P-Waveform Receiver Functions

2003

The Zagros Mountain belt of western Iran results from the collision of the Arabian and Central Iran continental blocks. The stage of the collision is unclear and the crustal structure of the Zagros is rather poorly known. In this study we investigate the velocity structure of the crust and upper mantle beneath the Ghir region located in the Central Zagros using data collected by a temporary local seismological network including a broad-band instrument. The structures of the sedimentary cover and the upper crystalline crust are estimated from the inversion of P and S traveltimes of local earthquakes recorded on a dense seismological network. The upper crust consists of an ∼11 km thick sedimentary layer (V p ∼ 4.70 km s −1) above a ∼8 km thick upper crystalline crust (V p ∼ 5.85 km s −1). The velocity of the lower crust and the depth of the Moho are found using receiver function analysis of teleseismic earthquakes. The lower crystalline crust is unusually slow (V p ∼ 6.5 km s −1) and ∼27 km thick. The upper bound for the total crustal thickness beneath the Ghir region is 46 ± 2 km. A comparison of the thickness of the crystalline crust of the Zagros with available information for the thickness of the crystalline crust of the Arabian Platform shows that, at present, the Zagros has a thinner crust. The current crustal thickness beneath the Zagros is comparable to the pre-collision crustal thickness of the Arabian Platform, suggesting that the Zagros is now in a very early stage of continental collision.

2011

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 (M b ≥5.5, epicentral distance from 30° to 95°) recorded at 12 three components short-period stations of Kermanshah, Isfahan and Yazd telemetry seismic networks. Our results obtained from P receiver functions indicate clear Ps conversions at the Moho 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.

Geophysical Journal International, 2009

We simultaneously estimated Q p −1 and Q s −1 by applying the extended coda-normalization method at 39 stations of three local networks in the East-Central Iran. We measured frequencydependent attenuation of both P and S waves for the frequency range of 1.5, 3.0, 6.0, 12 and 24 Hz. We have analysed 266 three-component seismograms of 53 local earthquakes, having focal depths less than 25 km, which occurred from 2003 December 28 to 2005 April 11. By fitting power-law frequency dependence to the estimated values over the whole stations, we obtained Q p −1 = (25 ± 3) × 10 −3 f (-0.99±0.04) and Q s −1 = (19 ± 2) × 10 −3 f (-1.02±0.06) in the upper crust of the East-Central Iran, where f is frequency in Hz. Our results are in the range of those estimated for Q p −1 and Q s −1 of the other seismically active regions.

Scientific Reports, 2019

Improving the resolution of seismic tomography by adding virtual seismometers is an ambitious aim in regions with poor instrumental coverage. In this study, inter-event empirical Green's functions (EGFs) were retrieved using cross-correlation of the vertical component of 630 earthquakes with M ≥ 4 which occurred around the collision-subduction transition zone in south Iran. To extract reliable inter-event EGFs and obtain stable tomographic results, we used about 1300 event pairs with good signal-to-noise ratio, each pair well aligned to a seismic station. Our results show that the retrieved inter-event EGFs agree well with those obtained from earthquakes in similar paths. The inverted velocity model presents two main layers including upper crust (up to ~16 km) and middle crust (deeper than ~18 km) in both sides of the Minab-Zendan-Palami transition zone. The upper crust contains two main layers: sedimentary and basement layers with thicknesses ~6 and ~10 km, respectively. Moreover, the main faults cause lateral variations in these main layers. The difference between the average velocities of the middle crust, between the collision and subduction zones, is about 0.5 km/s, delimited by faults. Also, an area with a 30 km width along these faults can be defined as the collision-subduction transition zone. Seismologists have applied many different techniques (e.g., receiver functions, ambient seismic noise, etc.) using seismic waveforms recorded by real receivers to study the Earth's interior. Traditional source-receiver observational methods in seismic tomography cannot fully exploit all information contained in a network of sparse stations because of possible ill-posed inversion problems (e.g. 1). Cells with few or no crossing raypaths lead to artifacts and smearing effects. Poor geometry of earthquakes and stations (non-uniform distribution) can happen near active faults, inside seismotectonic zones in local and/or regional scale tomography, and near plate boundaries in global scale tomography (see 2 and 3). Moreover, ambient seismic noise and teleseismic observations may not be sufficient to improve the coverage in poorly resolved cells. However, the resolution of tomography inversion in these regions can be improved using earthquakes as virtual seismometers 4. Earthquake event-pair cross-correlation (hereafter ECC) yields a trace similar to real empirical Green's function (EGF) of the inter-event path, as shown by 5. Based on source-receiver reciprocity theorem, one of the earthquakes is taken as a source, and the other one is considered a (virtual) receiver recording the waveform of the source. This earthquake interferometry approach provides inter-event EGF that is useful to study the Earth's crustal structure where it is impossible to directly record waveforms, such as in fault zone tomography (see 6 and 7). Convergence between the Arabian and Eurasian plates controls the main tectonic activity characterized by continent-continent collision in western Iran (along the Zagros Collision) and subduction in southern Iran along the Makran subduction (Fig. 1, inset map). A GPS-measured convergence rate varies from ~10 mm/y across the Main Zagros Thrust in Iran, part of the collisional system (with NNE shortening) to ~28 mm/y in the Makran subduction 8. The transition zone between the Zagros collision and Makran subduction is known as Minab-Zendan-Palami (hereafter MZP) fault zone with ~25 mm/y (see 9 and 10). This reverse and right-lateral, NNW trending fault system, offsets the topographical deformation in the Zagros and the Makran prisms 11. Many studies have been made on this transition zone, such as stress distribution (see 10,12 and 13), source parameters and/or slip distribution (e.g. 14), joint gravity and surface wave tomography 15 , and ambient seismic noise tomography (e.g. 16 and 17). However, few results about crustal structure have been published. Also, most of the

