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We produced a globally distributed catalog of earthquakes and nuclear explosions with calibrated hypocenters, referred to as the Global Catalog of Calibrated Earthquake Locations (GCCEL). This dataset currently contains 18,782 events in 289 clusters with >3.2 million arrival times observed at 19,258 stations. The term “calibrated” refers to the property that the hypocenters are minimally biased by unknown Earth structure. In addition, we calculate uncertainties using empirically determined variability of the arrival-time data itself, specific to each calibrated cluster of hypocenters. Outliers in the arrival-time dataset are removed based on measured variability of the data. In each cluster, we estimate the empirically determined uncertainty for each set of station-phase arrival times. We use a version of the hypocentroidal decomposition multiple event relocation algorithm specifically adapted for calibrated relocations of clusters of seismic events. Most clusters are calibrated by fitting the subset of direct crustal first arrivals (Pg and Sg) with a locally appropriate travel-time model to estimate the cluster hypocentroid. A few clusters are calibrated by aligning the pattern of relative locations in space and time with one or more events for which a ground-truth hypocenter is available from an independent source with known uncertainty, such as a nuclear explosion. Epicentral uncertainties in GCCEL typically range from 1 to 5 km with a 90% confidence interval. Most events have depth constraint from one or more sources, usually with an uncertainty of ≤5 km. GCCEL is a significant resource for research at local, regional, and global scales because it provides minimally biased absolute hypocenters, meaningful associated error estimates, and curated arrival times as a reference dataset that can be used as prior constraints in the development of new regional, national, and global earthquake catalogs; validation of new location techniques; and the generation of advanced Earth models.

The Makran subduction zone is unique in its wide onshore thick accretionary wedge, and a volcanic arc not parallel to the E-W trend of the Makran accretionary prism. To investigate the internal structure of the onshore accretionary wedge, the trend of the buried trench, and the crustal nature of fore-arc Jaz Murian Depression, we have calculated a 3D shear-wave velocity model for a region around the border between eastern and western Makran using ambient noise adjoint tomography and data from IASBS/CAM Makran temporary seismic network. Our 3D shear-wave velocity model clearly shows that the onshore accretionary prism consists of two southern low and northern high velocity zones with an average sedimentary cover of 22 and 30 km, respectively. The young age of the surface rocks of the high velocity part of the prism suggests the presence of a significant volume of igneous rocks scraped from the subducting oceanic slab. The 3D model indicates a continental crust of ~40 km with a thick sedimentary cover of ~20 km for the eastern part of Jaz Murian Depression. The presence of a NE-SE trending low velocity region at a depth interval of 40-60 km subparallel with the trend of the volcanic arc, intermediate-depth earthquakes, and geometry of the overriding plate might be related to the trend of the buried trench. This implies that the observed NE-SE trending volcanic arc might be related to the geometry of the buried trench and not the eastward reduction of the subduction angle.

SUMMARYTo estimate the maximum possible size of megathrust earthquakes, we calculate the thermal structure along two profiles in west and east Makran subduction zone by solving the steady-state 2-D energy equation. For the western profile, we derive the slab geometry from a recent receiver function study along IASBS (Institute for Advanced Studies in Basic Sciences) seismic profile in the onshore part of the Iranian Makran. For the eastern profile, the slab geometry is derived from a recent relocation of seismicity of Makran. Using the improved slab geometry and a force balance establishment in the accretionary wedge, the effective coefficient of friction, $\mu ^{\prime}$, is assumed to be equal to 0.03. We estimate the updip and downdip of the megathrust zone by simultaneously considering the seismicity related to the events with thrust and normal mechanisms and intersection between 100–150 and 350–450 °C isotherms and the subducting slab interface. Along the western profile, the megathrust updip locates ∼95 km north of the deformation front (DF) at the depth of ∼20 km and the downdip locates ∼300 km north of the DF at the depth of ∼35 km. Presence of normal mechanism events at deeper depths indicates that the downdip limit of the megathrust zone is consistent with the 350 °C isotherm. The megathrust width is ∼205 km along the western profile. Along the eastern profile, the megathrust updip locates ∼60 km north of the DF at the depth of ∼15 km and the downdip locates ∼280 km north of the DF at the depth of ∼35 km. The downdip limit of the megathrust zone is closely related to the 350 °C isotherm. The megathrust width is ∼220 km along the eastern profile. Assuming a segmentation of the thrust zone into the western and eastern parts, the areal size of the megathrust zones in west and east Makran is ∼82 000 and 88 000 km2, respectively. We estimate the magnitude of the largest possible megathrust earthquakes in the west and east Makran to be 8.65 ± 0.26 and 8.75 ± 0.26 Mw, respectively.

SUMMARYThe study of the dynamic triggering of earthquakes and tremors during large earthquakes at faraway distances is an active area of research. This type of remote dynamic triggering is often found in subduction zones. The Iranian plateau is part of the Alpine–Himalayan orogenic system and hosts different collision styles of deformation and significant strike-slip faults. Using 13 yr (26 December 2004–8 September 2017) of continuous data of Iranian National Seismic Network (INSN) and some dense temporary networks, for the first time we carried a systematic study of dynamic triggering in Iran during 47 recent large earthquakes with magnitude and depth ranges of 6.4–9.1 and 8–90 km, respectively. We explored the local catalogue of 124 805 events with a magnitude of completeness (Mc) of 1.8 for the study of dynamic triggering but did not find any convincing evidence of dynamic triggering from the catalogue. The waveform data of 24 hr duration around the main events were analysed to find possible dynamic triggering through manual analysis of the waveform, STA/LTA, and beta statistics and found the triggering. We found dynamic triggering in Iran during Sumatra, 26 December 2004, Mw 9.1; Tohoku-Oki, 11 March 2011, Mw 9.1; Indian Ocean, 11 April 2012, Mw8.6 and Baluchistan, 24 September 2013 earthquakes and also possible triggering during Sumatra, 12 September 2007, Mw8.5. Only ∼10 per cent of the analysed earthquakes produced dynamic triggering. The triggering initiates during the passage of high amplitude Love waves and continues through the passage of the Rayleigh waves. We found north, central and eastern regions are more probable for triggering than Zagros and Makran regions. The instances of triggering were not restricted to only a small region, but instead, occurred at multiple locations. We find the onset of tremor correlates with very small stress changes, on the order of 1 kPA. However, the amplitude of the dynamic stresses is not a sufficient condition since some of the areas with considerably larger dynamic stresses are not triggered any seismicity in the region. The backazimuth angle of ∼50° and ∼120° seems to play an important role in the triggering. Teleseismic waves most probable for triggering local earthquakes within NW and central Iran include incoming surface waves with an incident angle of ∼60°–90° with respect to the local fault fabric.

