Lateral Variations In Structure Research Articles

UK crustal structure close to the Iapetus Suture: a receiver function perspective

SUMMARY Teleseismic receiver functions have been calculated for data from 10 short-period three-component seismic recording stations across northern Britain to investigate variations in crustal and upper-mantle velocity structure. The stations straddle the Iapetus Suture zone, the inferred boundary between two of the continents fused together during the Caledonian Orogeny. The receiver function data shows that there is considerable azimuthal variation in both crustal and upper-mantle structure beneath several stations. The data are projected on to a 2-D profile, showing laterally continuous Moho Ps conversions at delay times between 3 and 4 s. Synthetic receiver functions, generated using the velocity model from a previous deep seismic reflection/refraction survey show Ps and PpPms phases comparable to the observed data. 1-D forward modelling of the data gives crustal thicknesses of ∼30 km. There is a significant velocity–depth trade-off in the receiver function method, and the crustal thicknesses have been constrained by a priori velocity information. Investigation of the data from station GIM close to the Iapetus Suture shows a sub-Moho phase, which is found only on data from northerly backazimuths. Phase modelling is consistent with the presence of a gradational low-velocity zone with a minimum Vp of 6.5 km s−1 at a depth of ∼43 km. This feature has similar characteristics to the wide-angle seismic reflection and refraction velocity model of the W-reflector in northern Scotland. The large-scale heterogeneity at GIM is attributed to structures associated with the Iapetus Suture. However, modelling has been performed using 1-D approximations, while the phase signature could result from complex 3-D morphology. We therefore conclude only that the results provide evidence of significant lateral variation in subcrustal structure.

Active tectonics in Central Italy: constraints from surface wave tomography and source moment tensor inversion

We investigate the lithosphere–asthenosphere structure and active tectonics of Central Italy, with emphasis on the Umbria–Marche area, by means of surface wave tomography and seismic moment tensor inversion. The data include: a large number of short period local and regional group velocity measurements sampling the Umbria–Marche Apennines and the Adria margin, respectively; incorporation of published phase velocity measurements sampling Italy and surroundings; results from deep seismic soundings which go through the Umbria–Marche area. The local group velocity maps, covering the area reactivated by the 1997–1998 Umbria–Marche earthquake sequence, suggest an intimate relationship between the lateral earth structure variations and the distribution of the active fault systems and related sedimentary basins. The upper crustal models reveal the importance of inherited compressional tectonics on the recent extensional deformation and associated seismic activity. Source inversion studies of the main events of the 1997 earthquake sequence show the dominance of normal faulting mechanisms, whereas selected aftershocks between the fault segments, at the step-over, reveal that the prevailing deformation is of strike-slip faulting type. At the regional scale, the crust exhibits clear layering and varies in thickness from about 25 km below the Tuscan Metamorphic Complex (TMC), to about 30 km below the Val Tiberina extensional thick sedimentary basin and reaches about 35 km below the Umbria–Marche geological domain (UMD). The lithospheric mantle (lid) is thin (about 30 km) below TMC, while it is about 70 km thick below UMD. A lithospheric root about 120 km wide, between the TMC and UMD, reaches a depth of at least 130 km. A low-velocity zone, defined mantle wedge ( V S less than 4.2 km/s) in the uppermost mantle overlying the high velocity lid is detached. This wedge is about 20 km thick and decouples the underlying lid from the crust. The retrieved crust and upper mantle structure beneath Central Italy is in agreement with Bouguer anomaly and heat flow data and supports a delamination process. The high velocity upper mantle underlying the mantle wedge is inferred to be subcrustal lithospheric material delaminated from the overlying crust.

