High-velocity Structure Research Articles

Shear-wave velocity structure beneath Europe, the northeastern Atlantic and western Asia from waveform inversions including surface-wave mode coupling

SUMMARY Waveforms of 449 seismograms have been inverted for S-wave velocity structures beneath Europe, the northeastern Atlantic, and western Asia down to a depth of 670 km. Recorded waveforms were used in the time window starting at the S-wave arrival and ending after the fundamental-mode Rayleigh-wave arrival. The inversion method is based on the partitioned waveform inversion (Nolet 1990), with the difference that synthetic seismograms are calculated taking surface-wave mode coupling into account in order to model body waves in laterally heterogeneous media more accurately. The partitioning of the inversion procedure makes non-linear optimization feasible, even for inversions including surface-wave mode coupling. The non-linear inversion of the waveforms resulted in linear constraints on the 3-D velocity structure. In a second step, these constraints were used in a linear inversion for the 3-D shear-wave velocity model beneath Europe, the northeastern Atlantic and western Asia. The EUR-SC'95 model is presented, which is characterized by a wide range of length-scales of the velocity structures. In central Europe, where the ray density is highest, small-scale structures are recovered, such as the presence of high velocities associated with the Hellenic subduction zone. On the edges of the inversion model, where the ray density (and therefore also the resolution) is poorer, we find larger-scale features, such as the relatively laterally homogeneous high-velocity structure beneath the Russian Platform to a depth of 300 km. In this paper we discuss the inversion method, data processing, parametrization difficulties due to the introduction of surface-wave mode coupling, spatial resolution of the model, and structures in the EUR-SC'95 model.

Litho—asthenospheric structures of northern Italy as inferred from teleseismic P-wave tomography

Regional three-dimensional inversions of teleseismic P-wave travel time residuals recorded by high-frequency regional and local seismic networks operating along the Western Alps and surrounding regions were carried out and lithosphere and upper mantle P-wave velocity models down to 300 km were obtained. Residuals of more than 500 teleseismic events, recorded by 98 fixed and temporary seismic stations, have been inverted. The comparison between real residuals and the ones obtained from tomographic model indicates that the method is able to solve the feature of the regional heterogeneities. Where the resolution is good, coherent lithospheric and upper mantle structures are imaged. In the shallower layers, high- and low-velocity anomalies follow the structural behaviour of the Alpine-Apenninic chains showing the existence of very strong velocity contrasts. In the deepest layers, velocity contrast decreases however two deep-seated high-velocity structures are observed. The most extended in depth and approximately trending NE-SW has been interpreted as a wreck of the oldest subduction responsible of the Alpine orogenesis. The second one, connected to the northwestern sector of the Apenninic chain, appears to vanish at depths greater than 180 km and is probably due to still active Apenninic roots. Cross-sections depict the spatial trend of perturbations and in particular outline the sub-vertical character of the Alpine and Apenninic anomalies. Under the Ligurian Sea, the 3-D inversion confirms the uplift of the asthenosphere in agreement with the tectonic evolution of the basin.

Shear-wave velocity variations in the upper mantle beneath central Europe

A new model, EUR-S91, of shear-wave velocity variations in the upper mantle beneath central Europe and surrounding regions, down to a depth of 670km, is presented. The model is derived from the inversion of the waveforms of 217 seismograms, using the partitioned waveform-inversion method for the seismogram in the time window from the S-wave arrival to the fundamental mode of the Rayleigh wave. The seismograms were mostly assembled from digitially recording long-period and broad-band stations in Europe. The resulting 3-D model accurately predicts most of the observed waveforms, in a wide band of frequencies. Body waves were fit for frequencies up to 60mHz. The fundamental Rayleigh mode, which at high frequencies is more prone to scattering and multipathing, was generally low passed at 25 mHz. The resolving power of the data set depends strongly on the density of available wave paths and varies as a function of geographical position. Small-scale heterogeneities like the subducted lithosphere in the Hellenic collision zone were imaged in the region with the highest density of wave paths, which indicates an optimum resolution of better than 200 km. The main new results of this study pertain to the transition between east and central Europe. We present new information about the structure below the Tornquist-Teisseyre Zone (TTZ). The TTZ is generally regarded as the boundary between the Precambrian crust of the Baltic Shield/Russian Platform and the younger crust of central Europe. Our 3-D S-velocity model reveals that below this line a sharp lateral boundary extends down to at least 140 km depth, with high velocities beneath the Baltic Shield and Russian Platform contrasting to the low velocities beneath the younger regions of Europe. The observed velocity contrast across the TTZ is largest between the Pannonian Basin and the Russian Platform, where it is equal to 12 per cent at 80 km depth. At depths of 300–400 km, the TTZ is underlain by a zone of low S velocity, which indicates a local thinning of the deeply rooted high-velocity structure or tectosphere that underlies most of eastern Europe. In addition to these new results on the transition region between east and central Europe, a number of features that are present in other tomographic results have been confirmed: the Precambrian provinces of Europe are characterized by high S velocities; low velocities are found beneath the Pannonian Basin, the western Mediterranean, northern Aegean Sea/Turkey and a small region south-west of the Massif Central. The lithosphere beneath the Paris Basin has a positive S-velocity anomaly. The resolution of all these anomalies has been established with sensitivity tests.

