Forward modelling receiver functions for crustal structure beneath station TBZ (Trabzon, Turkey)

We use teleseismic three-component digital data from the Trabzon, Turkey broadband seismic station TBZ to model the crustal structure by the receiver function method. The station is located at a structural transition from continental northeastern Anatolia to the oceanic Black Sea basin. Rocks in the region are of volcanic origin covered by young sediments. By forward modelling the radial receiver functions, we construct 1-D crustal shear velocity models that include a lower crustal low-velocity zone, indicating a partial melt mechanism which may be the source of surfacing magmatic rocks and regional volcanism. Within the top 5 km, velocities increase sharply from about 1.5 to 3.5 km s⁻¹. Such near-surface low velocities are caused by sedimentation, extending from the Black Sea basin. Velocities at around 20 km depth have mantle-like values (about 4.25 km s⁻¹ ), which easily correlate to magmatic rocks cropping out on the surface. At 25 km depth there is a thin low-velocity layer of about 4.0 km s⁻¹. The average Moho velocity is about 4.6 km s⁻¹, and its depth changes from 32 to 40 km. Arrivals on the tangential components indicate that the Moho discontinuity dips approximately southwards, in agreement with the crustal thickening to the south. We searched for the solution of receiver functions around the regional surface wave group velocity inversion results, which helped alleviate the multiple solution problem frequently encountered in receiver function modelling.
Station TBZ is a recently deployed broadband seismic station, and the aim of this study is to report on the analysis of new receiver function data. The analysis of new data in such a structurally complex region provides constraining starting models for future structural studies in the region.     

Upper crustal shear velocity models from higher mode Rayleigh wave dispersion in Scotland

Summary. Group velocities for first and second higher mode Rayleigh waves, in the frequency range 0.8–4.8 Hz, generated from a local earthquake of magnitude 3.7 M _L in western Scotland, are measured at stations along the 1974 LISPB line. These provide detailed information about the crustal structure west of the line. The data divide the region into seven apparently homogeneous provinces. Averaged higher mode velocity dispersion curves for each province are analysed simultaneously using a linearized inversion technique, yielding regionalized shear velocity profiles down to a depth of 17 km into the upper crust. Shear wave velocity is between 3.0 and 3.4 km s⁻¹ in the upper 2 km, with a slow increase to around 3.8 km s⁻¹. P -wave models computed using these results agree with profiles from the LISPB and LUST refraction experiments.     

Transitional continental–oceanic structure beneath the Norwegian Sea from inversion of surface wave group velocity data

We have analysed the fundamental mode of Love and Rayleigh waves generated by 12 earthquakes located in the mid-Atlantic ridge and Jan Mayen fracture zone. Using the multiple filter analysis technique, we isolated the Rayleigh and Love wave group velocities for periods between 10 and 50  s. The surface wave propagation paths were divided into five groups, and average group velocities calculated for each group. The average group velocities were inverted and produced shear wave velocity models that correspond to a quasi-continental oceanic structure in the Greenland–Norwegian Sea region. Although resolution is poor at shallow depth, we obtained crustal thickness values of about 18  km in the Norwegian Sea area and 9  km in the region between Svalbard and Iceland. The abnormally thick crust in the Norwegian Sea area is ascribed to magmatic underplating and the thermal blanketing effect of sedimentary layers. Maximum crustal shear velocities vary between 3.5 and 3.9  km  s⁻¹ for most paths. An average lithospheric thickness of 60  km was observed, which is lower than expected for oceanic-type structure of similar age. We also observed low shear wave velocities in the lower crust and upper mantle. We suggest that high heat flow extending to depths of about 30  km beneath the surface can account for the thin lithosphere and observed low velocities. Anisotropy coefficients of 1–5 per cent in the shallow layers and >7 per cent in the upper mantle point to the existence of polarization anisotropy in the region.     