Lateral variations of shallow shear-velocity structure in southwestern Taiwan inferred from short-period Rayleigh waves

Short-period fundamental-mode Rayleigh waves with periods of 1.1∼5.5 sec were used to investigate the lateral variations of shallow-depth shear-wave velocity structure up to a depth of 8 km under southwestern Taiwan. Through a priori regionalization, the region was divided into three subregions from the west to the east, and then the regionalized group velocity for each subregion was determined by a standard least-squares technique. By the structure inversion, the study region had obviously lateral velocity variations, which systemically increased from the west region to the east one. On the whole, the shear-wave velocity in the Western Foothills was higher than that in the Western Coastal Plain. Additionally, the three subregions all had a shear-wave velocity of less than 3 km/sec with the lowest one in the Western Coastal Plain near the coast, related to the thick sediments. These results were rather consistent with the geological features. For depths larger than 4 km, the velocity-gradient varying with depth in the Western Foothills was lower than that in the Western Coastal Plain. This is likely to interpret the reason that the seismic waves cannot be easily trapped within the Western Foothills; thus the short-period surface waves are poorly developed in that region.

Testing group velocity maps for Eurasia

SUMMARY Group velocity maps for seismic surface waves play an important role in monitoring the Comprehensive Test Ban Treaty so their accuracy is crucial. Group velocity anomalies can be modelled in terms of lateral variations in crustal and shallow mantle structure, the knowledge of which is important for understanding wave propagation and the blockage of regional phases. Accurate group velocity maps are also indispensable tools in attempts to lower the detection threshold for seismic events and to distinguish between explosions and earthquakes. This paper investigates the feasibility of validating existing maps using a relatively small data set of pathaveraged group traveltime data. We find that group velocity correction surfaces calculated for two sets of global maps and a set of regional maps in Eurasia exhibit significant differences. We compare our measurements with predictions from these maps and test whether any of these maps is consistent with our data. Large differences between measurements and predictions can occur for selected individual paths across Eurasia and we find that maps resulting from global inversions fit our data best. There are visually only subtle differences between global and regional maps but we speculate that the long-wavelength structure is relatively poorly constrained in the regional maps.

Extended time-term method for identifying lateral structural variations from seismic refraction data

A time-term method is a simple travel time inversion for seismic refraction crustal studies. Although this method is based on simple travel time calculation, it is robust in determining a basement velocity under the complicated surface structure. In the present paper, this method is extended to detect lateral velocity variation in the crystalline crust. We consider a model where a basement is composed of segments with a different velocity. Defining a likelihood function for unknown time-terms and refractor velocities, their best estimates are obtained by minimizing Akaike’s Bayesian Information Criterion (ABIC). To realize accurate velocity estimation, we also incorporate an effect of vertical velocity gradient within the refractor, which was ignored in the conventional algorithms. Numerical tests undertaken for realistic crustal structures show that our method successfully detects lateral velocity variation in a basement even for a case with undulated surface layers. It is particularly stable and reliable if the surface layers are not thick and their velocities are more than 30—40% smaller than the refractor velocities. Then, the refractor velocities are almost decoupled to the time-terms in the process of inversion. The incorporation of the effect of the velocity gradient is inevitably important when travel time data at far distances are included in the inversion.

Crustal and upper mantle seismic structure and lithospheric flexure along the Society Island hotspot chain

Summary Four wide-angle and refraction lines have been shot along the Society Island hotspot track. A 280 km long profile covers oceanic crust to the east of the islands, the active hotspot region, and the apron of Tahiti. The other profiles cover the region between Moorea and Huahine and the western archipelago of Huahine, Raiatea, Bora Bora, and adjacent oceanic crust. Traveltime modelling provided lateral variations in crustal structure along the hotspot chain. Removal of the contribution of the uppermost low-velocity layer, which represents the volcanic edifices and debris, suggests that the islands and atolls sit on normal oceanic crust and are surrounded by a large volcaniclastic apron. Seismic profiles running between Raiatea and Huahine, and between Raiatea and Moorea cover a region where extreme down-bending due to volcanic loading is expected. A nearly flat crust–mantle boundary indicates regional isostasy and clearly supports a stiff plate. Seismic structural models have been compared with calculated boundaries based on simple 3-D elastic plate models. A good fit to the seismic crust–mantle boundary was obtained for models with an effective elastic thickness of the lithosphere of 20–25 km. These values are in accordance with a model in which the elastic thickness depends on the thermal age of the lithosphere, and therefore contradict models that suggest that the region of French Polynesia has been thermally rejuvenated. Amplitude modelling of crustal and upper mantle arrivals suggests that crust on the hotspot trail has been modified with respect to the pre-hotspot crust. To explain the observations we suggest that, during the ongoing late-stage volcanic activity, the lower crust has been intruded by sills, and some melts are trapped in the crust–mantle boundary region. If late-stage volcanism and plutonism continue, prolonged crustal growth may form a large underplate or subcrustal plutonic core complex along the hotspot chain.