Neutral hydrogen and optical properties of three amorphous galaxies

We present new interferometric H I and optical observations of three amorphous galaxies, systems with a smooth, high surface brightness but an asymmetrical distribution of light. All three galaxies are forming stars and have LMC-like emission-line ratios, low dust content, and high H I velocity dispersions. NGC 1140 has a boxy inner morphology with a hook off one corner. At low light levels unusual extensions of starlight are seen curving to the northwest and southeast. The galaxy contains a very luminous central star-forming region and a small chain of H II regions that coincide with the hook. The central H II region has broad H(alpha) velocity profiles full width at half maximum (FWHM) less than or equal to 140 km/s, and it is a radio continuum source. There is a rotating H I gas disk, 40 kpc in radius, at a position angle 51 deg from the optical major axis. The central gas ridge follows the chain of H II regions, and the H I peak is on the hook. The outer gas on the southeast side curves away from the H I major axis. The central gas density is high, and the surface density declines very slowly with radius. The rotation velocity yields a mass of 1 x 10(exp 11) solar mass at 3.3 Holmberg radii (R(sub H)). NGC 1800 has a hook that coincides with a large H II region, and an r(exp 1/4) luminosity distribution. There are numerous H II regions along the major axis and extraordinary filaments of ionized gas. Emanating from the major axis on either side of the galaxy are H(alpha) fingers approximately 750 pc long. About 2.3 kpc to the north is a web of filaments approximately 3 kpc in extent. H(alpha) profiles of H II regions and filaments are narrow. The H I gas disk has a position angle that is approximately 13 deg different from that of the optical axis. There are two peaks near the center, one of which is near the largest H II region. Beyond the Holmberg radius to the west is a 6 x 10(exp 6) solar mass H I cloud. Its velocity indicates a mass of approximately 6 x 10(exp 9) solar mass for NGC 1800 at 1.5 R(sub H). At approximately R(sub 25) to the east there is a large H I shell. Also at approximately R(sub 25) on both sides the velocity gradient switches by 90 deg, and in the interior the rotation is about the major axis. The central gas density is low and falls off slowly. In the inner regions NGC 4670 resembles an S0/a galaxy seen rather edge-on. It contains a central supergiant H II region with very high velocity widths (FWHM less than or equal to 180 km/s) and complex velocity structures. It is a radio continuum source as well. The H I gas is a single spherical cloud or a disk at low inclination centered on the galaxy with a slight elongation along the optical major axis and rotation about the minor axis. The central gas density is high, and there is a high degree of concentration. The rotation speed indicates a total mass of 5 x 10(exp 10) solar mass at 1.1 R(sub H). We compare these characteristics with properties of gas in the presence of stellar bar potentials, gas warps, and interacting and merging galaxy models. Although there are inconsistencies and uncertainties, we conclude that NGC 1140 is a spiral of low surface brightness that has undergone a merger, while NGC 1800 and NGC 4670 are, respectively, probably an Im system and a spiral that had an encounter of the Noguchi (1988a) kind.

4[SUB]-1[/SUB] - 3[SUB]0[/SUB] E methonal emission in G1.6-0.025

The molecular cloud G1.6-0.025, close to the Galactic center, shows evidence of cloud-cloud interactions but has few signs of star formation. The cloud structure, as evidenced by studies of NH3 emission and 12 GHz methanol absorption against the 2.7 K background radiation, shows a main cloud at 50 km/s with a rotational temperature of 50 K, and a high-velocity component at + 165 km/s with temperature of about 120 K. G1.6-0.025 has been mapped in the 4<SUB>-1</SUB> - 3<SUB>0</SUB> E methanol transition (36.2 GHz). A triple structure can be seen in the thermal emission for the 50 km/s component, which is dissimilar to the structure seen in the NH3 or 12 GHz methanol lines. The emission region at 160 km/s is located close to the center of the low-velocity triple. Smaller knots within the low- and high-velocity structures are found to be superposed. Maser emission has been found in the knots at both velocities.