SUMMARYWe present the result of a 3-D Pg tomography in NW Iran to better understand the relationship between seismicity and velocity structure within the young continental collision system. In this regard, we have collected 559 07 Pg traveltime readings from 3963 well located earthquakes recorded by 353 seismic stations including 121 stations from four new temporary seismic networks. The most prominent feature of our Pg velocity model is a high correlation between the location of majority of large magnitude events and the location of low velocity regions within the seismogenic layer. The large instrumental and historical earthquakes with some limited exceptions tends to happen close to the borders of the low velocity regions. The Lorestan arc of Zagros has the thickest (∼20 km) low velocity region and Central Iran has the thinnest (less than 10 km) low velocity region where little seismicity is observed. Despite the relative increase of thickness of low velocity region in the uppermost part of the upper crust of Alborz, the average Pg velocity of the upper crust increases from Central Iran towards Alborz and reaches to its climax in the northern hills of Alborz, where the catastrophic Rudbar-Tarom 1990 event happened. The Pg velocity map shows presence of a low angle basement ramp in the Lorestan arc at the depth range of ∼10–20 km. The large low angle thrust Ezgele-Sarpolzahab 2017 earthquake and medium size high angle thrust events happened at the base and updip part of the velocity ramp, respectively. The calculated Pg velocity map shows low velocity regions at depths deeper than 11 and 20 km beneath the Sahand and Sabalan volcanoes, respectively.

Knowing the orientation of horizontal components of seismic sensors is important for many seismological applications such as waveform modeling, receiver function analysis, and shear-wave splitting. We determined the sensor orientations for broadband seismic stations belonging to the Iranian National Seismic Network (INSN) and the Iranian Seismological Center (IRSC) to enable such studies. For both networks, we have catalogs of event-based seismic waveforms of Iranian earthquakes. Sensor orientations were found by P-wave energy minimization on the transverse component and validated by long-period waveform modeling of events with well-constrained source parameters. We obtained stable sensor orientations for 28 (of 29) INSN sites and for 66 (of 92) IRSC sites. About 75% and 59% of all INSN and IRSC orientation estimates, respectively, are oriented within 15° of true north leaving many sites with largely misoriented sensors. We found temporally changing sensor orientations for 36 (of 121) sites.

The central Tethys realm including Anatolia, Caucasus and Iran is one of the mostcomplex geodynamic settings within the Alpine-Himalayan belt. To investigate thetectonics of this region, we estimate the depth to magnetic basement (DMB) as aproxy for the shape of sedimentary basins, and average crustal magneticsusceptibility (ACMS) by applying the fractal spectral method to aeromagnetic data.Magnetic data is sensitive to the presence of iron-rich minerals in oceanic fragmentsand mafic intrusions hidden beneath sedimentary sequences or overprinted byyounger tectono-magmatic events. Furthermore, a seismically constrained 2Ddensity-susceptibility model along Zagros is developed to study the depth extent ofthe tectonic structure.Comparison of DMB and ACMS demonstrates that the structural complexityincreases from the Iranian plateau into Anatolia.Strong ACMS show lineaments coincides with known occurrences of Magmatic-Ophiolite Arcs (MOA) and weak ACMS zones coincide with known sedimentarybasins in the study region, including Zagros. Based on strong ACMS anomalies, weidentify hitherto unknown MOAs below the sedimentary cover in eastern Iran and inthe SE part of Urima-Dokhtar Magmatic Arc (UDMA). Our results allow forestimation of the dip of the related paleo-subduction zones. Known magmatic arcs(Pontides and Urima-Dokhtar) have high-intensity heterogeneous ACMS. Weidentify a 450 km-long buried (DMB >6 km) magmatic arc or trapped oceanic crustalong the western margin of the Kirşehır massif in Anatolia from a strong ACMSanomaly. We identify large, partially buried magmatic bodies in the Caucasus LIP atthe Transcaucasus and Lesser Caucasus and in NW Iran. Strong ACMS anomaliescoincides with tectonic boundaries and major faults within the Iranian plateau whilethe ACMA signal is generally weak in Anatolia. The Cyprus subduction zone has astrong magnetic signature which extends ca. 500 km into the Arabian plate to thesouth of the Bitlis suture.We derive a 2D crustal-scale density-susceptibility model of the NW Iranian plateaualong a 500 km long seismic profile across major tectonic provinces of Iran from theArabian plate to the South Caspian Basin (SCB). A seismic P-wave receiver functionsection is used to constrain major crustal boundaries in the density model. Wedemonstrate that the Main Zagros Reverse Fault (MZRF), between the Arabian andthe overriding Central Iran crust, dips at ~13° angle to the NE and extends to a depthof ~40 km. The trace of MZRF suggests ~150 km underthrusting of the Arabian platebeneath Central Iran. We identify a new crustal-scale suture beneath the Taromvalley separating the South Caspian Basin crust from Central Iran. High density lowercrust beneath Alborz and Zagros may be related to partial eclogitization of crustalroots at depths deeper than ~40 km.

SUMMARY We inverted 3555 regional and teleseismic P-wave relative time residuals to resolve deep velocity structure beneath the NW part of the Zagros collision zone. The data were gathered by 46 seismic stations installed along a ∼520-km-long seismic profile crossing the Zagros Mountains, Central Iran and the western Alborz Mountains. The obtained tomogram reveals a high velocity lithospheric root beneath the Zagros Mountains and a low velocity wedge in the frontal edge of the Arabian Plate beneath the suture that might be interpreted as beginning of delamination of lower part of the Arabian mantle lithosphere from its upper part. A significant deep (depth >350 km) high velocity feature is observed in the lower part of the upper mantle to the north of the Zagros suture and beneath Central Iran. We interpret this feature as the remains of oceanic slab of the Neotethys lying in the lower portion of the upper mantle and the transition zone.

