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 important continuing goal of geophysical studies. There are a variety of geophysical methods (seismic refraction, seismic vertical reflection and seismic tomography) to investigate subsurfaces. The teleseismic P Receiver Function (RF) method has become a popular technique 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 station contain a wealth of information on the earthquake source, the earth structure in the vicinity of both the source and receiver, and mantle propagation effects. The resulting RF is obtained by removing the effects of the source and mantle path. The basic aspect of this method is that a small percentage of the incident P wave energy from teleseismic events with significant and relatively sharp velocity discontinuities in the crust and upper mantle will be converted to S wave (Ps), and arrive at the station within the P wave coda directly after the direct P wave. To obtain P-RF, the following steps are generally used: to utilize data recorded with different types of seismometers, the instrument responses have to be deconvolved from the original records. ZNE components are then rotated into the local LQT ray-based coordinate system (using the theoretical back azimuth and incidence angle).To eliminate the influence of the source and ray path, an equalization procedure is applied by deconvolving the Q component seismogram with the P signal on the L component. The resulting Q component data are named P-RF. An advantages of the RF method is that, because the P-to-S conversion point is close to the station (usually within 10 km laterally), the estimation is less affected by lateral velocity variations. The estimation provides a good point measurement at the station because of the steep incidence angle of the teleseismic P wave. Since the direct P arrival is used as a reference time, 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 ratio beneath the East of Iran (Birjand) and map out the lateral variation of Moho depth under this region. We selected data from teleseismic events (Mb ≥ 5.5, 30˚<Δ<95˚), recorded from 2005 to 2009 at 4 three-component short period stations from Birjand Seismic Telemetry Network. These stations are equipped with SS-1 seismometers with a natural frequency of 1 HZ. The data is recorded at 50-samples-per-second. First of all, is calculated 247 P-RFs for TEG, KOO and DAH stations and then estimated the Moho depth 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 receiver functions 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.