Seismic tomographic imaging of P- and S-waves velocity perturbations in the upper mantle beneath Iran

The inverse tomography method has been used to study the P - and S -waves velocity structure of the crust and upper mantle underneath Iran. The method, based on the principle of source–receiver reciprocity, allows for tomographic studies of regions with sparse distribution of seismic stations if the region has sufficient seismicity. The arrival times of body waves from earthquakes in the study area as reported in the ISC catalogue (1964–1996) at all available epicentral distances are used for calculation of residual arrival times. Prior to inversion we have relocated hypocentres based on a 1-D spherical earth's model taking into account variable crustal thickness and surface topography. During the inversion seismic sources are further relocated simultaneously with the calculation of velocity perturbations. With a series of synthetic tests we demonstrate the power of the algorithm and the data to reconstruct introduced anomalies using the ray paths of the real data set and taking into account the measurement errors and outliers. The velocity anomalies show that the crust and upper mantle beneath the Iranian Plateau comprises a low velocity domain between the Arabian Plate and the Caspian Block. This is in agreement with global tomographic models, and also tectonic models, in which active Iranian plateau is trapped between the stable Turan plate in the north and the Arabian shield in the south. Our results show clear evidence of the mainly aseismic subduction of the oceanic crust of the Oman Sea underneath the Iranian Plateau. However, along the Zagros suture zone, the subduction pattern is more complex than at Makran where the collision of the two plates is highly seismic.     

P- and S-velocity images of the lithosphere–asthenosphere system in the Central Andes from local-source tomographic inversion

About 50 000 P and S arrival times and 25 000 values of t * recorded at seismic arrays operated in the Central Andes between 20°S and 25°S in the time period from 1994 to 1997 have been used for locating more than 1500 deep and crustal earthquakes and creating 3-D P , S velocity and Qp models. The study volume in the reference model is subdivided into three domains: slab, continental crust and mantle wedge. A starting velocity distribution in each domain is set from a priori information: in the crust it is based on the controlled sources seismic studies; in slab and mantle wedge it is defined using relations between P and S velocities, temperature and composition given by mineral physics. Each iteration of tomographic inversion consists of the following steps: (1) absolute location of sources in 3-D velocity model using P and S arrival times; (2) double-difference relocation of the sources and (3) simultaneous determination of P and S velocity anomalies, P and S station corrections and source parameters by inverting one matrix. Velocity parameters are computed in a mesh with the density of nodes proportional to the ray density with double-sided nodes at the domain boundaries. The next iteration is repeated with the updated velocity model and source parameters obtained at the previous step. Different tests aimed at checking the reliability of the obtained velocity models are presented. In addition, we present the results of inversion for Vp and Vp/Vs parameters, which appear to be practically equivalent to Vp and Vs inversion. A separate inversion for Qp has been performed using the ray paths and source locations in the final velocity model. The resulting Vp , Vs and Qp distributions show complicated, essentially 3-D structure in the lithosphere and asthenosphere. P and S velocities appear to be well correlated, suggesting the important role of variations of composition, temperature, water content and degree of partial melting.     

Air-Coupled Seismic Waves at Long Range from Apollo Launchings

Summary Microphones and seismographs were co-located in arrays on Skidaway Island, Georgia, for the launchings of Apollo 13 and 14, 374 km to the south. Simultaneous acoustic and seismic waves were recorded for both events at times appropriate to the arrival of the acoustic waves from the source. Significant comparisons of the true signals are (1) the acoustic signal is relatively broadband compared to the nearly monochromatic seismic signal; (2) the seismic signal is much more continuous than the more pulse-like acoustic signal; (3) ground loading from the pressure variations of the acoustic waves is shown to be too small to account for the seismic waves; (4) the measured phase velocities of both acoustic and seismic waves across the local instrument arrays differ by less than 6 per cent and possibly 3 per cent if experimental error is included. It is concluded that the seismic waves are generated by resonant coupling to the acoustic waves along some 10 km of path on Skidaway Island. The thickness of unconsolidated sediment on the island is appropriate to a resonant ground wave frequency of 3.5 to 4 Hz, as observed. Under appropriate conditions, ground wave observations may prove more effective means of detecting certain aspects of acoustic signals in view of the filtering of wind noise and amplification through resonance.     