Shallow velocity structure along the Hirapur–Mandla profile using traveltime inversion of wide-angle seismic data, and its tectonic implications

In order to investigate the velocity structure, and hence shed light on the related tectonics, across the Narmada–Son lineament, traveltimes of wide-angle seismic data along the 240 km long Hirapur–Mandla profile in central India have been inverted. A blocky, laterally heterogeneous, three-layer velocity model down to a depth of 10 km has been derived. The first layer shows a maximum thickness of the upper Vindhyans (4.5 km s⁻¹ ) of about 1.35 km and rests on top of normal crystalline basement, represented by the 5.9 km s⁻¹ velocity layer. The anomalous feature of the study is the absence of normal granitic basement in the great Vindhyan Graben, where lower Vindhyan sediments (5.3 km s⁻¹ ) were deposited during the Precambrian on high-velocity (6.3 km s⁻¹ ) metamorphic rock. The block beneath the Narmada–Son lineament represents a horst feature in which high-velocity (6.5 km s⁻¹ ) lower crustal material has risen to a depth of less than 2 km. South of the lineament, the Deccan Traps were deposited on normal basement during the upper Cretaceous period and attained a maximum thickness of about 800 m.     

Comparison of the Upper Mantle Shear Wave Velocity Structure of the Indian Shield and the Tibetan Plateau and Tectonic Implications

Summary. Travel times and waveforms of long-period SH -waves recorded at distances of 10–30° and some SS waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the Indian Shield and the Tibetan Plateau. the shear velocity in the uppermost mantle beneath both the Indian Shield and the Tibetan Plateau is high and close to 4.7 km s⁻¹. the Indian Shield has a fairly thick high velocity lid, and the mean velocity between 40 and 250 km is between 4.58 and 4.68 km s⁻¹. In contrast, S -wave travel times and waveforms of S -waves, as well as a few for SS , show that the mean velocity between 70 and 250km beneath the central and northern part of the Tibetan Plateau is slower by 4 per cent or more than that beneath the Indian Shield and probably is between 4.4 and 4.5km s⁻¹. No large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of SH phases crossing Tibet or the Indian Shield. These results show that the structure of Tibet is not that of a shield and imply that the Indian plate is not underthrusting the whole of the Tibetan Plateau at the present time.     

Experimental study of the anisotropy of longitudinal and transverse waves from local earthquake records

Summary. The paper gives the results of a study of the anisotropy of seismic wave velocities within the Ashkhabad test field in Central Asia. The anisotropy was studied by analysing variations in the values of apparent velocities of first arrivals for epicentral distances ranging from 30 to 130 km and by analysing the delays (Δ t_s1-_s2 ) between the arrival times of shear waves with different polarizations.
The velocities of P -waves vary with azimuth from 5.3 to 6.27 km s^-1 and the velocities of S -waves vary from 3.15 to 3.5 km s^-1.
The delay times Δ t_S1 - _S2 depend on the direction of the propagation. The character of the variation of the propagation velocity of the longitudinal wave, the presence of two differently polarized shear waves S 1 and S 2 propagating at different velocities, and the character of the distribution of Δ t_S1 - _S2 on the stereogram suggest that the symmetry of the anisotropic medium is close to hexagonal with a nearly horizontal symmetry axis coinciding with the direction of maximal velocity. The azimuth of the symmetry axis of the medium is 140° and coincides with the direction of geological faults.     

Strain accumulation along an oblique plate boundary: the Reykjanes Peninsula, southwest Iceland

We use annual GPS observations on the Reykjanes Peninsula (RP) from 2000 to 2006 to generate maps of surface velocities and strain rates across the active plate boundary. We find that the surface deformation on the RP is consistent with oblique plate boundary motion on a regional scale, although considerable temporal and spatial strain rate variations are observed within the plate boundary zone. A small, but consistent increase in eastward velocity is observed at several stations on the southern part of the peninsula, compared to the 1993–1998 time period. The 2000–2006 velocities can be modelled by approximating the plate boundary as a series of vertical dislocations with left-lateral motion and opening. For the RP plate boundary we estimate left-lateral motion  18⁺⁴₋₃ mm yr⁻¹  and opening of  7⁺³₋₂ mm yr⁻¹  below a locking depth of  7⁺¹₋₂ km  . The resulting deep motion of  20⁺⁴₋₃ mm yr⁻¹  in the direction of  N(100⁺⁸₋₆)°E  agrees well with the predicted relative North America–Eurasia rate. We calculate the areal and shear strain rates using velocities from two periods: 1993–1998 and 2000–2006. The deep motion along the plate boundary results in left-lateral shear strain rates, which are perturbed by shallow deformation due to the 1994–1998 inflation and elevated seismicity in the Hengill–Hrómundartindur volcanic system, geothermal fluid extraction at the Svartsengi power plant, and possibly earthquake activity on the central part of the peninsula.     