The 12 November 2017 Mw 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic.

International audience

The Kashan-Ardestan sedimentary basin in Central Iran was initially formed by back-arc extension due to the subduction of Neo-Tethys oceanic lithosphere beneath the Iranian Plate during Eocene time. Following rifting and the onset of the Arabian-Central Iranian continental collision in the Oligocene, the basin was infilled by a sequence of continental clastic and evaporitic sediments referred to as the Lower Red Formation. Post-rift cooling and thermal subsidence led to the development of a shallow marine environment for the accumulation of Qom Formation carbonates and shales in the late Oligocene–early Miocene. The Qom Formation is the most significant hydrocarbon target in Central Iran, containing both source and reservoir rocks. The continental collision triggered the reactivation of pre-existing normal and strike-slip fault systems. The basin was subjected to compressional tectonism during the deposition of the Miocene Upper Red Formation and overlying Plio-Quaternary sediments. This long-lasting and multi-episodic tectono-sedimentary evolution of the Kashan-Ardestan Basin has led to the formation of a complex structural style, which must be resolved before petroleum system modeling and drilling of prospects can take place.In this study, several transverse and longitudinal 2D seismic lines were converted to depth and interpreted to define the deep-seated geometry of structures in the basin. The seismic lines were tied to the data from three exploration wells, reaching depths of ~ 4 km. In addition, ~ 15000 gravity and magnetic stations, covering the entire Kashan-Ardestan Basin, were integrated into our model.The results of our study indicate that two major strike-slip fault systems, including the Qom-Zefreh and Ardestan faults in the south and the Gazu fault zone in the north, control the geometry and evolution of the Kashan-Ardestan Basin. In this basin, the rheological profiles of the sedimentary sequences control the folding style and deformation mechanisms. Both basement-involved and thin-skinned faults developed in the basin and formed different types of fault-related anticlines. The reactivation of pre-existing strike-slip faults has produced positive flower structures during compression. There is some evidence that the Navab Anticline in the SW developed as a forced fold, with basement involvement. In addition, several thin-skinned detachment folds are observed above the evaporites of the Lower Red Formation at the base of the sedimentary cover. The Lower Red Formation thins and pinches out toward the eastern limit of the basin, where the Qom carbonates directly overly the Eocene volcanic basement. Meanwhile, the Upper Red Formation thins toward the north and northeastern limits of the basin, and towards the crests of anticlines. These syntectonic thickness variations allow us to define the geometric evolution of the Kashan-Ardestan Basin through geologic times, allowing for the burial history of the source rock and timing of trap formation at the reservoir level to be described.

The crustal structure of the Iranian Plateau bears important information about the details of the tectono-magmatic processes associated with the Neo-Tethys subduction and subsequent Arabia-Eurasia collision. Using a newly developed method of joint inversion of multi-frequency waveforms around and horizontal-to-vertical (H/V) ratios of the direct P arrivals in teleseismic P-wave receiver functions, we construct the shear-wave velocity image of the shallow crust (from surface up to 10-km depth below sea level) along a dense seismic array across the Zagros suture in the northwest Iranian Plateau. The most striking structural feature of the study region is the presence of low- and high-velocity anomalies (LVAs and HVAs) beneath the Zagros fold-and-thrust belt and the Iranian continent, respectively, indicating strong structural differences on the two sides of the suture. Systematic analysis on the velocity estimates and comparison with laboratory measurements and regional geology suggest that the LVAs and HVAs are representatives of Zagros sedimentary rocks and arc to intraplate magmatic rocks, respectively. The LVAs (1.3-2.0 km/s) are characterized by a series of faulted anti-form structures at ~1-7 km depths beneath Zagros. They are likely dominantly composed of shales and mudstones, and could have acted as mechanically weaknesses to accommodate different deformations of surroundings and give rise to the present-day depth-dependent seismicity. The HVAs beneath the central domain and Alborz in the Iranian continent present large ranges in both velocity (3.2-3.9 km/s) and depth (0-10 km), probably suggesting strong lithological variations in these areas. Most of the HVAs above 5-km depth have shear-wave velocities of 3.2 to 3.6 km/s, comparable to those of andesites and basalts dominated in the northwestern Iranian plateau. The deeper HVAs (below 5-km depth), which generally have greater velocities ~3.6-3.9 km/s falling into the velocity range of intrusive rocks such as granodiorites, diorites and diabases, appear to have much larger volumes at depth than that exposed on the surface in the study region. Moreover, the surface projections of the HVAs are spatially coincident with the major faults or tectonic boundaries of the region, suggesting a causal link. Our observations provide evidence for not only the lithology-controlled layering in both sedimentary structure and deformation in the Zagros passive margin but also the much more substantial magma generation and emplacement at depth than faulting-facilitated eruption and exposure on the surface in the Iranian active margin during the subduction and collision processes.

SUMMARY We present the results of a shear wave splitting analysis performed on the teleseismic SK(K)S and direct S wave recordings of 68 temporary broad-band stations to investigate the mantle deformation on the northern side of the Arabia–Eurasia collision zone in NW Iran. We used the Reference Station Technique to overcome potential contamination from the source-side anisotropy on the direct S wave signals. This method enabled us to expand our splitting measurement database beyond the usual SK(K)S phases. The average splitting delay time over the entire region was found to be 1.14 ± 0.42 s for the SK(K)S wave and 1.36 ± 0.26 s for the direct S wave. In most parts of the study area, the fast polarization directions for both shear phases are consistent and show a uniform NE–SW direction with an average of 36° and 37° for SK(K)S and S wave-derived results, respectively. This direction is in close agreement with the direction of the absolute plate motion vector in NW Iran (N39°E). The fast directions are associated with neither the surface geological trends, nor the geodetic strain fields. We propose that the observed anisotropy is mainly controlled by the LPO fabric developed due to the shearing of the asthenospheric layer in response to the motion of the lithosphere relative to the deeper mantle. Only in a narrow region near the tectonic boundaries of central Iran with NW Iran and the Alborz, NW–SE oriented SK(K)S fast directions tend to align with the major geological structures. Fast directions obtained from direct S wave indicate significantly smoother variations in the same regions and mostly continue to be aligned in the NE–SW direction. We attribute these differences to the change in the structure of the lithosphere in the tectonic boundary zone. The western margins of central Iran possess a strong deformational fabric as evidenced by the major active strike-slip zones there. Considering that the depth extent of this fabric expands over a relatively narrow zone in the mantle, it can locally influence the SK(K)S phases. The direct S waves, on the other hand, have a larger footprint and therefore average over a larger region, and relative to the SK(K)S phases, are influenced more strongly by the asthenospheric fabric due to their larger angles of incidence, which results in a larger zone of influence for station average anisotropy parameters.

