
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, thereafter removing mean and trend we examined appropriate band pass filters. In this study response of sensor in the our interest frequency(1-10HZ) is similar, thus we did not remove instrument response
Moreover, particle motion of PmP reflected phase is similar to P direct phase as compressional waves, particle motion of SmS and S phases show that these phase get to stations as shear waves. Thereafter picking Moho reflected and direct phases, if their travel time difference are less than 0.75 sec, so reflected picking phases are reliable. We found that Butterworth band pass filter with corner frequency 0.5 and 4 HZ is appropriate for providing clear PmP reflected phases and corner frequency with 0.2 and 4 HZ for SmS reflected phases. In general, the amplitudes of the reflection phases could be shorter than direct wave 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 direct phase, due to increment of Q values in lower crust relative to upper crust and source of earthquake. Therefore, to diagnose these phases, we often investigated, the cases that amplitude of the reflection phases will be larger relative to direct wave. We picked P and PmP from vertical component and considered the transverse and radial components for picking SmS reflected phase then read S phase 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 reading it from the identical components that used for picking SmS phase to avoid each time delays likely generated by anisotropy. Consistent with our research, curve dip of difference between reflected and direct phase 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 curve increases. In this base, 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.
کلیدواژهها [English]
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