2008

A primary objective of this project is to estimate the local and regional seismic velocity structures of north and northeastern Iraq, including the northern extension of the Zagros collision zone. Furthermore, earthquake source mechanisms will be investigated once a velocity model is derived for this region. Thus far, global seismic network coverage is poor throughout the region and extrapolated velocity models found in the literature lack sufficient accuracy to permit events to be located with significant precision. Ten three-component broadband stations composing the North Iraq Seismographic Network (NISN) were installed in late 2005. At present, waveforms from 290 teleseismic events, from November 30, 2005 through March 31, 2007, have been processed for P-wave receiver functions (RFs). Based on the USGS Preliminary Determination of Epicenters (PDE) bulletins, the epicentral distances of these teleseismic events to NISN stations range from 30º to 90º while their magnitudes equal or exceed 5.5. Results obtained to date indicate a lower-than-average shear wave velocity when compared to other crustal regions of the Earth. Additionally, Moho depths appear slightly shallower below the stations located along the foothills compared to the stations at higher elevation in the Zagros mountains. The Moho below the foothills is estimated at 40-50 km depth, while it dips down to a depth of 45-55 km below the northern extension of the Zagros zone. Common among the receiver functions is the presence of a significant velocity discontinuity at a depth of 15 km and 20 km for the stations below the foothills and Zagros mountains, respectively. The increase in velocity across this discontinuity lead to the observation of mid-crustal refracted body waves throughout NISN. Evaluation of the resulting models will be performed through relocation of recorded events, synthetic waveform analysis, and correlation with available geophysical and geological information.

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1978

Abstract A detailed microearthquake survey in a small region of the Zagros Mountains in Iran failed to detect any shocks whose depths were greater than 20 km. One third of the shocks in the same area have depths greater than 50 km when located using teleseismic observations. Because of poor azimuthal coverage and lack of local stations these teleseismic locations are probably in error. There is therefore no reliable seismic evidence for the existence of oceanic lithosphere beneath the Zagros fold belt.

Journal of Seismology, 2012

For the first time, we present the variation of crust-mantle boundary beneath the northeast Iran continental collision zone which is genetically part of the Alpine-Himalayan orogeny and beneath Central Iran which is a less-deformed tectonic block. The boundary was imaged by stacking teleseismic P-S converted phases and shows a strong variation of Moho from 27.5 km under Central Iran to 55.5 km beneath the Binalud foreland basin. The thickest crust is not located beneath the high topography of the Kopeh Dagh and Binalud mountain ranges suggesting that these mountain ranges are not supported by a crustal root. The simple gravity modeling of the Bouguer anomaly supports this idea.

Geophysical Journal International, 2006

More than 2000 instrumentally recorded earthquakes occurring in the Iran region during the period 1918-2004 have been relocated and reassessed, with special attention to focal depth, using an advanced technique for 1-D earthquake location. A careful review of starting depths, association of teleseismic depth phases, and the effects of reading errors on these phases are made and, when necessary, waveforms have been examined to better constrain EHB focal depths. Uncertainties in EHB epicentres are on the order of 10-15 km in the Iran region, owing to the Earth's lateral heterogeneity and uneven station distribution. Uncertainties of reviewed EHB focal depth estimates are on the order of 10 km, as compared to about 4 km for long-period P and SH body-waveform inversions. Nevertheless, these EHB depth estimates are sufficiently accurate to resolve robust differences in focal depth distribution throughout the Iran region and, within their errors, show patterns that are in agreement with the smaller number of earthquakes whose depths have been confirmed by body-wave modelling or local seismic networks. The importance of this result is that future earthquakes with apparently anomalous depths can easily be identified, and checked, if necessary. Most earthquakes in the Iranian continental lithosphere occur in the upper crust, with the crustal shortening produced by continental collision accommodated entirely by thickening and distributed deformation. In the Zagros Mountains nearly all earthquakes are confined to the upper crust (depths <20 km), and there is no evidence for a seismically active subducted slab dipping NE beneath central Iran. By contrast, in southeastern Iran, where the Arabian seafloor is being subducted beneath the Makran coast, low-level earthquake activity occurs in the upper crust as well as to depths of at least 150 km within a northward-dipping subducting slab. Near the Oman Line, a region transitional between the Zagros and the Makran, seismicity extends to depths of up to 30-45 km in the crust, consistent with low-angle thrusting of Arabian basement beneath central Iran. In north-central Iran, along the Alborz mountain belt, seismic activity occurs primarily in the upper crust but with some infrequent events in the lower crust, particularly in the western part of the belt (the Talesh), where the South Caspian basin underthrusts NW Iran. Earthquakes that occur in a band across the central Caspian, following the Apscheron-Balkhan sill between Azerbaijan and Turkmenistan, have depths in the range 30-100 km, deepening northwards. These are thought to be connected with either incipient or remnant northeast subduction of the South Caspian basin basement beneath the east-west trending Apscheron-Balkhan sill. Curiously, in this region of genuine mantle seismicity, there is no evidence for earthquakes shallower than 30 km.

Geophysical Journal …, 2011