Inversion of seismic attributes for velocity and attenuation structure

We have developed an inversion formuialion for velocity and attenuation structure using seismic attributes, including envelope amplitude, instantaneous frequency and arrival times of selected seismic phases. We refer to this approach as AFT inversion for amplitude, (instantaneous) frequency and time. Complex trace analysis is used to extract the different seismic attributes. The instantaneous frequency data are converted to t * using a matching procedure that approximately removes the effects of the source spectra. To invert for structure, ray-perturbation methods are used to compute the sensitivity of the seismic attributes to variations in the model. An iterative inversion procedure is then performed from smooth to less smooth models that progressively incorporates the shorter-wavelength components of the model. To illustrate the method, seismic attributes are extracted from seismic-refraction data of the Ouachita PASSCAL experiment and used to invert for shallow crustal velocity and attenuation structure. Although amplitude data are sensitive to model roughness, the inverted velocity and attenuation models were required by the data to maintain a relatively smooth character. The amplitude and t * data were needed, along with the traveltimes, at each step of the inversion in order to fit all the seismic attributes at the final iteration.     

Observations and origin of Rayleigh-wave amplitude anomalies

This is a report of observations of amplitude anomalies of fundamental-mode Rayleigh waves ( R 1) between periods of 17 and 100  s. The anomalies are with respect to amplitudes predicted by Rayleigh-wave excitation for a reference earth model and catalogued centroid earthquake source parameters, such as are used in large-scale waveform inversions. The observations indicate that the amplitude anomalies are consistent for nearby recordings of the same event, while there is no obvious relation between the observed anomalies and the paths travelled by the waves. This is in contrast to Rayleigh-wave phase anomalies, which are consistent for similar propagation paths, and hence form the input in many inversions for along-path structure. The observations in this paper show that a similar inversion of intermediate-period amplitude anomalies for along- and near-path structure is not warranted without eliminating source effects, since the amplitude anomalies are dominated by scattering off near-source earth structure and by possible uncertainties in the source parameters. Sensitivity kernels that take the coupling between the moment tensor and displacement field into account demonstrate that Rayleigh-wave amplitude sensitivity is largest near the source. This report argues that the interaction between source-radiated Rayleigh waves and near-source earth structure may not be ignored in amplitude inversion procedures.     

Seismo-acoustic analysis of the Buncefield oil depot explosion in the UK, 2005 December 11

A massive vapour cloud explosion occurred at the Buncefield fuel depot near Hemel Hempstead, UK, in the morning of 2005 December 11. The explosion was the result of an overflow from one of the storage tanks with the release of over 300 tons of petrol and generating a vapour cloud that spread over an area of 80 000 m², before being ignited. Considerable damage was caused in the vicinity of the explosion and a total of 43 people were injured. The explosion was detected by seismograph stations in the UK and the Netherlands and by infrasound arrays in the Netherlands. We analysed the seismic recordings to determine the origin time of 06:01:31.45 ±0.5 s (UTC) from P -wave arrival times. Uncertainties in determination of origin time from acoustic arrival times alone were less than 10 s. Amplitudes of P -, Lg and primary acoustic waves were measured to derive decay relationships as function of distance. From the seismic amplitudes we estimated a yield of 2–10 tons equivalent to a buried explosion. Most seismic stations recorded primary and secondary acoustic waves. We used atmospheric ray tracing to identify the various travel paths, which depend on temperature and wind speed as function of altitude, leading to directional variation. Refracted waves were observed from the troposphere, stratosphere and thermosphere with a good match between observed and calculated traveltimes. The various wave types were also identified through array processing, which provides backazimuth and slowness, of recordings from an infrasound array in the Netherlands. The amplitude of stratospheric refracted acoustic waves recorded by the array microbarometers was used to estimate a yield of 21.6 (±5) tons TNT equivalent. We have demonstrated through joint seismo-acoustic analysis of the explosion that both the seismic velocity model and the atmospheric model are sufficient to explain the observed traveltimes.     

Crustal Structure and Origin of Peake and Freen Deeps, NE Atlantic

Summary Peake and Freen Deeps are elongate structures some 30 nautical miles long by 7 miles wide situated near 43° N 20° W on the lower flanks of the Mid-Atlantic Ridge. Seismic reflection records show that underneath about 400 fm of layered sediment the bedrock lies at a depth greater than 3600 fm in Peake Deep and 3300 fm in Freen Deep; the surrounding seafloor is at about 2100 fm. Freen Deep is the eastern end of King's Trough, a flat floored feature some 400 fms deeper than the adjacent seafloor. The Trough extends 220 miles west-north-westwards towards the crest of the Mid-Atlantic Ridge. The area is aseismic and heat flow is normal; there is no displacement of the crest of the mid-ocean ridge on the projected line of King's Trough. Gravity and magnetic surveys have been made. With minor exceptions, magnetic anomalies are not due to bodies elongated parallel with the structure, which, therefore, cannot be a volcanic collapse caldera. Seismic refraction results in the Peake-Freen area show that the crust is not thinned under the deeps although the Moho may be depressed by 2 km. Bouguer anomalies also suggest that the Moho is flat and does not rise to compensate the deeps. Models consistent with gravity and seismic information suggest there is a dense block in the upper mantle under the area. Since no reason to ascribe the origin of the structure to tear faulting has yet been acquired, it is interpreted in terms of over thrusting perpendicular to the deeps, followed by inversion of the lower part of the thickened basaltic crust to eclogite, and its subsequent sinking into the mantle.     

