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New combined P receiver functions and seismicity data obtained from the EGELADOS network employing 65 seismological stations within the Aegean constrained new information on the geometry of the Hellenic subduction zone. The dense network and large data set enabled us to estimate the Moho depth of the continental Aegean plate across the whole area. Presence of a negative contrast at the Moho boundary indicating the serpentinized mantle wedge above the subducting African plate was seen along the entire forearc. Furthermore, low seismicity was observed within the serpentinized mantle wedge. We found a relatively thick continental crust (30–43 km) with a maximum thickness of about 48 km beneath the Peloponnese Peninsula, whereas a thinner crust of about 27–30 km was observed beneath western Turkey. The crust of the overriding plate is thinning beneath the southern and central Aegean and reaches 23–27 km. Unusual low Vp / Vs ratios were estimated beneath the central Aegean, which most likely represent indications on the pronounced felsic character of the extended continental Aegean crust. Moreover, P receiver functions imaged the subducted African Moho as a strong converted phase down to a depth of about 100 km. However, the converted Moho phase appears to be weak for the deeper parts of the African plate suggesting nearly complete phase transitions of crustal material into denser phases. We show the subducting African crust along eight profiles covering the whole southern and central Aegean. Seismicity of the western Hellenic subduction zone was taken from the relocated EHB-ISC catalogue, whereas for the eastern Hellenic subduction zone, we used the catalogues of manually picked hypocentre locations of temporary networks within the Aegean. Accurate hypocentre locations reveal a significant change in the dip angle of the Wadati–Benioff zone (WBZ) from west (~ 25°) to the eastern part (~ 35°) of the Hellenic subduction zone. Furthermore, a zone of high deformation can be characterized by a vertical offset of about 40 km of the WBZ beneath the eastern Cretan Sea. This deformation zone may separate a shallower N-ward dipping slab in the west from a steeper NW-ward dipping slab in the east. In contrast to hypocentre locations, we found very weak evidence for the presence of the slab at larger depths in the P receiver functions, which may result from the strong appearance of the Moho multiples as well as eclogitization of the oceanic crust. The presence of the top of a strong low-velocity zone at about 60 km depth in the central Aegean may be related to the asthenosphere below the Aegean continental lithosphere and above the subducting slab. Thus, the Aegean mantle lithosphere seems to be 30–40 km thick, which means that its thickness increased again since the removal of the mantle lithosphere about 15 to 35 Ma ago.

The Iranian Plateau is characterized by diverse tectonic domains, including the continental collisions (e.g. the Zagros and Alborz Mountains) and oceanic plate subduction (e.g. Makransubduction zone). To derive a detailed image of the crust–mantle (Moho) and lithosphere–asthenosphere (LAB) boundaries in some tectonically units of the Iranian Plateau, we used a large number of S receiver functions obtained from teleseismic events recorded at 68 national permanent stations (19 broadband and 49 short period stations). The S receiver functions clearly imaged the base of the crust and lithosphere and their variations within the different tectonic zones of the Iranian Plateau. Our new seismic images show a significant variation of the lithospheric thickness in the different geological features. The most complex structure was detected beneath the Zagros Mountains where the Arabian Plate is believed to underthrust beneath centralIran. We found the thickest crust under the Sanandaj-Sirjan metamorphic zone (SSZ)which proposes the overthrusting of the crust of central Iran into the Zagros crust along the main Zagros thrust (MZT), in agreement with the results of Paul et al., (2010). Furthermore, our results clearly show a shallow LAB at about 80-90 km depth beneath the Alborz, the central domain (CD) and central Iranian micro plate (CIMP) zones. Based on our results, the Arabian LAB, beneath the Zagros fold and thrust belt (ZFTB), SSZ and the Urumieh-Dokhtar magmatic assemblage (UDMA) is 200 km and may contain a dipping structure at depths ranging from 100 beneath the ZFTBto 150 km beneath the SSZ and the UDMA. This dipping structure interpreted as the presence of remnants of the fossil Neo-Tethys subduction. The location of the boundary between the Arabian and central Iranian lithospheres is beneath the UDMA, which is shifted northeastward relative to the surficial expression of the MZT.

