Crustal structure of the Peninsular India

Summary Earthquake parameters for the forty aftershocks of the main Koyna earthquake of 10 December, 1967, have been determined. Depths of the foci of the earthquakes have been found to vary between 2 to 17 km. The velocities for the phasesP _g ,P ^*,P _n have been observed to be 5.78±0.00, 6.58±0.04, 8.19±0.02 km/sec, and forS _g ,S ^*,S _n to be 3.42±0.00, 3.92±0.01 and 4.62±0.01 km/sec respectively. A two-layered crustal model has been interpreted for the Peninsular shield with the average thickness of the granitic layer as 20 km and that for the basaltic layer as 18.7 km. A plot of the epicenters suggests a NNE to SSW orientation of the fault.     

Ray parameters of diffracted long period P and S waves and the velocities at the base of the mantle

The derivation of P and S velocities at the core-mantle boundary (CMB) from long-period diffracted waves by the use of the simple ray-theoretical formulav _CMB=r _c /p (v _CMB=velocity at the CMB;r _c =core radius;p=ray parameter) yields apparent velocity values which differ from the true velocities. Using a dominant period of about 20 sec for calculating theoretical seismograms, we found a linear relation between the apparent velocity and the average velocity in a transition zone at the base of the mantle with fixed velocity on top.The ray parameters determined from long-period earthquake data are found to be 4.540±0.035 and 8.427±0.072 sec/deg for P_diff and S_diff, respectively. These values yield apparent velocities of 13.378±0.103 for P and 7.207±0.062 km/sec for S waves. By means of the theoretical relation between apparent and average velocity and under the assumption of linear variation of velocity with depth, one can invert the apparent velocities into true CMB velocities of 13.736±0.170 and 7.320±0.124 km/sec. These results imply positive velocity gradients at the base of the mantle and hence no significant departures from adiabaticity and homogeneity.Contribution No. 211 of the Geophysical Institute, University of Karlsruhe.     

A crust-mantle model for the NORSAR area

Summary Elastic waves from explosions were recorded at NORSAR and at a number of field stations, and the data were used for determining a crust-mantle model under the array. The number of explosions was eleven distributed on seven shot points. The total number of recording points was fifty-one, and the interpretation was based on 350 individual records.The velocities obtained for the crustal phases were 6.2, 6.6 and 8.2 km/sec for theP _g ,P _g andP _n waves respectively. A deep crustal phase with a velocity of about 7.4 km/sec was observed. The mean depths to the discontinuities within the crust were determined to be 17 and 26 km. The depth to Moho varied greatly across the array from 31.5 km in the central part to 38 km under the C-ring. The maximum dip observed for the Moho was 12^o.Contribution No. 57 to Norwegian Geotraverse Project.     

Variations in the upper mantle structure as derived fromP n andS n waves

Summary Observed travel-times ofP _n andS _n waves are used to determine the ratio ofP _n toS _n velocities, Poisson's ratio and the ratio of incompressibility to shear modulus for Europe, Japan, North America and Scandinavia. The analysis of the data reveals that theP _n toS _n velocity ratio decreases when theP _n velocity increases for Scandinavia and North America, while the velocity ratio increases when theP _n velocity increases for Europe and Japan.     

A study of the crustal structure beneath the Himalayas from body waves

Summary The crustal structure beneath the Himalayas has been investigated using body wave data from near earthquakes having epicentres over the Himalayas and recorded by the observatories situated over, or very near, the foothills of the mountains. A three-layered crustal model, without the top sedimentary layer, with velocities for theP wave group in Granite I, Granite II and the Basaltic layer as 5.48, 6.00 and 6.45 and for theS wave group as 3.33, 3.56 and 3.90 km/sec respectively, has been interpreted. The upper mantle velocity for theP wave has been observed to be 8.07 km/sec and for theS wave as 4.57 km/sec. Average thickness for the Granite I layer has been computed as 22.7 km, for the Granite II layer as 16.3 km and for the Basaltic layer as 18.7 km. Crustal and sub-crustal velocities indicate a lower trend under the mountain. A thicker crust has been obtained beneath the Himalayas.     