Global observation of off-great-circle propagation of Long-Period surface waves

In the current generation of global dispersion maps of surface waves, the long-wavelength structure seems to be very well determined. There is general agreement in the patterns of global phase velocity anomalies up to harmonic degree 16. However, the shorter-wavelength structure varies significantly between published maps, and it appears that this part of the models depends strongly on the inversion technique and on the data set of surface-wave dispersion (usually phase measurements). Polarization data depend on the lateral gradient of phase velocity and hence are more sensitive to shorter-wavelength structure than phase data; thus, including these data should enhance resolution. In this paper, I demonstrate that polarization data of long-period surface waves (80 s), as a function of frequency, can be reliably measured using a multitaper technique. the resulting off-great-circle arrival angles of the surface-wave packets are relatively easy to interpret within a ray-theoretical framework. Our data base of three-component recordings is now large enough to provide useful constraints on global dispersion maps, particularly on the shorter-wavelength parts. Apart from the phase velocity model itself, a possible misorientation of the horizontal components at each station is included in a non-linear inversion as an additional independent model parameter. This gives a significant improvement in the fit to the data. Misorientations of more than 3° are probable for at least four of the 37 stations investigated.     

What can be learned from rotational motions excited by earthquakes?

One answer to the question posed in the title is that we will have more accurate data for arrival times of SH waves, because the rotational component around the vertical axis is sensitive to SH waves although not to P-SV waves. Importantly, there is another answer related to seismic sources, which will be discussed in this paper.
Generally, not only dislocations commonly used in earthquake models but also other kind of defects could contribute to producing seismic waves. In particular, rotational strains at earthquake sources directly generate rotational components in seismic waves. Employing the geometrical theory of defects, we obtain a general expression for the rotational motion of seismic waves as a function of the parameters of source defects.
Using this expression, together with one for translational motion, we can estimate the rotational strain tensor and the spatial variation of slip velocity in the source area of earthquakes. These quantities will be large at the edges of a fault plane due to spatially rapid changes of slip on the fault and/or a formation of tensile fractures.     

Modelling and inversion of single-ended refraction data from the shot gathers of multifold deep seismic reflection profiling—an approach for deriving the shallow velocity structure

A multifold crustal-scale deep seismic near-vertical reflection profile generates a large number of single-ended shot gathers, which provide redundant data sets because of overlapping coverage of the shallow refractors. We present an approach for deriving the shallow velocity structure by modelling and inversion of single-ended seismic refraction first arrival traveltime data. We apply this method to a data set acquired with a 12-km long spread with 100 m spacing of shots and receivers, of the Neoproterozoic Marwar basin in the NW Indian shield. The approach is shown to be quite successful for delineating the shallow refractor depths, steep dips and velocities, even in the absence of regular reverse refraction profiles. The study reveals two-layered sedimentary formations, Malani volcanics and a complicated basement configuration of the Marwar basin, and provides a measure of resolution and uncertainty of the estimated model parameters. A seismic section of the near-trace gather is found to be qualitatively consistent with the derived structural features of the basin. The relative highs and lows, observed in the Bouguer gravity profile, further corroborate the derived velocity model. The present approach can be especially useful in offshore areas and elsewhere, where the single-ended multifold seismic profiles are the only available data sets.     

Regional tomographic inversion of the amplitude and phase of Rayleigh waves with 2-D sensitivity kernels

In this study, we test the adequacy of 2-D sensitivity kernels for fundamental-mode Rayleigh waves based on the single-scattering (Born) approximation to account for the effects of heterogeneous structure on the wavefield in a regional surface wave study. The calculated phase and amplitude data using the 2-D sensitivity kernels are compared to phase and amplitude data obtained from seismic waveforms synthesized by the pseudo-spectral method for plane Rayleigh waves propagating through heterogeneous structure. We find that the kernels can accurately predict the perturbation of the wavefield even when the size of anomaly is larger than one wavelength. The only exception is a systematic bias in the amplitude within the anomaly itself due to a site response.
An inversion method of surface wave tomography based on the sensitivity kernels is developed and applied to synthesized data obtained from a numerical simulation modelling Rayleigh wave propagation over checkerboard structure. By comparing recovered images to input structure, we illustrate that the method can almost completely recover anomalies within an array of stations when the size of the anomalies is larger than or close to one wavelength of the surface waves. Surface wave amplitude contains important information about Earth structure and should be inverted together with phase data in surface wave tomography.     