Cratons are extremely stable continental areas of the Earth’s crust, which have been formed and remained largely unchanged since Precambrian. However, their formation and how they survived destruction over billions of years remains a subject of debate. Seismic properties of the cratonic lithosphere re[U+FB02]ect its composition and physical state and obtain basic constraints on processes of the formation and evolution of continents. Insight on these issues may be gained by determining the depth and the nature of the Lithosphere-Asthenosphere Boundary (LAB), which is a necessary element of the plate tectonic theory. However, It has proved quite “elusive” beneath the oldest continental areas. What is missing to date is a consensus on the feature that would correspond to the LAB and whether such a feature exists everywhere beneath cratons. The relatively recently developed S receiver function technique employing S-to-P conversions appears promising for detecting the LAB with a sufficiently high resolution and density. A growing number of regional observations obtained from S receiver function studies has detected discontinuities characterized by a significant negative velocity contrast in the upper mantle. However, challenges still remain in detecting the S-to-P conversions from the LAB beneath the Precambrian cratons. Some recent SRF studies observed a deep (> 160 km) negative velocity contrast beneath cratons and interpreted it as the LAB. For example, a deep LAB at about 250 km was reported beneath the Kalahari craton by different authors. Similar results were also obtained beneath some parts of the Canadian shield, East European Craton, Australia, the Arabian Shield and Tanzania craton. In contrast, other SRF studies found no evidence for negative discontinuities at these depths in the North American craton, in Kalahari craton or in Australia. Instead they revealed a very sharp negative velocity gradient at much shallower depth (60-150 km), leading some authors to infer that the cratonic lithosphere may be considerably thinner than expected, contradicting tomographic and other geophysical or geochemical studies. Moreover, this finding contrasts also with other cratonic regions where SRFs clearly found an observable LAB discontinuity at depths of more than 160 km. To solve this problem, a concept of a “Mid-Lithospheric Discontinuity” (MLD) has been developed. The MLD is proposed as a global characteristic of Precambrian lithosphere, which had been consistently found as the 8_ discontinuity within the continental lithosphere in the analysis of long-range seismic profiles. The nature of the MLD remains, however, uncertain, and may be attributed to accumulated melts, presence of fluids, lithospheric stacking, remnants of fossil subduction interfaces or change in anisotropic properties. Such a feature would have implications for the formation and evolution of the continents. Indeed, issues related to the unambiguous detection of MLD and the controversial LAB beneath the cratonic regions of the globe still remain contentious and therefore further work is required to resolve this issue. I will concentrate on this topic and present some new results from Kalahari craton, Scandinavia and eastern Europe.

Based on joint consideration of S receiver functions and surface-wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the Kalahari Craton. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150–200 km, respectively. We found that a 50 km thick anisotropic layer, containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath, can account for the first MLD at about 85 km depth. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere, as inferred from chemical tomography, and those of our second MLD led us to characterize it as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. The deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the Thabazimbi-Murchison Lineament (TML), implying that the TML isolates the lithosphere of the Limpopo terrane from that of the ancient Kaapvaal terrane. The largest velocity contrast (3.6–4.7%) is observed at a boundary located at depths of 260–280 km beneath the Archean domains and the older Proterozoic belt. This boundary most likely represents the lithosphereasthenosphere boundary, which shallows to about 200 km beneath the younger Proterozoic belt. Thus, the Kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history, which left their imprint in its internal layering.

The Himalaya and the Tibetan Plateau are uplifted by the ongoing northward underthrusting of the Indian continental lithosphere below Tibet resulting in lithospheric stacking. The layered structure of the Tibetan upper mantle is imaged by seismic methods, most detailed with the receiver function method. Tibet is considered as a place where the development of a future craton is currently under way. Here we study the upper mantle from Germany to northern Sweden with seismic S receiver functions and compare the structure below Scandinavia with that below Tibet. Below Proterozoic Scandinavia, we found two low-velocity zones on top of each other, separated by a high-velocity zone. The top of the upper low-velocity zone at about 100 km depth extends from Germany to Archaean northern Sweden. It agrees with the lithosphere-asthenosphere boundary (LAB) below Germany and Denmark. Below Sweden it is known as the 8°discontinuity, or as a mid-lithospheric discontinuity (MLD), similar to observations in North America. Seismic tomography places the LAB near 200 km in Scandinavia, which is close to the top of our deeper low-velocity zone. We also observed the bottom of the asthenosphere (the Lehmann discontinuity) deepening from 180 km in Germany to 260 km below Sweden. Remnants of old subduction in the upper about 100 km below Scandinavia and Finland are known from controlled source seismic experiments and local earthquake studies. Recent tomographic studies indicate delamination of the lithosphere below southern Scandinavia and northern Germany. We are suggesting that the large-scale layered structure in the Scandinavian upper mantle may be caused by processes similar to the ongoing lithospheric stacking in Tibet.

