Variations of the Moho depth and Vp/Vs ratio beneath East Iran (Birjand) using P receiver function method
Characterization of the detailed structure of the crust and upper mantel is an importantcontinuing goal of geophysical studies. There are a variety of geophysical methods(seismic refraction, seismic vertical reflection and seismic tomography) to investigatesubsurfaces. The teleseismic P Receiver Function (RF) method has become a populartechnique to constrain crustal and upper mantle velocity under a seismic station.Teleseismic body waveforms recorded at a 3-component (Z, N-S, E-W) seismic stationcontain a wealth of information on the earthquake source, the earth structure in thevicinity of both the source and receiver, and mantle propagation effects. The resulting RFis obtained by removing the effects of the source and mantle path. The basic aspect of thismethod is that a small percentage of the incident P wave energy from teleseismic eventswith significant and relatively sharp velocity discontinuities in the crust and upper mantlewill be converted to S wave (Ps), and arrive at the station within the P wave coda directlyafter the direct P wave. To obtain P-RF, the following steps are generally used: to utilizedata recorded with different types of seismometers, the instrument responses have to bedeconvolved from the original records. ZNE components are then rotated into the localLQT ray-based coordinate system (using the theoretical back azimuth and incidenceangle).To eliminate the influence of the source and ray path, an equalization procedure isapplied by deconvolving the Q component seismogram with the P signal on the Lcomponent. The resulting Q component data are named P-RF. An advantages of the RFmethod is that, because the P-to-S conversion point is close to the station (usually within10 km laterally), the estimation is less affected by lateral velocity variations. Theestimation provides a good point measurement at the station because of the steepincidence angle of the teleseismic P wave. Since the direct P arrival is used as a referencetime, it can be shown that the result is not sensitive to crustal P velocity. We compute P receiver functions to investigate the crustal thickness and Vp/Vs ratiobeneath the East of Iran (Birjand) and map out the lateral variation of Moho depth underthis region. We selected data from teleseismic events (Mb ≥ 5.5, 30˚<Δ<95˚), recordedfrom 2005 to 2009 at 4 three-component short period stations from Birjand SeismicTelemetry Network. These stations are equipped with SS-1 seismometers with a naturalfrequency of 1 HZ. The data is recorded at 50-samples-per-second. First of all, iscalculated 247 P-RFs for TEG, KOO and DAH stations and then estimated the Mohodepth solely from the delay time of the Moho P-to-S conversion phases. Then, we used an H-Vp/Vs stacking algorithm to estimate crustal thickness and the Vp/Vs ratio under each station. The best value for the H and Vp/Vs ratio are found when the three phases (Ps and crustal multiples) are stacked coherently. The results obtained from the P receiverfunctions indicate clear conversions at the Moho boundary. A notable feature, which can be observed underneath all stations, is the presence of a significant sedimentary layer at about 0.7-1s delay time. The middle crustal layer at about 1.9-3.3s delay time can also be seen beneath all stations. The most coherent conversion, however, is the conversion at the Moho boundary arriving between 4.7-5.4s delay time. As a result of measurements using the Zhu and Kanamori (2000) method, the average Moho depth is found to be approximately 41 km and to vary from 38.5 to 44 km. The crust is relatively thin beneath the DAH station, whereas the thickest crust was observed beneath the KOO station, located southwest of the study area. The crust of Eastern Iran has an average Vp/Vs ratio of 1.76, with a higher ratio of 1.84 in the TEG station and lower ratio of 1.76 in the KOO station.
Iranian Journal of Geophysics, Volume:5 Issue:1, 2011
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