Attenuation Anomalies Research Articles

High S-wave attenuation anomalies and ringlike seismogenic structures in the lithosphere beneath Altai: Possible precursors of large earthquakes

This paper addresses inhomogeneities in the short-period S-wave attenuation field in the lithosphere beneath Altai. A technique based on the analysis of the amplitude ratios of Sn and Pn waves is used. High S-wave attenuation areas are identified in the West Altai, which are related to the source zones of recent large earthquakes, viz., the 1990 Zaisan earthquake and the 2003 Chuya earthquake. Associated with the Chuya earthquake, a large ringlike seismogenic structure had been formed since 1976. It is also found that ringlike seismogenic structures are confined to high S-wave attenuation areas unrelated to large historical earthquakes. It is supposed that processes paving the way for strong earthquakes are taking place in these areas. The magnitudes of probable earthquakes are estimated using the earlier derived correlation dependences of the sizes of ringlike seismogenic structures and the threshold values of magnitudes on the energy of principal earthquakes with prevailing focal mechanisms taken into consideration. The sources of some earthquakes are likely to occur near to the planned gas pipeline route from Western Siberia to China, which should be taken into account. The relationship of anomalies in the S-wave attenuation field and the ringlike seismogenic structures to a high content of deep-seated fluids in the lithosphere is discussed.

Gas hydrate and free gas detection using seismic quality factor estimates from high-resolution P-cable 3D seismic data

We have estimated the seismic attenuation in gas hydrate and free-gas-bearing sediments from high-resolution P-cable 3D seismic data from the Vestnesa Ridge on the Arctic continental margin of Svalbard. P-cable data have a broad bandwidth (20–300 Hz), which is extremely advantageous in estimating seismic attenuation in a medium. The seismic quality factor (Q), the inverse of seismic attenuation, is estimated from the seismic data set using the centroid frequency shift and spectral ratio (SR) methods. The centroid frequency shift method establishes a relationship between the change in the centroid frequency of an amplitude spectrum and the Q value of a medium. The SR method estimates the Q value of a medium by studying the differential decay of different frequencies. The broad bandwidth and short offset characteristics of the P-cable data set are useful to continuously map the Q for different layers throughout the 3D seismic volume. The centroid frequency shift method is found to be relatively more stable than the SR method. Q values estimated using these two methods are in concordance with each other. The Q data document attenuation anomalies in the layers in the gas hydrate stability zone above the bottom-simulating reflection (BSR) and in the free gas zone below. Changes in the attenuation anomalies correlate with small-scale fault systems in the Vestnesa Ridge suggesting a strong structural control on the distribution of free gas and gas hydrates in the region. We argued that high and spatially limited Q anomalies in the layer above the BSR indicate the presence of gas hydrates in marine sediments in this setting. Hence, our workflow to analyze Q using high-resolution P-cable 3D seismic data with a large bandwidth could be a potential technique to detect and directly map the distribution of gas hydrates in marine sediments.

Reverse-time prestack depth migration of GPR data from topography for amplitude reconstruction in complex environments

With increased computational power, reverse-time prestack depth migration (RT-PSDM) has become a preferred imaging tool in seismic exploration, yet its use has remained relatively limited in ground-penetrating radar (GPR) applications. Complex topography alters the wavefield kinematics making for a challenging imaging problem. Model simulations show that topographic variation can substantially distort reflection amplitudes due to irregular wavefield spreading, attenuation anomalies due to irregular path lengths, and focusing and defocusing effects at the surface. The effects are magnified when the topographic variations are on the same order as the depth of investigation—a situation that is often encountered in GPR investigations. Here, I use a full wave-equation RT-PSDM algorithm to image GPR data in the presence of large topographic variability relative to the depth of investigation. The source and receiver wavefields are propagated directly from the topographic surface and this approach inherently corrects for irregular kinematics, spreading and attenuation. The results show that when GPR data are acquired in areas of extreme topography, RT-PSDM can accurately reconstruct reflector geometry as well as reflection amplitude.

Acoustic investigation of NaBi(MoO4)2 and NaBi(WO4)2 crystals at high temperatures

The results from investigations of the acoustic properties of NaBi(MoO4)2 and NaBi(WO4)2 crystals above room temperature are presented. Variations in ultrasound velocity that indicate the occurrence of phase transitions are observed. Phase transitions for NaBi(MoO4)2 and NaBi(WO4)2 crystals are observed near 309 and 319 K, respectively. It is assumed that these phase transitions are associated with the ordering of Na+ and Bi3+ ions over cation sites. Observed anomalies in elastic properties and sound attenuation allow an interpretation based on the phenomenological Landau theory.