On August in 2012 a Mw6.4 earthquake hit the region near the town Ahar in NW Iran. With only 11 minutes delay it was followed by another large and close by Mw6.3 earthquake. The 2012 Ahar earthquakes have been unexpected in their large magnitudes and the activated faults are poorly studied. A mapped east-west striking surface rupture is attributed to the first earthquake, which shows a strike-slip mechanism. The second earthquake is reported to have a thrust mechanism and a deeper hypocenter, but is much more poorly constrained than the first earthquake. The short time interval between those two earthquakes made it impossible to distinguish their effects in the available static surface displacement data based on InSAR and GNSS, and difficult in global seismological records. Any source analysis using static displacement data and/or teleseismic waveforms therefore has to rely on the corresponding cumulative surface displacements and recorded waveforms of the first earthquake, respectively. In contrast, in regional waveform data, the seismic phase arrivals of both earthquakes are well separated in time. To tackle the coupling of the earthquakes we conducted a combined-data study that solves for the individual sources of the earthquake doublet simultaneously in a non-linear probabilistic finite-fault optimisation. In our combined-data study we improve the constraints on the doublet sources, particularly the second earthquake. We use InSAR data from RADARSAT-2 acquisitions and published co-seismic displacement vectors based on GNSS data. For the InSAR data, unfortunately, only measurements of an ascending orbit are available. The seismological data are teleseismic (distance larger than 1000 km) and regional waveform recordings (distances less than 1000 km). For the modelling we use Green’s functions of a layered regional velocity model and rectangular, constant-slip rupture models.Our non-linear, finite-fault optimisation makes use of Bayesian bootstrap data weighting, which enables a very efficient estimation of model parameter uncertainties. This method accounts for modelling and data errors and can sample non-Gaussian posterior probabilities of model parameters. Our results show that the two earthquakes activated two different faults. The first earthquake ruptured a shallow east-west striking dextral fault extending from the surface vertically down to approximately 8 km depth (6 to 14 km confidence). The second earthquake ruptured a north to north-east striking fault with a dip of about 40 degree with an oblique rupture mechanism. The fault activated by the second earthquake seems to be located below the first one, at levels deeper than 9 km and a bit shifted to the west.We verify our results with model-independent seismic multi-array backprojection of the radiated seismic energy.We used the python-based software toolbox Pyrocko for the data processing. The included module Grond implements the Bayesian bootstrap optimisation approach. Both are open-source under the GNU General Public License and available on pyrocko.org. The RADARSAT data used in this study have been provided through the RADARSAT-2 SOAR-EU loan agreement #16736. This research is further supported by the German Research Foundation DFG through an Emmy-Noether-Grant.

The present tectonics of Iran has resulted from the continental convergence of the Arabian and Eurasian plates. Our study area, in NW Iran comprises a part of this collision zone and consists of an assemblage of distinct lithospheric blocks including the central Iranian Plateau, the South Caspian Basin, and the Talesh western Alborz Mountains. A proper knowledge of mantle flow field is required to bettwer constrain mantle kinematics in relation to the dynamics of continental deformation in NW Iran. To achieve this aim, we examined splitting of teleseismic shear waves (e.g. SKS and S) arriving with steep arrival angles beneath the receiver, which provide excellent lateral resolution in the upper mantle. We used data from 68 temporary broadband stations with varying operation periods (4 to 31 months) along 3 linear profiles. We perfomed splitting analyses on SK(K)S and direct S waves. Resultant splitting parameters obtained from both shear phases exhibit broad similarities. Relatively large time delays observed for direct S-waves, however, are anticipated since these waves travel longer than SKS along a non-vertical propagation path in an anisotropic layer. Overall, the fast polarization directions (FPDs) in the Alborz, Talesh, Tarom Mountain and in NW Iran indicate a strong consistency with NE-SW anisotropic orientations. Besides, we observe a good accordance between S and SKS results. A comparison of splitting parameters with the absolute plate motion (APM) vector and structural trends in Iran and eastern Turkey suggests asthenospheric flow field as the dominant source for observed seismic anisotropy. The lithospheric layer beneath these regions is relatively thin (compared to the adjacent Zagros region), explaining why it appears to only make a partial contribution to the observed anisotropy. The stations located in central Iran just southwest of the Alborz yield angular deviations from the general NE-SW trend as this may be explained by changing style of deformation across the different tectonic blocks. These stations indicate significant misfit between SK(K)S and direct S-waves that could be caused by local heterogeneities developed due to a diffuse boundary from the flow organization in the upper mantle of central Iran. Another possibility for large differences between two types of waves might be reflect the anisotropic structure of a remnant slab segment or a foundered lithospheric root beneath central Iran with a volume small enough to be detected by SKS phases, but not by the direct S waves.

SUMMARY We present new SKS splitting measurements obtained from a temporary seismic broad-band network in western Iran across the Arabia–Eurasia collision zone. The average delay time over the entire network was found to be 1.27 ± 0.27 s. In the Zagros where the lithosphere attains its greatest thickness, the fast-axes are predominantly subparallel to the trend of the mountain ranges, suggesting a lithospheric origin of the anisotropy caused by transpressional deformation. In contrast in the Alborz, the fast-axes become subparallel to the absolute plate motion vectors and we propose that anisotropy is mainly controlled by the direction of the asthenospheric flow field. The central Iran region shows a more complex pattern of anisotropy that could be the result of variable and small-scale deformation fields in the crust and the shallow sublithospheric mantle.