Simultaneous Inversion For 3-D Crustal Structure and Hypocentres Including Direct, Refracted and Reflected Phases-Iii. Application to the Southern Rhine Graben Seismic Region, Germany

SUMMARY The new technique of simultaneous inversion for hypocentres and crustal structure (SSH) is applied to the southern Rhine Graben. P and S wave first and later arrival times of direct, refracted and reflected phases of 350 seismic events are used in the study. Substantial improvements of over 100 per cent in the non-linear residual fit are obtained for the 3-D and less for the 1-D models. Many of the structural results of the 3-D SSH models are consistent with major findings from other seismic refraction experiments and fit into the general geological, petrological and geodynamica1 frame of this area. Thus the thick sedimentary layer in the Rhine Graben proper manifests itself by large negative seismic velocity anomalies. For the middle crust the SSH models show a strong seismic differentiation between sections of the graben proper and the areas under its western and eastern shoulders of the Vosges and the Black Forest, respectively. Using a new a priori Bayesian inversion technique it has been attempted to substantiate the refraction-seismic low velocity zone of Gajewski & Prodehl (1987) in the middle crust under the Black Forest. The results demonstrate that such a velocity low is compatible with the observed traveltimes only if one assumes it as a more local crustal phenomenon in this area. In the lower crust the SSH models are able to clarify some controversies from earlier refraction experiments in the region. The models reveal many seismic features which are typical for continental rift zones such as high transitional velocity anomalies (-7.4 km s-l) under parts of the graben proper (‘rift-cushion’). In the upper mantle, the models show a ring-like high velocity structure of 8.30-8.40 km s-l, centred around a low velocity area (-7.9 km s-l) beneath the Kaiserstuhl. I conjecture this to be a ‘thermal fingerprint’ of a partially molten upwelling asthenospheric diapir bulging under the Moho. The simultaneously relocated hypocentres for the 1-D models show some systematic downward movements for the events in the lower crust of the southern Black Forest area, but otherwise do not differ too much from the initial ones to alter the present seismotectonic picture of the southern Rhine Graben. Thus the hypothesized phenomenon of the seismic gap along the southern section of the Graben Black Forest wedge can both structurally and seismically be substantiated. The relocated hypocentres suggest optically even a widening of this seismic gap. The later has thus to be accepted as a, possibly more general, characteristic of a graben formation.

Geometry and relative motion of the Philippine Sea Plate and Pacific Plate beneath the Kanto‐Tokai District, Japan

The recent acquisition of high‐density and improved seismic data by the Kanto‐Tokai (K‐T) Observational Network of the National Research Institute of Earth Science and Disaster Prevention requires the revision of conventional plate configuration models for the K‐T district. We propose a new interpretation of the configuration and relative motion of the Philippine Sea (PHS) plate, Pacific (PAC) plate, and Eurasian (EUR) plate on the basis of the distribution of hypocenters, velocity structure, and focal mechanisms in the K‐T district. The new model for the configuration of the PHS and PAC slabs clearly delineates the PHS slab subducting northwestward beneath the K‐T district from the Sagami and Suruga troughs and the PAC slab subducting westward beneath the PHS slab from the Japan trench. The thickness of the descending PHS slab is estimated to be 30 ± 5 km. The proposed configuration of the PHS slab is significantly different from those obtained by other researchers, including our own previous study. The new model is fully consistent with global plate motion data. The new model leads to an interpretation of the tectonic processes taking place beneath the K‐T district. For the Sagami trough wing of the PHS slab, the northeastern portion of the slab is dipping northwestward in direct contact with the upper surface of the PAC slab; the southwestern portion carrying the Izu Peninsula has been bent upward, because of its buoyancy, producing the observed contorted shape. For the Suruga trough wing, the dip direction of the PHS slab gradually changes from northwestward at the eastern portion to northward at the western portion. The new model also indicates the presence of offset within the PHS slab between the Sagami and Suruga trough wings. This area is characterized by a low level of seismic activity, the absence of a high‐velocity structure, and the presence of Quaternary volcanoes. It seems reasonable to speculate that the distribution of Quaternary volcanic edifice construction is controlled by the location of the offset within the PHS slab. In view of this new model, the tectonic processes of large earthquakes which occurred recently in the studied area are also reexamined.

Tomographic study of crustal velocity structure in southern Finland

An inverse tomographic problem of the construction of a 3-D velocity structure has been solved for depths of 10, 20, 30, and 40 km using the P-wave arrivals registered by the seismic array in southern Finland and on the DSS lines Sveka (1981) and Baltic (1982). The interpreted results verify and enlarge DSS data obtained earlier for a high-velocity structure of the crust and explain some small lateral velocity variations. The concordant tomographic and DSS data specify a 3-D model for a complex geological structure in the region.