Velocity structure of upper-mantle transition zones beneath central eurasia from seismic inversion using genetic algorithms

We present velocity constraints for the upper-mantle transition zones beneath Central Siberia based on observations of the 1982 RIFT Deep Seismic Sounding (DSS) profile. The data consist of seismic recordings of a nuclear explosion in north-western Siberia along a 2600 km long seismic profile extending from the Yamal Peninsula to Lake Baikal. We invert seismic data from the mantle transition zones using a non-linear inversion scheme using a genetic algorithm for optimization and the WKBJ method to compute the synthetic seismograms. A statistical error analysis using a graph-binning technique was performed to provide uncertainty values in the velocity models.
Our best model for the upper-mantle velocity discontinuity near 410 km depth has a two-stage velocity-gradient structure, with velocities increasing from 8.70–9.25 km s⁻¹ over a depth range of 400–415 km, a gradient of 0.0433 s⁻¹, and from 9.25–9.60 km s⁻¹ over a depth range of 415–435 km, a gradient of 0.0175 s⁻¹. This derived model is consistent with other seismological observations and mineral-physics models. The model for the velocity discontinuity near 660 km depth is simple, sharp and includes velocities increasing from 10.15 km s⁻¹ at 655 km depth to 10.70 km s⁻¹ at 660 km depth, a gradient of 0.055 s⁻¹.     

Continuous GPS measurements across the Western Alps, 1996–1998

Global Positioning System (GPS) data are analysed from three permanent sites, two in the Western Alps (Grasse, France, and Zimmerwald, Switzerland) and the third in the Po Basin (Torino, Italy), for the 2.5-year period from 1 January 1996 to 1 July 1998. An analysis of the stochastic properties of the position time series reveals a significant amount of spatially and temporally correlated noise, which best fits a model combining white noise and flicker noise. The coloured noise is drastically reduced by spatially filtering the time series, suggesting that it is not due to site-specific effects such as monument motion, but rather to noise sources common to the three sites, such as reference frame, satellite orbit or Earth orientation errors. We find velocity uncertainties (95 per cent confidence interval) of 2.7  mm  yr⁻¹ at GRAS, 2.3  mm  yr⁻¹ at ZIMM, and 2.9  mm  yr⁻¹ at TORI. The residual velocity in the Eurasian reference frame is statistically greater than zero at ZIMM only, with 2.4 ± 2.3  mm  yr⁻¹ of motion in a NW direction. These uncertainties place an upper bound on the expected deformation in the Western Alps. Velocities relative to stable Eurasia do not exceed 3  mm  yr⁻¹, corresponding to a maximum strain rate of 0.03  μstrain  yr⁻¹ over the GRAS–TORI–ZIMM triangle.     

Simultaneous inversion of seismic data

Summary. The resolving power of different data sets, consisting of surface-wave dispersion measurements and S travel times, are compared for a continental structure. The shear velocity in the low-velocity zone can be resolved in some detail with higher-mode phase-velocity data. Sufficient resolution for small density contrasts (0.03 g cm⁻³) until depths of ∼ 300 km can be reached if higher-mode group velocities are available as well, even at a precision as low as 0.10 km/s. At greater depths the density is not resolved, and here travel-time data are superior to higher modes in resolving the shear velocity.     