Tertiary magmatic activities in Iran are the main magmatic events responsible for porphyry copper deposit occurrences. The Urumieh-Dokhtar magmatic belt (UDMB), the Ahar-Arasbaran, the Taleghan-Tarom, the Kavir-Sabzevar zones, and the Lut Block contain the greatest volume of the Tertiary magmatic rocks in Iran. Many porphyry copper deposit indications have been found, especially in the south-east and north-west of the UDMB in the Kerman and the Ahar-Arasbaran zones, respectively. New porphyry copper mineralisation found in central parts of the UDMB and the East Iran magmatic belt (the Lut Block), in Tertiary rocks, have made these areas more interesting for porphyry copper prospecting, where before that they were not noteworthy. In this paper, we used the combination of upward continuation and edge enhancement filters to find and predict the pattern of distribution of porphyry copper deposits in the Tertiary magmatic belts. The results show the high correlation between magnetic lithologies/structures and porphyry copper deposits at a regional scale and new attractive areas have been proposed for prospecting based on deep geophysical features.

Regional-scale magnetic and gravity data have been enhanced to identify major structures and distinguish geophysical blocks in order to evaluate the distribution of major mineral deposits and prospects in Central Iran. Major structures and coherent magnetic blocks have been defined from magnetic data enhanced using a combination of upward continuation and edge enhancement filters. Broad-scale regional structural architecture and coherent density blocks have been defined from gravity data enhanced using the tilt angle. Crustal thickness was calculated from Bouguer gravity data. Integration of the structures, magnetic, density and depth characteristics derived from the complementary geophysical techniques have been used to correlate and interpret the regional distribution of mineral deposits and occurrences to assist future exploration. Porphyry Cu deposits in the Kerman metallogenic zone are shown to be closely associated with major structures in high magnetic and gravity blocks that are coincident with the Tertiary volcanic units, and in areas where the crustal zone was interpreted to be more than 50 km thick. Pb–Zn deposits in Central Iranian Microcontinent are shown to be spatially associated with major structures, predominantly outside of the high magnetic response blocks, and commonly overlay the low gravity and thick crustal zones. In the north of the study area, one group of Fe deposits display a close correlation to blocks with both a high magnetic and high gravity response. Along strike to the southeast, a second group cluster is seen in an area with a moderate magnetic and a low gravity response.

Meteorological investigations using the global positioning system (GPS) are based on expensive permanent networks and they are not developed globally on the Earth. In this study it is confirmed that single station GPS meteorology is feasible where there is no possibility for development of a sophisticated dense GPS network. Since 1 January 2011 a GPS station has been installed in the Institute for Advanced Studies in Basic Sciences in the province of Zanjan, Iran, where upper air meteorological data are not available. The GPS data were processed in order to estimate the zenith total delay (ZTD) of GPS signals due to the troposphere. The estimated ZTD was then transformed to precipitable water vapour (PWV) using the ERA-Interim globally available humidity and temperature vertical profiles. Three kinds of validation were applied to the estimated PWV and all of them reasonably proved the validity of the GPS results: (1) the measured surface water vapour pressure and dew point temperature show 87.8 and 86.6% correlation respectively with the estimated PWV; (2) the PWV measured using radiosondes in three neighbouring cities of Zanjan (Tabriz, Tehran and Kermanshah) with nearly the same climatic regime show 81.1, 71.7 and 66.4% correlation respectively with the GPS PWV time series, and (3) the global reanalysis datasets for Zanjan show 89.2% correlation with the GPS results. These validations indicate that, in the absence of permanent GPS networks, if proper data processing strategies are adopted the low cost single station GPS meteorology can be considered as a possibility for meteorological monitoring.

A triplet of Mw ∼6 earthquakes on 2017 December 1–12 occurred ∼50 km north of Kerman, Iran, in an area of mountainous topography where several major right-lateral strike-slip fault systems—the Gowk, Nayband, Lakar Kuh and Kuh Banan faults—converge. Here we assess their source parameters and surficial expression using regional and teleseismic waveforms and arrival times, synthetic aperture radar interferometry, optical satellite image correlation and field observations. All three main shocks occurred on shallow reverse faults associated with the southern termination of the Lakar Kuh right-lateral strike-slip fault. The first two main shocks on 1 December and 12 December (08:43 UTC) likely ruptured and reruptured a previously unrecognized, blind, NE-dipping fault beneath the Mian Kuh range. Slip in both earthquakes extends much further along strike than down dip, hinting at structural or stratigraphic controls on rupture dimensions. The third main shock on 12 December (21:41 UTC) is perhaps the most interesting of the three events. It ruptured a conjugate SW-dipping thrust in the hangingwall of the first fault, generating a sinuous fault scarp in the alluvial plain north of the Mian Kuh range, consistent with its unusually shallow centroid depth of ∼2 km. Its high ratio of net surface slip (average ∼1.5 m and maximum ∼2.5 m) to length (∼7 km) and its narrow down-dip width (∼6 km) implies a very high stress drop. The surface rupture aligns along-strike with larger scarps that contain uplifted and incised fan surfaces in their hangingwalls, but this subtle expression of active faulting had not been fully recognized prior to these earthquakes. The clustering in space and time of large, shallow earthquakes on hidden faults is of broad concern for seismic hazard assessment in mountainous parts of Iran and in other collisional settings.

We compute a 2-D tomogram using the P wave arrival time readings from a temporary seismic experiment to study the seismic structure of the crust and upper mantle in NW Iran. The study area includes the western margins of the South Caspian Basin (SCB), and the Sahand and Sabalan post-collisional volcanoes in NW Iran. We invert 2780 regional and teleseismic relative P wave arrival times recorded by 23 stations along the seismic profile extending from the western shoreline of the Caspian Sea to Lake Urumieh. Our tomographic results show a higher-velocity region beneath the SCB. The observed higher velocities strongly correlate with the observed positive gravity anomalies over the southwestern margins of the Caspian Sea, suggesting an oceanic like nature for the SCB lithosphere. The tomographic results also show several lower-velocity anomalies in the crust. The Sabalan volcano is underlain by a low-velocity zone in the lower crust, which is most likely thermal in nature. In the Sahand region, the lower velocities are considerably shallower in depth and might be controlled by shallow sedimentary structures, as well as an anomalously warm upper crust. The shallow low-velocity regions are connected with deeper low-velocity zones 60–100 km deep in the upper mantle. This pattern points to a possible mantle source of post-collisional volcanism in NW Iran, i.e. the melting of a subducted slab.