Constraints on a plume in the mid-mantle beneath the Iceland region from seismic array data

Teleseismic P waves passing through low-wave-speed bodies in the mantle are refracted, causing anomalies in their propagation directions that can be measured by seismometer arrays. Waves from earthquakes in the eastern Pacific and western North America arriving at the NORSAR array in Norway and at seismic stations in Scotland pass beneath the Iceland region at depths of ∼ 1000–2000 km. Waves arriving at NORSAR have anomalous arrival azimuths consistent with a low-wave-speed body at a depth of ∼ 1500 km beneath the Iceland–Faeroe ridge with a maximum diameter of ∼250 km and a maximum wave-speed contrast of ∼ 1.5 per cent. This agrees well with whole-mantle tomography results, which image a low-wave-speed body at this location with a diameter of ∼ 500 km and a wave-speed anomaly of ∼ 0.5 per cent, bearing in mind that whole-mantle tomography, because of its limited resolution, broadens and weakens small anomalies. The observations cannot resolve the location of the body, and the anomaly could be caused in whole or in part by larger bodies farther away, for example by a body imaged beneath Greenland by whole-mantle tomography.     

The variation of delays in stress-induced anisotropic polarization anomalies

Summary. Polarization anomalies in seismic shear wavetrains, diagnostic of propagation through anisotropic media, have now been observed in dilatancy zones in seismic regions. Stress-induced dilatancy will open cracks with preferred orientations, which will be effectively anisotropic to short-period seismic waves. The polarization anomalies are due to the shear waves splitting, in propagation through anisotropic media, into components with different polarizations and different velocities. This writes characteristic signatures into the shear wavetrains. The paper examines ways in which the differential shear-wave anisotropy (the delay between the split shear-waves) varies with direction by plotting stereograms of the relative delays, and their polarizations, for possible dilatancy symmetry-systems. It seems likely, that if sufficient observations of these anomalies can be obtained at each stage of the dilatancy episode, it will be possible to estimate the symmetry directions of the dilatancy and the geometry of the stress-induced crack-system, as well as monitoring the progress of the dilatancy episode.     

3-D magnetotelluric inversion and model validation with gravity data for the investigation of flood basalts and associated volcanic rifted margins

20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.     

Double-planed deep seismic zone and upper-mantle structure in the Northeastern Japan Arc

Summary. The ScSp wave converted from the ScS wave at the boundary between the descending lithospheric slab and the mantle above it was clearly observed from a nearby deep earthquake with magnitude 7.7 at some stations of the seismic network of Tohoku University which covers the Tohoku District, the northeastern part of Honshu, Japan. By applying the three-dimensional seismic-ray tracing method, the location of this boundary was determined from the difference in arrival time between the ScS and ScSp waves. The result shows that the upper boundary of the descending slab lies exactly on the upper plane of the double-planed deep seismic zone found in the Northeastern Japan Arc.
There is an additional evidence that the boundary is located on the upper plane of the double-planed deep seismic zone. The hypocentre distribution of intermediate-depth earthquakes located by the small-scale seismic-array observation is extremely different from that obtained by the relatively large-scale seismic network. The discrepancy in the distribution of hypocentres of the same earthquake independently located is well explained by the inclined lithospheric slab model derived from the difference in arrival time between the ScS and ScSp waves.
The earthquakes with reverse faulting or with down-dip compressional stresses occur at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down-dip extensional stresses also occur in a very narrow zone from 30 to 40 km below the dipping boundary in the depth range from 50 to about 200 km, and these shocks form the lower plane of the double-planed deep seismic zone.     

Three-dimensional inversion without blocks

Summary. We propose a method for solving non-linear inverse problems in the case where the unknown is a function of the spatial coordinates and the data set is discrete and finite. The method is based on a generalized leastsquares criterion, it is defined directly for non-linear problems (without previous linearization of the forward problem), and in the particular linear case it gives the same results (although slightly more general) than the Backus & Gilbert approach. As an example, we apply the method to the three-dimensional seismic velocity inverse problem, using as data the arrival times of seismic waves. The following paper (Nercessian et al .) shows some esults obtained using the present method.     