Geodetic observations of ice flow velocities over the southern part of subglacial Lake Vostok, Antarctica, and their glaciological implications

In the austral summer seasons 2001/02 and 2002/03, Global Positioning System (GPS) data were collected in the vicinity of Vostok Station to determine ice flow velocities over Lake Vostok. Ten GPS sites are located within a radius of 30 km around Vostok Station on floating ice as well as on grounded ice to the east and to the west of the lake. Additionally, a local deformation network around the ice core drilling site 5G-1 was installed.
The derived ice flow velocity for Vostok Station is  2.00 m a⁻¹± 0.01 m a⁻¹  . Along the flowline of Vostok Station an extension rate of about 10⁻⁵ a⁻¹ (equivalent to 1 cm km⁻¹ a⁻¹) was determined. This significant velocity gradient results in a new estimate of 28 700 years for the transit time of an ice particle along the Vostok flowline from the bedrock ridge in the southwest of the lake to the eastern shoreline. With these lower velocities compared to earlier studies and, hence, larger transit times the basal accretion rate is estimated to be 4 mm a⁻¹ along a portion of the Vostok flowline. An assessment of the local accretion rate at Vostok Station using the observed geodetic quantities yields an accretion rate in the same order of magnitude. Furthermore, the comparison of our geodetic observations with results inferred from ice-penetrating radar data indicates that the ice flow may not have changed significantly for several thousand years.     

Absolute S-velocity estimation from receiver functions

We present a novel method to recover absolute S velocities from receiver functions.
For a homogeneous half-space the S velocity can be calculated from the horizontal slowness and the angle of surface particle motion for an incident P wave. Generally, the calculated S velocity is an apparent half-space value which depends on model inhomogeneity and P -waveform. We therefore, suggest to calculate such apparent half-space S velocities from low-pass filtered (smoothed) receiver functions using a suite of filter-parameters, T . The use of receiver functions neutralize the influence of the P -waveform, and the successive low-pass filterings emphasize the variation of S velocity with depth.
We apply this   V _{S ,app.}( T )  technique to teleseismic data from three stations: FUR, BFO and SUM, situated on thick sediments, bedrock and the Greenland ice cap, respectively. The observed   V _{S ,app.}( T )  curves indicate the absolute S velocities from the near surface to the uppermost mantle beneath each station, clearly revealing the different geological environments. Application of linearized, iterative inversion quantify these observations into   V _S ( z )  models, practically independent of the S -velocity starting model. The obtained models show high consistency with independent geoscientific results. These cases provide also a general validation of the   V _{S ,app.}( T )  method.
We propose the computation of   V _{S ,app.}( T )  curves for individual three-component broad-band stations, both for direct indication of the S velocities and for inverse modelling.     

An anomalous high-velocity layer at shallow crustal depths in the Narmada zone, India

The Narmada zone in central India is a zone of weakness that separates the region of Vindhyan (Meso-Neoproterozoic) deposition to the north from Gondwana (Permo-Carboniferous–lower Cretaceous) deposits to the south. The reinterpretation of analogue seismic refraction data, acquired during the early 1980s, using 2-D ray-tracing techniques reveals a basement (velocity 5.8–6.0 km s⁻¹ ) topography suggesting that the Narmada zone, bounded by the Narmada North and Narmada South faults is a region of basement uplift. A layer of anomalously high velocity (6.5–6.7 km s⁻¹ ) at depths between 1.5 and 9.0 km appears to be present in the entire region. Within the Narmada zone this layer occurs at shallower depths than outside the Narmada zone. At two places within the Narmada zone this layer is at a depth of about 1.5 km. This layer cannot be considered as the top of the lower crust because in this case it should have produced large positive gravity anomalies at the shallowest parts. Instead, these parts correspond to Bouguer gravity lows. Furthermore, lower crust at such shallow depths has not been reported from any other part of the Indian shield. Therefore, this layer is likely to represent the top of a high-velocity mafic body that has different thicknesses in different places.     

Crustal and uppermost mantle structure in southern Africa revealed from ambient noise and teleseismic tomography