Investigation of the thickness of continental roots, which migrate coherently with plates belongs to the most systematic keys in order to understand the continental evolution. South Africa’s lithosphere preserves a nearly uninterrupted geological history of more than 3.5 billion years. It was formed during the break-up of the supercontinent Gondwana over a period of 80 million years and therefore is the longest, best-preserved geological record of the planet Earth. To estimate the LAB and other major lithospheric discontinuities beneath South Africa, we used the novel technique of S receiver function, which employs S-to-P conversions and appears promising for detecting the LAB. This technique has already proven its power for mapping the LAB in the tectonically different regions. Although the South African craton has been extensively studied in recent years, especially by SRFs, the depth extent of the lithosphere and its nature varies somewhat between studies. It seems that several differences in methodology and data selection criteria leading to variations in the SRFs obtained. Some authors observed Sto-P conversions at 25-30 s (_250-300 km) and interpreted them as being from the LAB. In contrast, some other authors found these conversions at shallower depths (_150 km). We calculated SRFs for the data of more than 120 stations within South Africa. Such a huge amount of data has not been yet applied for SRF studies in South Africa. Our results clearly shows 3 different LVZs at about 100 (_10s), 150-220 (15-22 s) km and 300-330 km (30-33 s), which were not seen in any of the previous studies. Based on our preliminary results, the deep and sharp LAB phase at 300-330 km is significantly imaged beneath the cratons (Kaapvaal and Zimbabwe cratons; > 2.7 Ga) and the oldest belt (Limpopo belt _ 2.7 Ga). This phase may not be visible northward and southward beneath the much younger mobile belts (Mozambique and Namaqua-Natal belt; _ 1.1 Ga). Instead, a shallower and less sharper boundary at 150-220 km depth can be identified across the whole region. This boundary may show an intralithospheric discontinuity beneath cratons and may reveal the LAB beneath the younger belts. In addition, a 100 km LVZ can be also observed, which may confirm the so-called “8 degree discontinuity” seen in dense, long-range seismic profiles.

The dense deployment of seismic stations so far in the western half of the United States within the USArray project provides the opportunity to study in greater detail the structure of the lithosphere-asthenosphere system. We use the S receiver function technique for this purpose which has higher resolution than surface wave tomography, is sensitive to seismic discontinuities and has no problems with multiples like P receiver functions. Only two major discontinuities are observed in the entire area down to about 300km depth. These are the crust-mantle boundary (Moho) and a negative boundary which we correlate with the lithosphere-asthenosphere boundary (LAB) since a low velocity zone is the classical definition of the seismic observation of the asthenosphere by Gutenberg (1926). Our S receiver function LAB is at a depth of 70-80km in large parts of westernmost North America. East of the Rocky Mountains its depth is generally between 90 and 110km. Regions with LAB depths down to about 140km occur in a stretch from northern Texas over the Colorado Plateau to the Columbia Basalts. These observations agree well with tomography results in the westernmost USA and at the east coast. However, in the central cratonic part of the USA the tomography LAB is near 200km depth. At this depth no discontinuity is seen in the S receiver functions. The negative signal near 100km depth in the central part of the USA is interpreted by Yuan and Romanowicz (2010) or Lekic and Romanowicz (2011) as a recently discovered mid lithospheric discontinuity (MLD). A solution for the discrepancy between receiver function imaging and surface wave tomography is not yet obvious and requires more high resolution studies at other cratons before a general solution may be found. Our results agree well with petrophysical models of increased water content in the asthenosphere, which predict a sharp and shallow LAB also in continents (Mierdel et al. 2007). We are comparing our results from North America with other regions (South Africa, South America, Europe).

The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere-asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid-lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert-Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid-lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.