Highlights • The Radially averaged power spectrum method is applied to calculate average magnetic susceptibility in Iran. • The results demonstrate that known occurrences of Magmatic-Ophiolite Arcs (MOA) correlate with high average susceptibility areas. • We interpret two parallel, hitherto unknown, MOAs in eastern Iran which developed in a steeply dipping (>60° dip) subduction zone. • Neo-Tethys subduction angle was shallow (60° dip) in SE Iran which indicates slab tearing. • We define a new outline of the economically important Tabas sedimentary basin. Abstract The Iranian plateau is one of the most complex geodynamic settings within the Alpine-Himalayan belt. The Paleo-Tethys and Neo-Tethys ocean subduction is responsible for the formation of several magmatic arcs and sedimentary basins within the plateau. These zones mostly are separated by thrust faults related to paleo-suture zones, which are highlighted by ophiolites. Sediment cover and overprint of a different magmatic phase from late Triassic to the Quaternary impede identification of some magmatic arcs and ophiolite belts. We track the known magmatic arcs, such as the Urmia-Dokhtar Magmatic Arc (UDMA), and unknown, sediment covered magmatic arcs by aeromagnetic data. We present a new map of average susceptibility calculated by the radially averaged power spectrum method. High average susceptibility values indicate the presence of a number of lineaments that correlate with known occurrences of Magmatic-Ophiolite Arcs (MOA), and low average susceptibility coincides with known sedimentary basins like Zagros, Makran, Kopeh-Dagh, and Tabas. In analogy to Zagros, low average susceptibility values indicate sedimentary basins to the south of the Darouneh fault and in the northern part of the Lut, Tabas and Yazd blocks. We interpret the Tabas basin as a pull-apart or back-arc basin. We identify hitherto unknown parallel MOAs in eastern Iran and the SE part of UDMA which both indicate steeply dipping (>60° dip) paleo-subduction zones. In contrast, we interpret shallow subduction (

We present a new 2D crustal-scale model of the northwestern Iranian plateau based on gravity–magnetic modeling along the 500 km long China–Iran Geological and Geophysical Survey in the Iranian plateau (CIGSIP) seismic profile across major tectonic provinces of Iran from the Arabian plate into the South Caspian Basin (SCB). The seismic P-wave receiver function (RF) model along the profile is used to constrain major crustal boundaries in the density model. Our 2D crustal model shows significant variation in the sedimentary thickness, Moho depth, and the depth and extent of intra-crustal interfaces. The Main Recent Fault (MRF) between the Arabian crust and the overriding central Iran crust dips at approximately 13° towards the northeast to a depth of about 40 km. The geometry of the MRF suggests about 150 km of underthrusting of the Arabian plate beneath central Iran. Our results indicate the presence of a high-density lower crustal layer beneath Zagros. We identify a new crustal-scale suture beneath the Tarom valley between the South Caspian Basin crust and Central Iran and the Alborz. This suture is associated with sharp variation in Moho depth, topography, and magnetic anomalies, and is underlain by a 20 km thick high-density crustal root at 35–55 km depth. The high-density lower crust in Alborz and Zagros may be related to partial eclogitization of crustal roots below about 40 km depth. The gravity and magnetic models indicate a highly extended continental crust for the SCB crust along the profile. Low observed magnetic susceptibility of the Kermanshah ophiolites likely indicates that the ophiolite rocks only form a thin layer that has been thrust over the sedimentary cover.

Because of the outdated methods of common landfill selection, it is imperative to reevaluate the usage suitability. To assess the suitability of the existing waste landfill in Zanjan, Iran, we have used a combination of the analytical hierarchy process (AHP) and GIS techniques, along with fieldwork surveys. Four major criteria and 12 subcriteria were considered, and the AHP was applied to assign the relative importance weights of criteria and subcriteria to each other. Finally, a landfill suitability map was generated and ranked based on the final suitability scores. The results show that the unsuitable areas are around Zanjan, in the middle parts of the plain. By contrast, the most suitable areas are uncultivated areas, located mostly in the west, north, and south. The results also indicate that the present landfill is a highly suitable site. After desk studies, geoelectrical surveys and infiltration measurements were conducted to make the final decision. Double-ring permeability tests confirm the landfill is an acceptable site. The electrical sounding shows that the leachate plume has a width of about ~ 450 m, spreads to a depth of about ~ 55 m, and migrates towards the northeast. Considering the groundwater depth, dry climate, and a low infiltration rate of the landfill soils, it can be concluded that leachate plumes will not contaminate groundwater within this decade. The proposed method can be implemented to reevaluate the suitability of any old operating reservoir such as oil reservoirs, petrol filling stations, heavy industrial tanks, and landfills, containing liquid hazardous materials.

On 2012 August 11, a pair of large, damaging earthquakes struck the Varzaghan-Ahar region in northwest Iran, in a region where there was no major mapped fault or any well-documented historical seismicity. To investigate the active tectonics of the source region we applied a combination of seismological methods (local aftershock network, calibrated multiple event relocation and focal mechanism studies), field observations (structural geology and geomorphological) and inversions for the regional stress field. The epicentral region is north of the North Tabriz Fault. The first main shock is characterized by right-lateral strike-slip motion on an almost E-W fault plane of about 23 km length extending from the surface to a depth of about 14 km. The second main shock occurred on an ENE-striking fault that dips at 60-70A degrees to the NW. Independent inversions of focal mechanisms and geologically determined fault kinematic data for the active stress state yield a transpressional tectonic regime with sigma(1) oriented N132E. For the region northeast of the North Tabriz Fault, the presence of rigid lithosphere of the South Caspian Basin implies the kinematic adjustment by northward transferring of the contracted masses through both distributed deformation and structural deflections. Our results suggest that the kinematic adjustment inside a contracting wedge may occur along interacting crosswise or conjugate faults to accommodate low rates of internal deformation. At a global scale, our results indicate that despite the basic assumption of 'rigid blocks' in geodetic plate modelling, internal deformation of block-like regions could control the kinematics of deformation and the level of seismic hazard within and around such regions of low deformation rate.

Predictive capabilities of the four updated NGA ground-motion models (NGA-WEST2) are evaluated in this study for acceleration response spectra using observed ground motions for August 11, 2012 Varzaghan-Ahar events in Iran. The predicted results were compared with those of regional attenuation equations and NGA08 models as well. The results of analyses revealed that the models of NGA-WEST2 improve prediction performance of NGA08 models for the two studied moderate events. All of models underestimate the recorded spectra of the second event for regionally significant period of 0.2 s at source distances below 50 km. For this distance range, almost all of the models underestimate the recorded spectra for periods larger than 1 s.