Rayleigh wave phase velocity maps in southern Africa are obtained at periods from 6 to 40 s using seismic ambient noise tomography applied to data from the Southern Africa Seismic Experiment (SASE) deployed between 1997 and 1999. These phase velocity maps are combined with those from 45 to 143 s period which were determined previously using a two-plane-wave method by Li & Burke. In the period range of overlap (25–40 s), the ambient noise and two-plane-wave methods yield similar phase velocity maps. Dispersion curves from 6 to 143 s period were used to estimate the 3-D shear wave structure of the crust and uppermost mantle on an 1°× 1° grid beneath southern Africa to a depth of about 100 km. Average shear wave velocity in the crust is found to vary from 3.6 km s^–1 at 0–10 km depths to 3.86 km s^–1 from 20 to 40 km, and velocity anomalies in these layers correlate with known tectonic features. Shear wave velocity in the lower crust is on average low in the Kaapvaal and Zimbabwe cratons and higher in the surrounding Proterozoic terranes, such as the Limpopo and the Namaqua-Natal belts, which suggests that the lower crust underlying the Archean cratons is probably less mafic than beneath the Proterozoic terranes. Crustal thickness estimates agree well with a previous receiver function study of Nair et al. . Archean crust is relatively thin and light and underlain by a fast uppermost mantle, whereas the Proterozoic crust is thick and dense with a slower underlying mantle. These observations are consistent with the southern African Archean cratons having been formed by the accretion of island arcs with the convective removal of the dense lower crust, if the foundering process became less vigorous in arc environments during the Proterozoic.     

A seismic study of deep geological structure in the Bristol Channel area, SW Britain

Summary. Results from eight seismic refraction lines, 35–90 km long, in the Bristol Channel area are presented. The data, mostly land recordings of marine shots, have been interpreted mainly by ray-tracing and time-term modelling. Upper layer velocities through Palaeozoic rocks usually fall within the range 4.8–5.2 km s⁻¹. Below the Carboniferous Limestone with a normal velocity of 5.1–5.2 kms⁻¹, the Old Red Sandstone with a velocity of 4.7–4.8 kms⁻¹ acts as a low velocity layer, as do parts of the underlying Lower Palaeozoic succession. In the central South Wales/Bristol Channel area and the Mendips, a 5.4–5.5 km s⁻¹ refractor is correlated with a horizon at or near the top of the Lower Palaeozoic succession. Under the whole area, except for north Devon, a 6.0–6.2 km s⁻¹ basal refractor has been located and is correlated with Precambrian crystalline basement rocks. In general, this refractor deepens southwards from a series of basement highs, which existed before the major movements of the Variscan orogeny in South Wales, resulting in a southerly thickening of the pre Upper Carboniferous supra-basement sequence. In north Devon, a 6.2 km s⁻¹ refractor at shallow depth, interpreted as a horizon in the Devonian or Lower Palaeozoic succession, overlies a deep reflector that may represent the Precambrian crystalline basement.     

Kinematic multistation discriminator between local quarry blasts and earthquakes

It has long been known that S waves on seismograms of local explosions are often accompanied by strong low-frequency, low-velocity, Rg surface wave trains, often significantly diminished for earthquakes. We utilize this fact to construct a new formal discriminator between earthquakes and explosions by measuring the S -surface-wave group velocity. The method is based on analysing the velogram; that is, the display of the envelope of ground motion versus group velocity V = R/T , where R is the epicentral distance and T  the traveltime. We examine the distribution of seismic energy in time and space using envelopes of records from the Israel Seismic Network (ISN), from which we compute the velograms and observe differences in the velograms of quarry blasts and earthquakes. The data include 143 seismic events occurring in three areas (Galilee, Dead Sea, and Gilad) monitored by the ISN; the magnitude range is M _L = 1.0–2.8 at distances of 15–310 km. From the velograms we measure the group velocity, V _{m s} , within the 1–4 km s⁻¹ range at which the velogram reaches its maximum for each available station. The resulting V _{m s} (R) function is closely fitted by the empirical relationship a + b  ln  R , with a and b coefficients varying from event to event. A simple linear function c = b + 0.33a at a threshold C = 0.69 completely separates ( a,b ) pairs for the 67 Galilee events, and, for the 76 remaining events, one earthquake and four explosions are wrongly classified. After data validation and application of the Fisher linear discriminator, adapted to the events from Galilee, only two misidentified events remain for the whole data set.     