Inversion of seismic refraction and wide-angle reflection traveltimes for three-dimensional layered crustal structure

We present a method for the determination of crustal structure by simultaneous inversion of seismic refraction and wide-angle reflection traveltimes for 3-D interface geometry and layer velocity. Crustal structure is represented by layers in which velocity varies linearly with depth, separated by smooth interfaces with a cubic B-spline parametrization. Lateral variations in structure are therefore represented by variations in interface depth only. The model parametrization we have chosen means that ray paths consist of piecewise circular arc segments for which analytic expressions of trajectory and traveltime are calculated. The two-point problem of finding the first-arrival ray path and traveltime of a specified phase between a given source and receiver is solved using a shooting technique. A subspace inversion method is used to solve the inverse problem, which is formulated as a non-linear optimization problem in which we seek to minimize an objective function that consists of a data residual term and a regularization term. Before performing the inversion, each data pick must be assigned as a refraction or reflection from a particular layer or interface. Since our method represents structure in terms of interfaces, fewer parameters would generally be used in a reconstruction compared to an equivalent 3-D variable-velocity inversion. The method is well suited to wide-angle surveys that consist of many sources and relatively few receivers (or vice versa), such as marine shot lines used in conjunction with land-based receivers. Data coverage in this kind of survey is often sparse and, especially if near-offset ray paths are unavailable, highly variable. A 3-D synthetic test with an array consisting of eight sources lying within a three-sided square of 79 receivers is described. The test model consists of a three-interface structure that includes a layer pinch-out, and the synthetic data set comprises 987 refraction and 930 reflection travel times contaminated with 75 ms of data noise. Six iterations of an 18-D subspace method demonstrate that the method can produce an accurate reconstruction that satisfies the data from a 1-D starting model. We also find that estimates of a posteriori model covariance and resolution obtained from linear theory are useful in analysing solution reliability despite the non-linear nature of the problem. Application of the method to data collected as part of the 1995 TASGO project in Tasmania shows that the method can satisfy 1345 refraction and reflection traveltime picks with a geologically reasonable and robust 254-parameter three-interface model. The inversion results indicate that the Moho beneath NW Tasmania varies in depth from 27 km near the coast to 37 km near central Tasmania, with the major increase in depth occurring across the Arthur Lineament.

Global Event Location with Full and Sparse Data Sets Using Three-dimensional Models of Mantle P-wave Velocity

In order to improve on the accuracy of event locations at teleseismic distances it is necessary to adequately correct for lateral variations in structure along the ray paths, either through deterministic model-based corrections, empirical path/station corrections, or a combination of both approaches. In this paper we investigate the ability of current three-dimensional models of mantle P-wave velocity to accurately locate teleseismic events. We test four recently published models; two are parameterized in terms of relatively long-wavelength spherical harmonic functions up to degree 12, and two are parameterized in terms of blocks of constant velocity which have a dimension of a few hundreds of km. These models, together with detailed crustal corrections, are used to locate a set of 112 global test events, consisting of both earthquakes and explosions with P-wave travel-time data compiled by the International Seismological Centre (ISC). The results indicate that the supposedly higher resolution block models do not improve the accuracy of teleseismic event locations over the longer wavelength spherical harmonic models. For some source locations the block models do not predict the range of observed travel-time residuals as well as the longer wavelength models. The accuracy of the locations largely varies randomly with geographic position although events in central Asia are particularly well located. We also tested the effect of reduced data sets on the locations. Multiple location iterations using 30 P-wave travel times indicate that teleseismic events may be located within an area of 1000 km’ of the true location 66% of the time with only the model-based corrections, and increasing to 75% if calibration information is available. If as few as 8 phases are available then this is possible only 50% of the time. Further refinement in models and/or procedure, such as the addition ofP“phases, azimuth data, and consideration of P-wave anisotropy may provide further improvement in the teleseismic location of small events.