We use Pn -tomography to map lithospheric mantle velocity and anisotropy at the Arabia–Eurasia plate boundary, namely Makran and Zagros. We use catalogue events recorded by Oman, UAE, Saudi Arabia and Iran networks, the International Seismological Centre and the National Earthquake Information Center. Events of 1.8–16 degree distances were used for this Pn -tomography. In this study we show that the northeastern Arabia plate is characterized by cold and stable lithospheric mantle. Contrastingly, Eurasia is underlain by hot unstable lithospheric mantle. The Arabia–Eurasia lithospheric suture follows the Zagros collision surface suture within c. 70 km lateral proximity. At the southernmost Zagros collision, the Arabia lithosphere is inferred to extend further NE beneath Lut Block. This may be indicative of extended subduction of Arabia beneath Eurasia in southernmost Zagros. We find that eastern Makran shows typical subduction characteristics, with inferred oceanic lithosphere underlying the eastern Oman Sea and hot unstable lithospheric mantle below overriding Helmand Block. Contrastingly, the western Makran subduction zone including Arabia and Eurasia continental sides is underlain by a low- Pn -velocity anomaly, indicative of hot unstable lithospheric mantle. Surface evidence show that western, southern and eastern boundaries of western Makran low- Pn -velocity anomaly may represent a Late Neogene reactivated Precambrian terrane boundary in north Oman.

We selected 136 earthquakes recorded at stations of the Institute of Geo- physics of the University of Tehran (IGUT) during 1996-2007, in order to estimate the average attenuation parameters and variations of station corrections across the central Alborz mountains, the northern extension of the Alpine-Himalayan orogeny in wes- tern Asia. The selected events provide 1260 high signal-to-noise ratio (SNR) records with relatively good spatial coverage. We graphically find the distances at which the nature of geometrical spreading attenuation (R b ) changes significantly using a locally weighted scatterplot smoothing (LOWESS, local regression smoothing method) called robust LOWESS. A trilinear function with hinges at distances of about 80 and 160 km describes the geometric spreading attenuation with distance. By regressing to the hinged trilinear function, we found that b1 � −1:15 � 0:21, b2 � 0:09 � 0:31, and a fixed b3 � −0:5 minimize the average absolute value of the Fourier spectrum amplitude residuals. Using an anelastic attenuation coefficient at different frequencies, the direct quality factor Q in the central Alborz region is obtained as Q � 109 � 2f 0:64� 0:04 . The geographical distribution of station corrections at 1 Hz can be clearly separated along a dividing line connecting the North Tehran, Mosha, and Attari faults into two regions: the northern low-attenuating and the southern high attenuating regions. The stations with strong positive station corrections clearly align along the dividing line. This suggests the existence of a higher attenuation zone south of the dividing line. The presence of thick Tertiary-Quaternary sedimentary rocks with thick layers of salt in the southern region may explain the observed greater attenuation of seismic waves.

The M w 6.1 Silakhour earthquake occurred on 31 March 2006 in the Zagros region of Iran. It was preceded by two relatively large foreshocks of m b 4.8 and 5.2 on 30 March and followed by an extensive aftershock sequence. The earthquake sequence occurred along the right‐lateral strike‐slip Main Recent Fault (MRF). To understand the spatial relationship between the mainshock and the high level of foreshock and aftershock activity, we have merged all seismic data recorded by Iranian seismic networks with data from the global catalogs and relocated the events using a multiple‐event relocation method. We also conducted source studies of the mainshock and larger foreshocks and aftershocks, which confirm dominantly right‐lateral strike‐slip faulting for the mainshock, the two large foreshocks, and the largest aftershock. The relocated epicenters of the 2006 earthquake sequence lie close to the MRF and spread parallel to the MRF over a distance of ∼55 km, much longer than the 15–20‐km rupture length to be expected of a magnitude 6.1 strike‐slip earthquake. The relocated epicenters reveals that the seismic sequence is divided into two parts, an eastern patch on the Borujerd–Dorud segment of the MRF that is directly associated with the rupture of the mainshock and a second patch, 20–25 km long, on the Nahavand–Borujerd segment of the MRF. The two patches are offset by ∼5 km in the same sense as the MRF. The pattern of the western patch of seismicity strongly suggests a seismically active lineation that does not correspond closely to the mapped faults of the area. We take this as evidence of ongoing evolution of fault geometry on this section of the MRF. Coulomb stress calculations suggest that the seismicity on the Nahavand–Borujerd segment may be a separate earthquake sequence, triggered by static stress changes related to the mainshock rupture.

We investigate the effects of a huge impact called Borealis impact, possibly occurred at 4.5 Ga and created the northern lowland of Mars, on the thermal evolution of Mars using 2D axi-symmetric convection in a spherical shell of temperature- and pressure-dependent viscosity. A suite of models are calculated to study effects of different physical parameters on the impact-induced mantle dynamics. We use the conventional scaling laws of basin diameter and impactor size, the impact shock pressure in the interior of Mars and the foundering mechanism of impact heating to calculate the temperature increase in the mantle. The mantle is allowed to melt as its temperature surpasses the solidus. To assess the effects of the impact on the thermal evolution of Mars, we compare the thermal evolution of the models with impact to the equivalent models having no impact. It is shown that the impact increases the rate of crustal production for a period of about 50 m.y. after the impact. The initial impact-induced melt volume is about 3.44 × 108 km3, whereas the impact induced mantle upwelling beneath the impact site results in the production of an extra 8.8 × 108 km3 of melt. The models with no impact also produce melt in due time and the total volume of melt at 4.6 Gyr is not significantly sensitive to the impact. This observation suggests that large impacts can significantly alter the rate of crustal production of Mars but have minor effects on the total volume of crust or melt production.