Waveform modelling of teleseismic S, Sp, SsPmP, and shear-coupled PL waves for crust- and upper-mantle velocity structure beneath Africa

We describe a waveform modelling technique and demonstrate its application to determine the crust- and upper-mantle velocity structure beneath Africa. Our technique uses a parallelized reflectivity method to compute synthetic seismograms and fits the observed waveforms by a global optimization technique based on a Very Fast Simulated Annealing (VFSA). We match the S , Sp, SsPmP and shear-coupled PL phases in seismograms of deep (200–800 km), moderate-to-large magnitude (5.5–7.0) earthquakes recorded teleseismically at permanent broad-band seismic stations in Africa. Using our technique we produce P - and S -wave velocity models of crust and upper mantle beneath Africa. Additionally, our use of the shear-coupled PL phase, wherever observed, improves the constraints for lower crust- and upper-mantle velocity structure beneath the corresponding seismic stations. Our technique retains the advantages of receiver function methods, uses a different part of the seismogram, is sensitive to both P - and S -wave velocities directly, and obtains helpful constraints in model parameters in the vicinity of the Moho. The resulting range of crustal thicknesses beneath Africa (21–46 km) indicates that the crust is thicker in south Africa, thinner in east Africa and intermediate in north and west Africa. Crustal P - (4.7–8 km s⁻¹) and S -wave velocities (2.5–4.7  km s⁻¹) obtained in this study show that in some parts of the models, these are slower in east Africa and faster in north, west and south Africa. Anomalous crustal low-velocity zones are also observed in the models for seismic stations in the cratonic regions of north, west and south Africa. Overall, the results of our study are consistent with earlier models and regional tectonics of Africa.     

Lithospheric structure of the continent–continent collision zone: eastern Turkey

We infer the lithospheric structure in eastern Turkey using teleseismic and regional events recorded by 29 broad-band stations from the Eastern Turkey Seismic Experiment (ETSE). We combine the surface wave group velocities (Rayleigh and Love) with telesesimic receiver functions to jointly invert for the S -wave velocity structure, Moho depth and mantle-lid (lithospheric mantle) thickness. We also estimated the transverse anisotropy due to Love and Rayleigh velocity discrepancies. We found anomalously low shear wave velocities underneath the Anatolian Plateau. Average crustal thickness is 36 km in the Arabian Plate, 44 km in Anatolian Block and 48 km in the Anatolian Plateau. We observe very low shear wave velocities at the crustal portion (30–38 km) of the northeastern part of the Anatolian Plateau. The lithospheric mantle thickness is either not thick enough to resolve it or it is completely removed underneath the Anatolian Plateau. The shear velocities and anisotropy down to 100 km depth suggest that the average lithosphere–asthenosphere boundary in the Arabian Plate is about 90 and 70 km in Anatolian block. Adding the surface waves to the receiver functions is necessary to constrain the trade-off between velocity and the thickness. We find slower velocities than with the receiver function data alone. The study reveals three different lithospheric structures in eastern Turkey: the Anatolian plateau (east of Karliova Triple Junction), the Anatolian block and the northernmost portion of the Arabian plate. The boundary of lithospheric structure differences coincides with the major tectonic boundaries.     

The crustal structure of the eastern Marmara region, Turkey by teleseismic receiver functions

We study the crustal structure of eastern Marmara region by applying the receiver function method to the data obtained from the 11 broad-band stations that have been in operation since the 1999 İzmit earthquake. The stacked single-event receiver functions were modelled by an inversion algorithm based on a five-layered crustal velocity model to reveal the first-order shear-velocity discontinuities with a minimum degree of trade-off. We observe crustal thickening from west (29–32 km) to east (34–35 km) along the North Anatolian Fault Zone (NAFZ), but we observe no obvious crustal thickness variation from north to south while crossing the NAFZ. The crust is thinnest beneath station TER (29 km), located near the Black Sea coast in the west and thickest beneath station TAR (35 km), located inland in the southeast. The average crustal thickness and S -wave velocity for the whole regions are  31 ± 2  km and  3.64 ± 0.15 km s⁻¹  , respectively. The eastern Marmara region with its average crustal thickness, high heat flow value (101 ± 11 mW m⁻²) and with its remarkable extensional features seems to have a Basin and Range type characteristics, but the higher average shear velocities (∼3.64 km s⁻¹) and crustal thickening from 29 to 35 km towards the easternmost stations indicate that the crustal structure shows a transitional tectonic regime. Therefore, we conclude that the eastern Marmara region seems to be a transition zone between the Marmara Sea extensional domain and the continental Anatolian inland region.