ALBERT

Description: Receiver functions are widely employed to detect P-to-S converted waves and are especially useful to image seismic discontinuities in the crust. In this study we used the P receiver function technique to investigate the velocity structure of the crust beneath the Northwest Zagros and Central Iran and map out the lateral variation of the Moho boundary within this area. Our dataset includes teleseismic data (Mb ≥ 5.5, epicentral distance from 30◦ to 95◦) recorded at 12 three component short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at theMoho boundary. The Moho depths were firstly estimated from the delay time of the Moho converted phase relative to the direct P wave beneath each network. Then, we used the P receiver function inversion to find the properties of the Moho discontinuity such as depth and velocity contrast. Our results obtained from PRF are in good agreement with those obtained from the P receiver function modeling. We found an average Moho depth of about 42 km beneath the Northwest Zagros increasing toward the Sanandaj-Sirjan Metamorphic Zone and reaches 51 km, where two crusts (Zagros and Central Iran) are assumed to be superposed. The Moho depth decreases toward the Urmieh-Dokhtar Cenozoic volcanic belt and reaches 43 km beneath this area. We found a relatively flat Moho beneath the Central Iran where, the average crustal thickness is about 42 km. Our P receiver function modeling revealed a shear wave velocity of 3.6 km/s in the crust of Northwest Zagros and Central Iran increasing to 4.5 km/s beneath the Moho boundary. The average shear wave velocity in the crust of UDMA as SSZ is 3.6 km/s, which reaches to 4.0 km/s while in SSZ increases to 4.3 km/s beneath the Moho.

Description: We compute P and S receiver functions to investigate the crustal and lithospheric thickness as well as the Vp/Vs ratio beneath the Northwest of Iran and map out the lateral variations of these discontinuities under this region.We selected data from teleseismic events (Mb ≥ 5.5, epicentral distance between 30°-95° for P receiver functions and Mb 〉 5.7, epicentral distance between 60°-85° for S receiver functions) recorded since 1995 to present at 8 three component short period stations from Tabriz Telemetry Network. First of all, we calculated PRFs for each station and then the Moho depth will be estimated only from the delay times of the Moho p-to-s conversion phases. Then, we used an H-Vp/Vs stacking algorithm of Zhu & Kanamori to estimate crustal thickness and Vp/Vs ratio under each station. The average Moho depth is estimated to be about ˜48 Km and varies from 38.5 to 53 km. Deeper and shallower Moho are found under the western and eastern stations, beneath SHB and SRB, respectively. The obtained average Vp/Vs ratio is 1.76, with higher ratio of 1.82 beneath the TBZ station and lower ratio of 1.73 beneath the AZR station. The crustal structure beneath these stations is also determined by the modeling of P receiver functions.We obtained a 3 layered model for the crust beneath this region for all stations. The thickness of layers are estimated to be 7-11, 18-35 and 38-53 km, respectively. The average of the crustal shear wave velocity is 3.39 km/s and it reaches 4.32 km/s below the Moho discontinuity. The crustal thicknesses derived from these data are in good agreement with the results obtained from our S receiver functions. In addition, Clear negative signals from the Lithosphere-Asthenosphere boundary (LAB) are also observed (˜8.7 s) in our S receiver functions. They map a clear LAB showing a thin continental lithospheric thickness of 84 Km and varies from 77 to 103 km. Deeper and shallower lithosphere–astenosphere boundary are found under the western and eastern parts of the study area. The P to S converted phases from 410 and 660 km upper mantle discontinuities are delayed more than 2 and 1 s with respect to the IASP91 global reference model, indicating an slower upper mantle and show higher temperature than the standard earth model. Because the 410 and 660 km discontinuities do not show the same delay, the transition zone is also considered to be thinner than that predicted by the IASP91 reference model. This could mean that the upper mantle in the region is still influenced by several geodynamical processes involving rifting, uplifting and magmatism.

Description: We computed P and S receiver functions to investigate the lithospheric structure beneath the northwest Iran and compute the Vp/Vs ratio within the crust of this seismologically active area. Our results enabled us to map the lateral variations of the Moho as well as those of the lithosphere–asthenosphere boundary (LAB) beneath this region. We selected data from teleseismic events (Mb 〉 5.5, epicentral distance between 30° and 95° for P receiver functions and Mb 〉 5.7, epicentral distance between 60° and 85° for S receiver functions) recorded from 1995 to 2008 at 8 three-component short-period stations of Tabriz Telemetry Seismic Network. Our results obtained from P receiver functions indicate clear conversions at the Moho boundary. The Moho depth was firstly estimated from the delay time of the Moho converted phase relative to the direct P wave. Then we used the H-Vp/Vs stacking algorithm of Zhu and Kanamori to estimate the crustal thickness and Vp/Vs ratio underneath the stations with clear Moho multiples. We found an average Moho depth of 48 km, which varies between 38.5 and 53 km. The Moho boundary showed a significant deepening towards east and north. This may reveal a crustal thickening towards northeast possibly due to the collision between the Central Iran and South Caspian plates. The obtained average Vp/Vs ratio was estimated to be 1.76, which varies between 1.73 and 1.82. The crustal structure was also determined by modeling of P receiver functions. We obtained a three-layered model for the crust beneath this area. The thickness of the layers is estimated to be 6–11, 18–35, and 38–53 km, respectively. The average of the shear wave velocity was calculated to be 3.4 km/s in the crust and reaches 4.3 km/s below the Moho discontinuity. The crustal thickness values obtained from P receiver functions are in good agreement with those derived by S receiver functions. In addition, clear conversions with negative polarity were observed at ~8.7 s in S receiver functions, which could be related to the conversion at the LAB. This may show a relatively thin continental lithosphere of about 85 km implying that the lithosphere was influenced by various geodynamical reworking processes in the past.

Description: The Kopeh-Dagh fold belts are among the most seismically active areas in Northeastern Iran,which build the northern part of the Alpine–Himalayan orogen in western Asia. They act as the abrupt northeastern limit to active deformation in Iran. We perform a combined P and S receiver function analysis to detect the major discontinuities within the lithosphere beneath Northeast Iran. Our results obtained from 12 short period and broadband seismological stations significantly map the lateral variations of the Moho boundary. Based on P receiver functions, we show that the Moho depth varies from∼43 km beneath the southern Kopeh-Dagh foreland basin to ∼49 km below the northern part of the basin. S receiver functions reliably reveal an averageMoho depth of ∼50–55 km beneath the Kopeh-Dagh mountain range showing the regional shortening in response to the collision of Arabia with Eurasia. Furthermore, we observe clear conversions with negative polarity at ∼8.5–9.5 s in S receiver functions, which could be related to the conversion at the lithosphere–asthenosphere boundary. This may show a relatively thin continental lithosphere of about 85–95 km beneath the Kopeh-Dagh implying that the lithosphere was influenced by geodynamical reworking processes in the past.