Geographic location of Iran plateau between Arabia and Eurasia plates caused the Iranian plate to be in a high pressure condition. Configuration and tectonic structures procedure in this region is affected by motions of these plates. With respect to this fact that the Iran plateau is in a high pressure situation, it is expected that the main construction of contractions and faults in this region to be reverse or thrust. Kope Dagh region has the same configurations and tectonic structures process. So, this region considered as an active tectonic region. Thus, the study of its crust is essential to locate the future earthquakes. The depth of Moho is an important parameter to characterize the overall structure of a crust and can often be related to geology and tectonic evolution of the region. In this study, we use the teleseismic receiver function technique to determine the crustal thickness and Vp/Vs ratios for 6 stations in Kope Dagh region. The teleseismic receiver function represents the structural response near a recording station to the incoming teleseismic P wave. The first-order information about the crustal structure under a station can be derived from the radial receiver function which is dominated by P-to-S converted energy from a series of velocity discontinuities in the crust and upper mantle. Because of the large velocity contrast at the crust-mantle boundary, the Moho P-to-S conversion is often the largest signal following the direct P. In this work, the teleseismic waves, which had been registered by the 6 stations of the Ghuchan and Mashhad seismic networks have been processed using the P receiver function method to calculate the Moho depth beneath the Kope Dagh region. We used the available short period data. Our results showed a Moho depth of approximately 45 km on average, which varies between 41 and 49 km. These results confirmed the previous results obtained from other studies in this region. We calculated also P and S receiver functions for a single broadband station and compared our results with those obtained from short period stations in Kope Dagh.

Fast convergence between the oceanic Nazca and the continental South American plate is accommodated by recurrent rupture of large segments of the two plates’ interface. The resulting earthquakes are among the largest and, for their sizes, most frequent on Earth. Along the Chilean and southern Peruvian margin, all segments have ruptured at least once in the past 150 years for which there exist historic and/or instrumental records. The one segment that is most mature for re-rupture stretches for more than 500 km along the northernmost Chilean coast between roughly -23° and -18° latitude. It last broke in 1877 in a magnitude ~8.8 earthquake, triggering a major Tsunami. From the historical record, it has been known to have a recurrence cycle of approximately 110 years. The adjoining segments to the north and south broke rather recently in 1995 and 2001 in M>8 earthquakes and an M 7.7 earthquake encroached the southern part of the gap in 2007. The IPOC project intends to investigate this segment of the Nazca-South American plate boundary, on which a strong to devastating earthquake is expected to occur within the next years, by monitoring at a variety of time-scales deformation, seismicity, and magnetotelluric fields in the subduction zone at the closing stages of the interseismic cycle before and possibly during occurrence of a big earthquake. For that purpose, installation of long-term observatories in Northern Chile started in 2006 in a close cooperation of the Universidad de Chile (Santiago, Chile), the Universidad Catolica del Norte (Antofagasta, Chile), the Institut de Physique du Globe de Paris (Paris, France), and the German Research Centre for Geosciences (GFZ, Potsdam, Germany). Currently we are operating 14 modern seismological stations equipped with STS-2 broadband seismometers and accelerometers (EPI sensor). At least two more stations will be installed in the near future. To cope with the high resolution and dynamic of the sensors and data acquisition, site installation was accomplished with special care. At each station a cavern was blasted into the bedrock up to 5 meters deep to ensure stable conditions for measurements. Currently five stations are additionally recording continuously GPS signals, another five are also recording meteorological data, and another seven are equipped with Magneto-Telluric (MT) probes (fluxgate magnetometers and electrode lines). It is planned to extend the multi-parameter observation to as many stations as possible. So far ten of the stations are sending continuous data via satellite links (VSAT) to the GEOFON data host at the GFZ. We will be reporting first results on seismicity, transient deformation and MT from the first two years of recording.

Characterization of the detailed structure of the crust and upper mantel is an important goal of geophysical studies. Teleseismic body waveforms have been used to infer crust and upper mantel structure. In this study we use the P receiver function method to determine the crustal thicknesses and Vp/Vs ratios and map out the lateral variation of Moho depth under northwest of Zagros (Kermanshah region, western Iran). The data used in this study are selected teleseismic data (epicentral distance between 30°-95°, mb> 5.5) recorded at six short-period three component stations of Kermanshah Network (2003-2007). Our results revealed a Moho depth of approximately 40 km beneath this area. The thinnest crust was found beneath DHR in west part of the study area (36 ± 1 km), whereas the thickest crust was observed in the eastern part of the area beneath VIS (51.5 ± 2 km) related to the overthrusting system existed along the MZT. We observed that Moho depth increases from west to east. The average VP/VS ratio was estimated to be 1.76 and varies from 1.72 ± 0.04 to 1.83 ± 0.03. In addition the crustal structure was also determined by the modeling of the P receiver functions. The main phases, which were observed in our final P receiver functions, are the Moho conversions, their multiples in the crust as well as the conversions at the base of the sedimentary layer. The crustal thickness derived from inversion of these data is approximately 42.5 km and is in good agreement with our first results obtained from the Zhu and Kanamori method as well as with those obtained from other geophysical studies. The average shear wave velocity was estimated to be 3.69 km/s in the crust reaching 4.80km/s in the upper mantle.