Large impacts not only create giant basins on terrestrial planets but also heat their interior by shock waves. We investigate the impacts that have created the largest basins existing on the planets: Utopia on Mars, Caloris on Mercury, Aitken on Moon, all formed at � 4 Ga. We determine the impact-induced temperature increases in the interior of a planet using the ‘‘foundering’’ shock heating model of Watters et al. (Watters, W.A., Zuber, M.T., Hager, B.H. [2009]. J. Geophys. Res. 114, E02001. doi:10.1029/ 2007JE002964). The post-impact thermal evolution of the planet is investigated using 2D axi-symmetric convection in a spherical shell of temperature-dependent viscosity and thermal conductivity, and pressure-dependent thermal expansion. The impact heating creates a superheated giant plume in the upper mantle which ascends rapidly and develops a strong convection in the mantle of the sub-impact hemisphere. The upwelling of the plume rapidly sweeps up the impact-heated base of the mantle away from the core–mantle boundary and replaces it with the colder surrounding material, thus reducing the effects of the impact-heated base of the mantle on the heat flux out of core. However, direct shock heating of the core stratifies the core, suppresses the pre-existing thermal convection, and cripples a pre-existing thermally-driven core dynamo. It takes about 17, 4, and 5 Myr for the stratified cores of Mars, Mercury, and Moon to exhaust impact heat and resume global convection, possibly regenerating core dynamos.

We have calculated empirical attenuation curves for the local magnitude scale (ML) in the Alborz region of northern Iran. The maximum trace amplitudes de- rived from synthetic Wood-Anderson seismograms of 1290 records of 59 events in the distance range of 8.5-550 km recorded by the short-period Iranian Seismic Telemetry Network (ISTN) were inverted for the attenuation curve, magnitudes, and station cor- rections. The earthquakes ranged from ML3:25 to 5.65 and were recorded at 26 sta- tions in the region. We used both nonparametric and parametric least-squares methods for inversion. The resulting parametric equation is logA0 1:1725logR 0:0021R 0:4450. The two methods yielded very similar results and are very close to Richter's original attenuation curve for southern California. The station corrections vary between 0:42 and 0:47 suggesting that local site effects may have a strong influence on the amplitudes.

Interference of reflections from upper and lower interfaces of geological units is a common source of interpretational ambiguity. This paper utilizes the fractal dimension of instantaneous seismic attributes to study this interfering phenomenon. Using a 1D synthetic seismic trace, we examine the divider, Hurst, spectral and variance fractal methods. Since each method utilizes a moving window technique as part of its analysis, a series of optimum window lengths needs to be determined. The four methods are applied to the instantaneous amplitude, phase and frequency attributes of a series of 1D synthetic seismic traces. To study wavelet interference on 2D seismic sections, we generate four 2D synthetic seismic sections by convolving a zero phase and a minimum phase wavelet with two different wedge models. For the first model, the reflection coefficient of the upper wedge interface is positive and the lower interface negative, whereas for the second model, the polarities are positive. Our results suggest that fractal analysis of the instantaneous phase, using the Hurst method, can separate two opposite polarity wavelets within a distance of λ/16 or greater and of λ/4 or greater when they have the same polarity. Our results are not sensitive to random noise when the signal-to-noise-ratio is greater than 10.

The Tehran Seismic Telemetry Network commenced operation in 1996, but it has never published any bulletins and there has been no systematic evaluation of the precision and reliability of its seismic recording. In this paper a standard approach is presented to evaluate the quality of operation of the Network. Our findings show no serious gaps in data coverage and the threshold of completeness is about magnitude 2 within the Network. We show that incorrect phase reading is the major reason for errors in identifying the phases, and the large scatter in arrival times. We propose a number of suggestions for the improvement of network operation. q 2004 Elsevier Ltd. All rights reserved.

The D″ layer is a dense and chemically distinct layer at the base of the convecting mantle. Numerical modeling of the entrainment of this layer by mantle convection requires the solution of the advective transport equation without introducing numerical diffusion across sharp material boundaries. We use our improved second moment numerical method to solve the equation. The method conserves the amount of material and the first and second moments of material distribution in each control volume. We first consider two examples of isothermal Rayleigh–Taylor instability to illustrate the performance of our method by comparing our results with those of a number of field, tracer and marker chain methods. We show that the performance of our method in minimizing the numerical diffusion is better than the field methods and comparable to the tracer and marker chain methods. We then study the instability of the dense D″ layer and its interaction with the overlying mantle. A range of density contrast between the D″ layer and the mantle, layer thickness, and the Rayleigh number, Ra , is examined. We show that for higher values of these parameters, the amount of entrainment decreases and the layer remains stable over longer periods of time. For very thick D″ layers and high Ra values, internal convection can take place within the layer.

The second moment numerical method (SMM) of Egan and Mahoney [Numerical modeling of advection and diffusion of urban area source pollutant. Journal of Applied Meteorology 1972; 11: 312–322] is adapted to solve for the pure advection transport equation in a variety of flow fields. SMM eliminates numerical diffusion by employing a procedure that takes into account the first and second moments of mass distribution in each grid element. For pure translational flow fields, the method is conservative, positive definite and shape-preserving. In rotational and/or shear flows, the accuracy of SMM is significantly reduced. Two improvements are presented to make the SMM applicable to a wider range of flow problems. It is shown that the improved SMM (ISMM) is less diffusive and more shape-preserving than the SMM in rotational and/or deformational flows. The ISMM can also be used to solve for a color function in compressible flow fields. The computational efficiency of this method is compared with that of other methods and, for a given accuracy, it is shown that ISMM is a cost-effective, non-diffusive and shape-preserving method. Copyright © 2000 John Wiley & Sons, Ltd.

Summary Using a two-phase flow model, we investigate the formation of a high-percentage melt layer beneath the oceanic lithosphere and focusing of the melt towards the ridge axis, taking into account freezing and melt extraction. Melt migration is modelled dynamically within a viscous permeable media that includes the asthenosphere and viscous part of the lithosphere. Due to a much faster melt migration in the brittle part of the lithosphere, the melt migration is simulated by instantaneous melt extraction from an assigned melt extraction region beneath the ridge axis. It is shown that a high-percentage melt layer forms and successfully focuses melt to a narrow zone beneath the mid-ocean ridge. Performance of the melt focusing mechanism is not significantly sensitive to the size of the melt extraction region, the melt extraction threshold and the spreading rate. In all of our models, about half of the total melt production freezes beneath the base of the lithosphere and the rest is focused towards the ridge and forms the crust.