Elastic properties of polycrystalline diopside (CaMgSi2O6)

A polycrystalline specimen of clinopyroxene diopside has been hot-pressed at P = 15 kbar and T = 850°C in a piston-cylinder apparatus. Compressional (ν_P) and shear (ν_S) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The velocities at 7.5 kbar are ν_P = 8.06 km/sec and ν_S = 4.77 km/sec. These data are consistent with velocity-density trends for orthopyroxenes due to the compensating effects of the monoclinic structure (positive) and Ca content (negative). With the addition of the new data for diopside, it is possible to calculate directly the velocities of various upper-mantle mineral assemblages.     

Features of deep crustal structure and the onshore-offshore transition at the Iberian flank of the Valencia Trough (Western Mediterranean)

First results are presented of a recent onshore seismic survey complementary to the Valsis-2 Cruise, which consisted of ESP, COP and CDP marine seismic profiles across the Valencia Trough (Western Mediterranean).The marine energy source used was an airgun array of 5800 cubic inch recorded at 2 land stations on the western flank of the Valencia Trough, at distances between 10–120 km.The experiment has resulted in an extended sampling of the deep crustal structure of the eastern Mediterranean flank of the Iberian peninsula, as well as the offshore-onshore transition.Three transverse NW-SE profiles have been interpreted. Local thinning of the sedimentary cover has been determined towards the centre of the basin which, together with the shallow high velocities observed on the southern profile, could be related to volcanic episodes.A seismic continental basement has been found at depths between 3 and 5 km. A thin lower crust (3–5 km) with velocities around 6.8 km/s has been identified in the northern part of the basin. Alternative crustal models considered for the 3 profiles have been tested, not only from arrival times but also from relative amplitude distributions. A first-order Moho discontinuity fits the data best. The welldefined Moho boundary results in energetic P_MP reflections, and a clear updoming is observed towards the interior of the basin, from depths about 20–21 km inshore of Barcelona to 15–17 km depths 60 km offshore. An anomalous upper mantle with low P_n velocities of about 7.7 km/s is confirmed in most of the sampled areas.     

Determining Q of near-surface materials from Rayleigh waves

High-frequency (≥2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity (V_S), compressional (P)-wave velocity (V_P), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor Q_P and S-wave quality factor Q_S in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting Q_S from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from Q_P cannot be ignored when Vs/V_P reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert Q_P from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to Q_P than to Q_S. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.     

Upper mantle structure of the Apulian plate from Rayleigh waves

Phase velocities of Rayleigh waves for the Adriatic Sea area are obtained in the period range 25–190 sec along the path (l'Aquila-Trieste) AQU-TRI and 20–167 sec along the path (Trieste-Bari) TRI-BAI.The phase velocities are systematically higher than the known values for the surrounding regions. The data inversion indicates the presence of a lithosphere typical of stable continental areas with clear high-velocity lid (V _s 4.6 km/sec) overlying a well developed low velocity zone (V _s 4.2 km/sec).P. F. Geodinamica C.N.R., Roma Pubbl. N. 189.     

Ultrasonic Velocities, Acoustic Emission Characteristics and Crack Damage of Basalt and Granite

Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of P-wave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of double-couple events indicating shear, suggests shear cracks linking previously formed tensile cracks.     