Lithospheric memory and stress field controls on polyphase deformation of the Pannonian basin–Carpathian system

The Pannonian basin–Carpathian system displays pronounced lateral variation in lithopsheric structure and sedimentary basin configuration. The tectonic history of the system is characterised by a polyphase history with Late Miocene extension in the Pannonian basin and simultaneously occurring compression in the Carpathian arc. Pre-existing structures play a key role in subsequent basin reactivation, explaining anomalous features in subsidence characteristics and inferred thinning factors. The area exhibits a relatively high level of neotectonic activity, imaged by high resolution seismic data, intraplate seismicity and stress indicator data. Major parts of the system have experienced a rapid late Neogene uplift and erosion, amounting up to several kilometres in the Romanian Carpathians. A major part of the present basin configuration, previously primarily attributed to basin formation mechanisms such as stretching in the Pannonian basin and lithospheric flexure in the Carpathians, appears to be strongly affected by recent deformation, postdating basin formation of the Pannonian basin and the Carpathian foredeep. These findings affect modelling predictions of basin subsidence, sediment sourcing, thermal evolution and hydrodynamic regime with implications for hydrocarbon play-concepts in the area.

Lateral variations of the upper mantle structure in Eurasia from group velocities of surface waves

Group velocities of both Rayleigh and Love waves in the 20–70 s period range along paths across Asia and the Eastern Europe are used to obtain group velocity maps of the region. For this purpose, we developed a method for tomographic inversion on spherical surface, which is practically free of a priori assumptions. Checker-board test shows that velocity anomalies of 10°×10° size can be resolved. The ‘local’ dispersion curves are inverted to velocity-depth curves up to 160 km depth along three profiles: (1) Tibet, the South China Platform and the southern part of the Japan Sea; (2) the West Siberian Platform, the Tienshan and the Pamir; (3) the East European Platform and the Black Sea Basin. Lateral S-wave velocity variations are presented at the depths of 60, 90 and 120 km. A low velocity zone in asthenosphere under Japan Sea is extended under the South China and the North China–Korean Platforms. The upper mantle structure of the Russian (East European) and the West Siberian Platforms is different: a low-velocity zone at 120–140 km depths is observed under the West Siberian Platform, whereas the velocity under the Russian Platform increases monotonously with depth. A low velocity layer appears under the Black Sea indicating an oceanic structure of the Basin. Low surface wave velocities in Tibet are caused not only by a thick crust, but also by low S-wave velocities in the upper mantle. The Precambrian West Siberian, North Indian and North China–Korean Platforms are characterized by high velocities at the depth of 120 km unlike the younger tectonic units.

Deep crustal structure in the southwestern Yilgarn Craton, Western Australia

The Archaean rocks of the Yilgarn Craton comprise the largest geological entity in Western Australia and one of the largest areas of Archaean crust anywhere in the world. However, poor outcrop has prevented a thorough understanding of the structure of the crust in the region, a subject of particular interest being the relationship between high-grade metamorphic rocks in the west, and lower-grade granitoid-greenstone terrains further east. Interpretation of previously unpublished seismic refraction data, combined with the re-interpretation of published data, allows crustal velocity structure in the southwest of the Craton to be resolved. Overall, the structure is similar to shield areas elsewhere in the world, the data defining a two-layered crust with an average thickness of about 35 km. However, significant lateral variations in velocity structure are also observed, and these coincide with a terrane boundary previously postulated on the basis of geological observations and seismic reflection data. The terrane boundary dips to the east, extends through the entire crust, and is notable for an area of anomalously high seismic velocity in the lower crust. Two possible interpretations of the high-velocity zone are proposed. Firstly, it is due to the presence of mafic to ultramafic intrusions. Secondly, and the preferred interpretation, the high-velocity zone is a fault-bounded mega-sliver, which may be a suspect terrane in its own right, perhaps with oceanic affinities. The results of the seismic experiment strongly support tectonic models for Yilgarn Craton based on terrane tectonics. The possibility that the terrane boundary is being reactivated by contemporary stresses also provides a plausible explanation for the local intra-plate seismicity.