Surface-wave propagation across the Mexican Volcanic Belt and the origin of the long-period seismic-wave amplification in the Valley of Mexico

A network of nine broad-band seismographs was operated from March to May 1994 to study the propagation of seismic waves across the Mexican Volcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band al the stations situated in the southern part of the MVB.
The group velocities of the fundamental mode of the Rayleigh wave in the period range 2–13 s are found to be lower in the southern part of the MVB than in its northern part and in the region south of the MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S -wave velocity of 1.7 km s^-1 and an average thickness of 2 km) beneath the southern part of the MVB. This low-velocity zone is associated with the region of active volcanism.
Numerical simulations show that this superficial low-velocity layer causes a regional amplification of 8–10 s period signals, which is of the same order as the amplification measured from the data. This layer also increases the signal duration significantly because of the dispersion of the surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB     

Analogue modelling of inverted domino‐style basement fault systems

Inversion of pre‐existing extensional fault systems is common in rift systems, back‐arc basins and passive margins. It can significantly influence the development of structural traps in hydrocarbon basins. The analogue models of domino‐style basement fault systems shown in this paper produced, on extension, characteristic hangingwall growth stratal wedges that, when contracted and inverted, formed classic inversion harpoon geometries and asymmetric hangingwall contractional fault‐propagation folds. Segmented footwall shortcut faults formed as the basement faults were progressively back‐rotated and steepened. The pre‐existing extensional fault architectures, basement fault geometries and the relative hangingwall and footwall block rotations exerted fundamental controls on the inversion styles. Digital image correlation (DIC) strain monitoring illustrated complex vertical fault segmentation and linkage during inversion as the major faults were reactivated and strain was progressively transferred onto footwall shortcut faults. Hangingwall deformation during inversion was dominated by significant back‐rotation as the inversion progressed. The mechanical stratigraphy of the cover sequences strongly influenced the fold and fault evolution of the reactivated fault systems. The implications of the experimental results for the interpretation and analysis of inversion structures are discussed and are compared with natural examples of inverted basement‐involved extensional faults observed in seismic datasets.     

New insight into the crust and upper mantle structure under Alaska

To better understand the seismic structure of the subducting Pacific plate under Alaska, we determined the three-dimensional P-wave velocity structure to a depth of approximately 200 km beneath Alaska using 438,146 P-wave arrival times from 10,900 earthquakes. In this study an irregular grid parameterization was adopted to express the velocity structure under Alaska. The number of grid nodes increases from north to south in the study area so that the spacing between grid nodes is approximately the same in the longitude direction. Our results suggest that the subducting Pacific slab under Alaska can be divided into three different parts based on its geometry and velocity structure. The western part has features similar to those in other subduction zones. In the central part a thick low-velocity zone is imaged at the top of the subducting Pacific slab beneath north of the Kenai Peninsula, which is believed to be most likely the oceanic crust plus an overlying serpentinized zone and the coupled Yakutat terrane subducted with the Pacific slab. In the eastern part, significant high-velocity anomalies are visible to 60–90 km depth, suggesting that the Pacific slab has only subducted down to that depth.     

Sequential integrated inversion of refraction and wide-angle reflection traveltimes and gravity data for two-dimensional velocity structures

A new algorithm is presented for the integrated 2-D inversion of seismic traveltime and gravity data. The algorithm adopts the 'maximum likelihood' regularization scheme. We construct a 'probability density function' which includes three kinds of information: information derived from gravity measurements; information derived from the seismic traveltime inversion procedure applied to the model; and information on the physical correlation among the density and the velocity parameters. We assume a linear relation between density and velocity, which can be node-dependent; that is, we can choose different relationships for different parts of the velocity–density grid. In addition, our procedure allows us to consider a covariance matrix related to the error propagation in linking density to velocity. We use seismic data to estimate starting velocity values and the position of boundary nodes. Subsequently, the sequential integrated inversion (SII) optimizes the layer velocities and densities for our models. The procedure is applicable, as an additional step, to any type of seismic tomographic inversion.
We illustrate the method by comparing the velocity models recovered from a standard seismic traveltime inversion with those retrieved using our algorithm. The inversion of synthetic data calculated for a 2-D isotropic, laterally inhomogeneous model shows the stability and accuracy of this procedure, demonstrates the improvements to the recovery of true velocity anomalies, and proves that this technique can efficiently overcome some of the limitations of both gravity and seismic traveltime inversions, when they are used independently.
An interpretation of field data from the 1994 Vesuvius test experiment is also presented. At depths down to 4.5 km, the model retrieved after a SII shows a more detailed structure than the model obtained from an interpretation of seismic traveltime only, and yields additional information for a further study of the area.