Body wave velocity distribution in the Benioff zone of central Kamchatka during aftershocks of the Kronotskii earthquake of 1997 (M = 7.9)

The distribution of the P and S velocities in the Benioff zone of central Kamchatka during the period of aftershocks (1997–2004) of the disastrous Kronotskii earthquake of 1997 (M = 7.9, M_W = 7.7) has been determined. Based on the data for the foreshock period immediately preceding the earthquake (1991–1997), a sharp increase in the body wave velocities in the Benioff zone below the Kronotskii Peninsula (up to 9.5–9.7 km/s for V _P and 5.1–5.3 km/s for V _S) has been determined at depths of 55–140 km in the subvertical region. Based on observations during the period of aftershocks comparable with the last period of foreshocks (about 7 years), it has been established that the body wave velocities calculated for the Benioff zone below the Kronotskii Peninsula returned to the initial values typical of the beginning of that period. This indicates that stresses relaxed around the head part of the Kronotskii earthquake rupture zone after its origination. This conclusion is confirmed by a sharp decrease in the number of earthquakes with M = 2.3–4.9 in the Benioff zone below the Kronotskii Peninsula. Moreover, taking the velocity distribution during the period of aftershocks into account, it has been determined that a second stress relaxation zone is located at the southwestern flank of the Kronotskii earthquake rupture zone where the largest (M = 6.7) aftershock occurred. According to these data, it is concluded that two stress concentration centers could have existed during the preparation of the Kronotskii earthquake.     

Constraining the crustal structure under the central and western Tian Shan based on teleseismic receiver functions and gravity anomalies

The Tian Shan is a vast range that spans several countries in Asia. Understanding its evolutionary history may provide valuable insights into intracontinental orogenic dynamics. In this study, we explored the crustal characteristics of the Tian Shan and their relationships to the tectonic evolution of the region. A new H-stacking method that combines the P receiver function and gravity anomalies was used to estimate the thickness and ratio of P- to S-wave velocities (v_P/v_S) for 91 broadband seismic stations in the central and western Tian Shan. Our results revealed significant lateral variations in crustal thickness and v_P/v_S. A ~45-km-thick crust and an intermediate-high v_P/v_S (~1.74–1.84) were found in the Kazakh Shield and Tarim Basin, which we interpreted to indicate a mafic crystalline basement and lower crust. The central Tian Shan varied greatly in crustal thickness (40–64 km) and v_P/v_S ratio (1.65–2.00), which may be due to crustal shortening, mafic underplating, and crustal melting. In contrast, we observed a relatively thin crust (42–50 km) with an intermediate v_P/v_S ratio (~1.78) in the western Tian Shan. The differences in the crustal structures between the western and central Tian Shan imply that the Talas-Fergana Fault may be trans-lithospheric.     

Simultaneous inversion of velocity structures and hypocentral locations: Application to the Friuli seismic area NE Italy

A layeredP- andS-wave velocity model is obtained for the Friuli seismic area using the arrival time data ofP- andS-waves from local earthquakes. A damped least-squares method is applied in the inversion.The data used are 994P-wave arrival times for 177 events which have epicenters in the region covered by the Friuli seismic network operated by Osservatorio Geofisico sperimentale (OGS) di Trieste, which are jointly inverted for the earthquake hypocenters andP-wave velocity model. TheS-wave velocity model is estimated on the basis of 978S-wave arrival times and the hypocenters obtained from theP-wave arrival time inversion. We also applied an approach thatP- andS-wave arrival time data are jointly used in the inversion (Roecker, 1982). The results show thatS-wave velocity structures obtained from the two methods are quite consistent, butP-wave velocity structures have obvious differences. This is apparent becauseP-waves are more sensitive to the hypocentral location thanS-waves, and the reading errors ofS-wave arrival times, which are much larger than those ofP-waves, bring large location errors in the joint inversion ofP- andS-wave arrival time. The synthetic data tests indicated that when the reading errors ofS-wave arrivals are larger than four times that ofP-wave arrivals, the method proposed in this paper seems more valid thanP- andS-wave data joint inversion. Most of the relocated events occurred in the depth range between 7 and 11 km, just above the biggest jump in velocity. This jump might be related to the detachment line hypothesized byCarulli et al. (1982). From the invertedP- andS-wave velocities, we obtain an average value 1.82 forV _p /V _s in the first 16 km depth.     