A reappraisal of postglacial decay times from Richmond Gulf and James Bay, Canada

Decay times inferred from relative sea-level (RSL) histories of previously glaciated regions provide a potentially important constraint on mantle rheology. We present a new compilation of RSL data from Richmond Gulf and James Bay, Canada. This recompilation reveals errors in previous compilations that led to inaccurate estimates for the Richmond Gulf decay time in a series of recently published articles. We derive updated estimates for the decay time at Richmond Gulf and James Bay using a methodology that incorporates errors in both the age and the height of the sea-level markers. This exercise is guided by a series of synthetic RSL calculations that show that decay time estimates in the region can be significantly biased if the RSL time-series are not corrected for global eustatic sea-level trends, or if the estimates are based on composite RSL histories derived by combining data from both the Richmond Gulf and the James Bay regions. Our decay time analysis for Richmond Gulf applies the pioneering approach of Walcott (1980) to a large database and we derive a value of 4.0-6.6 kyr, where the range is defined by a misfit tolerance 10 per cent higher than the minimum. Our analysis for James Bay is based on the uplift curve derived by Hardy (1976), and we estimate a decay time of about 2.0-2.8 kyr. The difference between our estimates for Richmond Gulf and James Bay may be due to errors in the observational record from these regions, but could also be influenced by lateral variations in lithospheric structure associated with the assembly of Laurentia.

Slow differential rotation of the Earth's inner core indicated by temporal changes in scattering

The finding that the Earth's inner core might be rotating faster than the mantle has important implications for our understanding of core processes, including the generation of the Earth's magnetic field. But the reported signal is subtle--a change of about 0.01 s per year in the separation of two seismic waves with differing paths through the core. Subsequent studies of such data have generally supported the conclusion that differential rotation exists, but the difficulty of accurately locating historic earthquakes and possible biases induced by strong lateral variations in structure near the core-mantle boundary have raised doubt regarding the proposed inner-core motion. Also, a study of free oscillations constrained the motion to be relatively small compared to previous estimates and it has been proposed that the interaction of inner-core boundary topography and mantle heterogeneity might lock the inner core to the mantle. The recent detection of seismic waves scattered in the inner core suggests a simple test of inner-core motion. Here we compare scattered waves recorded in Montana, USA, from two closely located nuclear tests at Novaya Zemlya, USSR, in 1971 and 1974. The data show small but coherent changes in scattering which point toward an inner-core differential rotation rate of 0.15 degrees per year--consistent with constraints imposed by the free-oscillation data.

Seismic signature of Variscan and Alpine tectonics in NW Iberia: Crustal structure of the Cantabrian Mountains and Duero basin

The seismic structure of the crust of the NW Iberian Peninsula, along the Cantabrian Mountains and their southern foreland, the Duero basin, is investigated from the first data set available from refraction and wide‐angle reflection profiles. The velocity‐depth distributions obtained along E‐W and N‐S transects evidence lateral variations in crustal structure that can be associated to Variscan and Alpine tectonic episodes. The western part of the range, toward the Variscan hinterland zones, as well as the Duero basin below the sediments, show a crustal structure similar to that of the Variscan belt elsewere in Europe. The total crustal thickness is 30–32 km, and three main layers (upper, middle, and lower crust with average velocities 6.0, 6.25, and 6.8 km/s) are resolved. Some Paleozoic structures such as the basal detachment of the Cantabrian Zone are expressed in the seismic models. The crust appears to vary eastward in the external zones. The most outstanding feature is a crustal root beneath the highest Cantabrian summits where the Moho is found at 47 km depth. A prominent Alpine reworking of the crust in the eastern part of the Cantabrian Zone is thus revealed, and the N‐S seismic transects delineate a geometry similar to the one found across the central Pyrenees in the Etude Continentale et Océanique par Reflexion et Refraction Sismique (ECORS) profile, revealing the importance of the Alpine deformations in the northern part of the Iberian plate.