Deep velocity structure of crust and upper mantle in the central parts of Balkan Region

Some velocity characteristics of the crust and upper mantle in the central part of the Balkan region are studied on the basis of about 1000 local and teleseismic earthquakes, registered on the 32 seismic stations. An approach for estimation of the large anisotropic structures effects is used together with the general consideration about the connection between the time residuals and velocity inhomogeneities. The main features of the inhomogeneities in the crust and upper mantle are drscussed in relation with some gravity, heat flow and seismotectonic data. The distribution of the crustal inhomogeneities in general corresponds to the configuration of the morphotectonic structures in Bulgaria. The subcrustal inhomogeneities are discordant with the surface structures, but their orientation is in coincidence with the Trans-Balkan seismolineament system. This fact indicates that the crustal seismicity in the region is probably controlled by the upper mantle structures. The high-velocity structures in the deep upper mantle beneath the Rhodope massif probably represent a paleosubduction at a depth more than 300 km.

Analysis of near-source contributions to early P-wave coda for underground explosions. II. Frequency dependence

An analysis of dispersion in more than 1600 teleseismic short-period P waves from 46 underground explosions has established that near-source effects are responsible for systematic frequency-dependent variations observed in the first 15 sec of the P signals. Explosions from the Nevada, Amchitka, and Novaya Zemlya test sites exhibit a common magnitude dependence of the dispersive behavior, with smaller events having relatively enriched low-frequency (0.4 to 0.8 Hz) energy in the coda. For the Nevada and Amchitka sites, the larger events have relatively enhanced high-frequency (0.8 to 1.1 Hz) energy in the coda as well, which may actually be a consequence of diminished high-frequency content of the direct arrivals. The dispersive behavior also correlates well with known source depths for the Nevada Test Site and Amchitka events, and with estimated pP delay times for the Novaya Zemlya events, indicating that burial depth and/or explosion size are important factors. Pahute Mesa tests show a secondary dependence on position in the site, with centrally located events having stronger dispersion, as well as more pronounced slowly varying azimuthal patterns in the frequency dependence. Stations at azimuths to the north-northeast from the Mesa have particularly strong dispersion for centrally located events. The spatial and azimuthal variations for Pahute Mesa events do not appear to be the result of aftershock radiation, as suggested by Douglas (1984), but instead are associated with frequency-dependent defocusing and scattering from a high-velocity structure beneath the test site. Some of the dispersion from Novaya Zemlya events appears to result from deep path properties; however, the strong magnitude/depth dependence for all of the test sites indicates that very near-source conditions have the strongest influence on the frequency dependence. A likely explanation for the dispersive behavior is a combination of relatively enhanced surface wave excitation and scattering for shallower events, and frequencydependent defocusing for sites overlying anomalous high-velocity structures, as is the case for both Pahute Mesa and Amchitka.

Seismic structure of Iran from surface and body wave data

Summary An investigation of the lithospheric structure of the Iranian plateau has been undertaken in order to find complementary information bearing on the tectonic evolution of the plateau. The phase velocity dispersion of the fundamental mode Rayleigh waves between the WWSSN stations Shiraz, Tabriz and Mashad, and between the sites of the Iranian Long Period Array and MAIO SRO have been obtained from well-dispersed wavetrains generated by eight great circle earthquakes. The aftershocks of the destructive Tabas earthquake in central Iran which were accurately located by a field survey recorded clear P and S phases at MAIO providing supplementary travel-time information for the interpretation of the phase velocity data. The uppermost mantle P-wave velocities beneath Iran obtained in a previous study (Asudeh) were also used in the analysis. An Alpine type crust was found for the Iranian plateau while uppermost mantle velocities ranging from low-velocity mantle in eastern Iran to a high-velocity shield-like structure in the west of the country.

The crust and upper mantle structure beneath southern Italy by earthquakes and DSS data

This paper describes a travel—time analysis performed for the Italian seismic stations, in particular those operating in southern Italy, in order to study the crust and upper mantle properties in the region. Average P-wave residuals of teleseisms in the distance range 30°–95° with respect to Jeffreys-Bullen tables, at thirteen permanent and temporary stations of southern Italy, are coherent with a high velocity zone beneath Calabria and northern Sicily and low velocity material in the mantle beneath the Eolian Islands. Travel—time residuals from Tyrrhenian intermediate earthquakes show a high velocity structure which extends in a NW direction from a depth of at least 200 km down to 450 km. A damped least-squares inversion applied to DSS data confirms the existence of low velocity zones in the crust beneath the Eolian Islands, at 8–12 km depth, that agrees with previous results and with the lack of S waves from local earthquakes.