Geochemical constraints on the model of the composition and thermal conditions of the Moon according to seismic data

A new method of reconstruction of the temperature profile in the lunar mantle from the velocities of seismic P- and S-waves for different models of chemical composition is developed. The procedure of the solution of an inverse problem is realized with the help of the minimization of the Gibbs free energy and the equations of state of a mantle substance, taking into account phase transformations, anharmonicity, and the effects of inelasticity. The geophysical and geochemical constraints on composition and temperature distribution in Moon’s mantle are established. The upper mantle can be composed of olivine pyroxenite, depleted by low-volatile oxides (∼2 wt % of CaO and Al₂O₃). On the contrary, the lower mantle must be enriched by low-volatile oxides (∼4–6 wt % of CaO and Al₂O₃). Its composition can be represented by a mineral association of the olivine + clinopyroxene + garnet or olivine + orthopyroxene + clinopyroxene + garnet type, which is close in composition to pyrolite. The temperature distribution at depths 50–1000 km are approximated by the equation: T(°C) = 351 + 1718[1–exp (−0.00082H)]. The constraints inferred make it possible to conclude that the published values of the velocities of P- and S-waves for the lunar mantle, obtained by processing the data of seismic experiments of the Apollo lunar mission are inconsistent with each other at depths below 300 km. Otherwise, the variations in the velocities of P- and S-waves disturb the symmetry between the petrological model (composition), the temperature profile, and the seismic profile.     

Crustal thickness,VP/VS ratio,and shear wave velocity structures beneath Myanmar and their tectonic implications

The crustal structure in Myanmar can provide valuable information for the eastern margin of the ongoing Indo-Eurasian collision system. We successively performed H–k stacking of the receiver function and joint inversion of the receiver function and surface wave dispersion to invert the crustal thickness (H), shear wave velocity (V_S), and the V_P/V_S ratio (k) beneath nine permanent seismic stations in Myanmar. H was found to increase from 26 ?km in the south and east of the study area to 51 ?km in the north and west, and the V_P/V_S ratio was complex and high. Striking differences in the crust were observed for different tectonic areas. In the Indo-Burma Range, the thick crust (H ?～ ?51 ?km) and lower velocities may be related to the accretionary wedge from the Indian Plate. In the Central Myanmar Basin, the thin crust (H ?= ?26.9–35.5 ?km) and complex V_P/V_S ratio and V_S suggest extensional tectonics. In the Eastern Shan Plateau, the relatively thick crust and normal V_P/V_S ratio are consistent with its location along the western edge of the rigid Sunda Block.     

Velocities,elastic moduli and weathering-age relations for pacific layer 2 basalts

Compressional (V_p) and shear (V_s) wave velocities have been measured to 10 kb in 32 cores of basalt from 14 Pacific sites of the Deep Sea Drilling Project. Both V_pandV_s show wide ranges (3.70to6.38km/sec forV_pand1.77to3.40km/sec forV_sat0.5kb) which are linearly related to density and sea floor age, confirming earlier findings by Christensen and Salisbury of decreasing velocity with progressive submarine weathering based on studies of basalts from five sites in the Atlantic. Combined Pacific and Atlantic data give rates of decreasing velocity of ?1.89and?1.35km/sec per100my forV_pandV_s respectively. New analyses of oceanic seismic refraction data indicate a decrease in layer 2 velocities with age similar to that observed in the laboratory, suggesting that weathering penetrates to several hundred meters in many regions and is largely responsible for the extreme range and variability of layer 2 refraction velocities.     