Moho depth variation in southern California from teleseismic receiver functions

The number of broadband three‐component seismic stations in southern California has more than tripled recently. In this study we use the teleseismic receiver function technique to determine the crustal thicknesses and Vp/Vs ratios for these stations and map out the lateral variation of Moho depth under southern California. It is shown that a receiver function can provide a very good “point” measurement of crustal thickness under a broadband station and is not sensitive to crustal P velocity. However, the crustal thickness estimated only from the delay time of the Moho P‐to‐S converted phase trades off strongly with the crustal Vp/Vs ratio. The ambiguity can be reduced significantly by incorporating the later multiple converted phases, namely, the PpPs and PpSs+PsPs. We propose a stacking algorithm which sums the amplitudes of receiver function at the predicted arrival times of these phases by different crustal thicknesses H and Vp/Vs ratios. This transforms the time domain receiver functions directly into the H‐Vp/Vs domain without need to identify these phases and to pick their arrival times. The best estimations of crustal thickness and Vp/Vs ratio are found when the three phases are stacked coherently. By stacking receiver functions from different distances and directions, effects of lateral structural variation are suppressed, and an average crustal model is obtained. Applying this technique to 84 digital broadband stations in southern California reveals that the Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are found under the eastern Transverse Range, the Peninsular Range, and the Sierra Nevada Range. The central Transverse Range, however, does not have a crustal root. Thin crusts exist in the Inner California Borderland (21–22 km) and the Salton Trough (22 km). The Moho is relatively flat at the average depth in the western and central Mojave Desert and becomes shallower to the east under the Eastern California Shear Zone (ECSZ). Southern California crust has an average Vp/Vs ratio of 1.78, with higher ratios of 1.8 to 1.85 in the mountain ranges with Mesozoic basement and lower ratios in the Mojave Block except for the ECSZ, where the ratio increases.

Long‐wavelength topographic relaxation for self‐gravitating planets and implications for the time‐dependent compensation of surface topography

The support of planetary surface topography is controlled by the thickness of the elastic lithosphere and thus has implications for the thermomechanical structure of shallow planetary interiors. Previous analyses of the support of long‐wavelength topography have generally utilized elastic formulations, which preclude consideration of the time evolution of lithospheric stresses and thermal state. Here we formulate a viscoelastic model for the support of topography on a spherical planet. Our stress relaxation model employs a multilayer linear viscoelastic rheology and includes the effect of self‐gravitation. In these models we approximate internal rheology with an olivine flow law, and we estimate the internal thermal structure assuming simple conductive cooling. We analyze how relief at the surface and internal density interfaces evolves with time in response to a surface or internal load. Our analysis demonstrates that crustal compensation is strongly dependent on planetary radius, with a tendency for a large terrestrial planet like Earth to reach complete Airy isostasy at long wavelengths within a time scale controlled by the mantle viscosity. The correlation of degree of compensation with planetary radius is consistent with previous work [Turcotte et al., 1981], which showed that membrane stress can support significant long‐wavelength gravity anomalies. Applied to the Moon, the degree of compensation associated with loading of 100‐Ma lithosphere is less than 0.3, and the load Love numbers are greater than −0.6 for all harmonics including degree 2, even after 109 years of relaxation. This conclusion does not change significantly even with a moderately low viscosity for the crust. The tendency for a small body like the Moon to support long wavelength stresses may be relevant in understanding the anomalously large degree 2 terms in the lunar shape. The results also suggest that major impact basins would not be compensated throughout lunar geological history (>4 b.y.), even if the surfaces on which the basins were initially formed were relatively young (i.e., 100–300 Ma). The fact that the South Pole‐Aitken basin is largely compensated suggests that the lateral variations in thermal and/or mechanical structure induced by impact processes play significant roles in the compensation of this structure.

InGaN/GaN quantum well growth on pyramids of epitaxial lateral overgrown GaN

InGaN/GaN quantum wells (QW) were grown by metalorganic chemical vapor deposition (MOCVD) on pyramids of epitaxial lateral overgrown (ELO) GaN samples. The ELO GaN samples were grown by MOCVD on sapphire (0001) substrates that were patterned with a SiNx mask. Scanning electron microscopy and cathodoluminescence (CL) imaging experiments were performed to examine lateral variations in structure and QW luminescence energy. CL wavelength imaging (CLWI) measurements show that the QW peaks on the top of the grooves are red-shifted in comparison with the QW emission from the side walls. The results show that In atoms have migrated to the top of the pyramids during the QW growth. The effects of V/III ratio, growth temperature as well as ELO GaN stripe orientation on the QW properties are also studied.

Three-dimensional crustal structure beneath the TOR array and effects on teleseismic wavefronts

The temporary seismic station array (TOR) was designed to study the lithosphere–asthenosphere system across the northwestern part of the Trans-European Suture Zone (TESZ) by teleseismic tomography. Teleseismic wavefronts, when propagating through complex crustal structure, undergo severe distortion that may result in travel time residual anomalies of significant amplitude. The inversion of teleseismic travel time residuals for deep structures without accounting for such crustal-related anomalies may erroneously map these travel time anomalies into features at greater depth. In this study we apply a three-dimensional (3-D) technique to estimate effects of a priori known 3-D crustal structure on travel times of teleseismic waves observed at the TOR seismic array across the TESZ to correct for these effects in future tomographic studies.A uniform 3-D crustal model is developed by use of published two-dimensional crustal models from previous active seismic surveys. The parameterization of this 3-D crustal model is designed to adequately represent those crustal structures that mostly influence the propagation of teleseismic wavefronts. The 3-D model includes lateral variation in velocity structure, Moho topography, and large and deep sedimentary basins. The teleseismic forward problem for this local 3-D model is solved by calculation of travel times to the base of the model using a standard whole Earth model and by subsequent propagation of spherical wavefronts using finite difference methods. Travel time calculations for an event near Japan reveal significant lateral variations in the range between −0.3s and +0.5s due to crustal structures. Being able to obtain the full travel time field at the surface of the model has the additional advantage of improving the identification and timing of seismic phases observed at the TOR seismic array.

Crustal Shortening on the Margins of the Tien Shan, Xinjiang, China

We present the results of mapping selected cross-sections across the margins of the Chinese Tien Shan, an intracontinental mountain belt that formed in response to the India-Eurasia collision. This belt contains significant lateral variation in topography, structure, and stratigraphy at all scales, and our estimated rates of shortening also reveal a distribution of shortening that varies laterally. At the largest scale, it consists of two major high mountain ranges in the west that merge eastward into a complex, single high mountain belt with several distinct ranges, then separates farther eastward into several low mountain ranges in the south and a single narrow high mountain range in the north. Active fold-and-thrust belts along parts of the north and south flanks of the Tien Shan involve only Mesozoic and Cenozoic sedimentary cover, which varies in both stratigraphy and structure from east to west. The southern fold-and-thrust belt decreases in width and complexity from west to east and ends before rea...

Teleseismic P-residual study in the Italian region - inferences on large scale anisotropic structure of the subcrustal lithosphere

Jeffreys-Bullen (absolute) and relative P-wave travel-time residuals were analyzed over Italy and its surrounding using P arrival times from the ISC bulletins supplemented by the data from local observatories. We analyzed the travel-time station corrections by two independent methods to obtain information on lateral variations of the velocity structure over the area and a view of possible upper mantle anisotropy. In the first method, the station corrections are computed as a constant and two cosine terms with appropriate phase shifts. Besides a static term, the second method allows us to study the relative residuals in dependence both on azimuths and incidence angles and thus to investigate their spatial variations and to map lateral variations of anisotropic structure of the subcrustal lithosphere. The high and low-velocity directions inferred from the spatial distribution of the relative residuals as well as the high- and low-velocity upper mantle heterogeneities reflect the geodynamic development of the region, governed by the collision between the African and Eurasian plates