Building and Testing an <Emphasis Type="Italic">a priori</Emphasis> Geophysical Model for Western Eurasia and North Africa

We construct and evaluate a new three-dimensional model of crust and upper mantle structure in Western Eurasia and North Africa (WENA) extending to 700 km depth and having 1° parameterization. The model is compiled in an a priori fashion entirely from existing geophysical literature, specifically, combining two regionalized crustal models with a high-resolution global sediment model and a global upper mantle model. The resulting WENA1.0 model consists of 24 layers: water, three sediment layers, upper, middle, and lower crust, uppermost mantle, and 16 additional upper mantle layers. Each of the layers is specified by its depth, compressional and shear velocity, density, and attenuation (quality factors, Q _P and Q _S ). The model is tested by comparing the model predictions with geophysical observations including: crustal thickness, surface wave group and phase velocities, upper mantle _n velocities, receiver functions, P-wave travel times, waveform characteristics, regional 1-D velocities, and Bouguer gravity. We find generally good agreement between WENA1.0 model predictions and empirical observations for a wide variety of independent data sets. We believe this model is representative of our current knowledge of crust and upper mantle structure in the WENA region and can successfully be used to model the propagation characteristics of regional seismic waveform data. The WENA1.0 model will continue to evolve as new data are incorporated into future validations and any new deficiencies in the model are identified. Eventually this a priori model will serve as the initial starting model for a multiple data set tomographic inversion for structure of the Eurasian continent.     

Inversion of seismic and gravity data for the composition and core sizes of the Moon

We model the internal structure of the Moon, initially homogeneous and later differentiated due to partial melting. The chemical composition and the internal structure of the Moon are retrieved by the Monte-Carlo inversion of the gravity (the mass and the moment of inertia), seismic (compressional and shear velocities), and petrological (balance equations) data. For the computation of phase equilibrium relations and physical properties, we have used a method of minimization of the Gibbs free energy combined with a Mie-Gr@uneisen equation of state within the CaO-FeO-MgO-Al₂O₃-SiO₂ system. The lunar models with a different degree of constraints on the solution are considered. For all models, the geophysically and geochemically permissible ranges of seismic velocities and concentrations in three mantle zones and the sizes of Fe-10%S core are estimated. The lunar mantle is chemically stratified; different mantle zones, where orthopyroxene is the dominant phase, have different concentrations of FeO, Al₂O₃, and CaO. The silicate portion of the Moon (crust + mantle) may contain 3.5–5.5% Al₂O₃ and 10.5–12.5% FeO. The chemical boundary between the middle and the lower mantle lies at a depth of 620–750 km. The lunar models with and without a chemical boundary at a depth of 250–300 km are both possible. The main parameters of the crust, the mantle, and the core of the Moon are estimated. At the depths of the lower mantle, the P and S velocities range from 7.88 to 8.10 km/s and from 4.40 to 4.55 km/s, respectively. The radius of a Fe-10%S core is 340 ± 30 km.     

The model of the depth variations in the properties and state of the rocks in the upper,middle, and lower continental crust of the Kola-Norwegian block,Kola Peninsula

Based on the experimental and calculated data, the model of depth variations in density, as well as the velocities of longitudinal (P) and transverse (S) waves, within the upper, middle, and lower crusts of the Kola-Norwegian block down to a depth of about 40 km is suggested. The variations in density and the velocities of the P- and S-waves are primarily caused by the changes in the mineral composition of the rocks. The relative reduction in the velocities of the P- and S-waves under the action of the increasing pressure and temperature in the depth interval from 5 to 37 km is estimated at ～2%.     

Estimation of Sedimentary Thickness in Kachchh Basin, Gujarat Using SP Converted Phase

An inexpensive method using natural earthquake data is utilized for determining the sedimentary thickness in Kachchh. The Institute of Seismological Research (ISR) is operating a network of broadband seismographs and strong motion accelerographs in Gujarat. We used data from 13 broadband seismographs and two strong motion accelerographs in the study. The stations are within 5 to 80?km from the epicenters. In this study the S-to-P converted phase, S_P, is used. This phase is generated due to large impedance contrast between sediments and basement. This phase is clear in the vertical component. The difference in the travel times of S and S_P phases and velocities of P and S waves is used for determining the sedimentary layer thickness. The thickness of sediments beneath each of these 15 stations was determined covering an area of 23,500?sq km.