Coda waves’ tomography for surficial exploration

This work extrapolates the application of coda-waves’ imaging, in order to determine lateral anomalies of attenuation for surficial targets, and providing experimental elements that justify its use in an economic way with a relatively good resolution. A numerical procedure for the spatial inversion of the attenuation, inside a small region, has been established. The numerical effectiveness of the inversion was evaluated carrying out a field experiment, which detects, based on attenuation contrasts, the presence of a tunnel built in the Colombian Eastern Cordillera. The experimental work allows the validation of the proposed method, and the acquisition of associated information about the geotechnical conditions at the experimental site. © 2016. Acad. Colomb. Cienc. Ex. Fis. Nat. All rights reserved.

Decoupling of Pacific subduction zone guided waves beneath central Japan: Evidence for thin slab

AbstractThe fine‐scale seismic structure of the northeast Japan subduction zone is studied based on waveform analyses of moderate‐sized (M4.5–6), deep‐focus earthquakes (h >350 km) and the finite difference method (FDM) simulation of high‐frequency (up to 8 Hz) wave propagation. Strong regional S wave attenuation anomalies for specific source‐receiver paths connecting the cluster of events occurring in central part of the Sea of Japan recorded at fore arc stations in northern and central Japanese Islands (Honshu) are used to model the deeper structure of the subducting Pacific Plate, where recent teleseismic tomography has shown evidence for a possible slab tear westward beneath the Sea of Japan. The character of the observed anomalous S wave attenuation and the following high‐frequency coda can be captured with the two‐dimensional (2‐D) FDM simulation of seismic waves in heterogeneous plate model, incorporating the thinning of the plate at depth, which is also compared with other possible causes of dramatic attenuation of high‐frequency S wave due to low‐Q or much weaker heterogeneities in the slab. The results of simulation clearly demonstrate that the dramatic loss of high‐frequency S wavefield from the plate into the surrounding mantle occurred due to the variation in the plate geometry (i.e., thinning of the plate) at depth near the source rather than due to variation in physical properties, such as due to the lowered‐Q and weaker heterogeneities in the plate. The presence of such a thin zone defocuses the high‐frequency slab‐guided S wave energy from the subducting plate into the surrounding mantle and acts as a geometric antiwaveguide. Based on the sequence of simulation results obtained, we propose thinning of Pacific Plate at depth subducting beneath northeastern Japan, localized to central part of Honshu, in agreement with the observations.

Ultrasonic investigation of the organic spin-liquid compoundEtMe3Sb[Pd(dmit)2]2

Ultrasonic measurements performed on the spin-liquid organic compound $\mathrm{Et}{\mathrm{Me}}_{3}\mathrm{Sb}[\mathrm{Pd}(\text{dmit}{)}_{2}{]}_{2}$ reveal an inhomogeneous elastic behavior above 40 K due to the rotational motion in the $\mathrm{Et}{\mathrm{Me}}_{3}\mathrm{Sb}$ cations. Two velocity softening anomalies and concomitant attenuation peaks at 40 K and 160 K are respectively attributed to the rotation of methyl and ethyl groups. At lower temperatures, the elastic character is homogeneous and an anomaly is observed below 2 K. A magnetic field study suggests that two kinds of excitations contribute to this anomaly. First, a softening of the velocity toward the lowest temperatures which is not dependent on magnetic field implies other degrees of freedom than spins. Second, spin excitations coupled to longitudinal acoustic phonons yield a velocity softening anomaly that completely disappears for field values higher than 7 T. Consistent attenuation anomalies are also observed for both contributions. These data suggest that only one kind of spin excitation exists in the spin-liquid ground state below 1.5 K. A spinon-phonon interaction likely explains the magnetic field dependent elastic anomalies. However, no spin-gap-like excitations related to a pairing instability of the spinons could be detected in this experiment.

Detection of sharp lateral discontinuities through the analysis of surface-wave propagation

Surface-wave (SW) techniques are mainly used to retrieve 1D velocity models and are therefore characterized by a 1D approach, which might prove unsatisfactory when relevant 2D effects are present in the investigated subsurface. In the case of sharp and sudden lateral heterogeneities in the subsurface, a strategy to tackle this limitation is to estimate the location of the discontinuities and to separately process seismic traces belonging to quasi-1D subsurface portions. We have addressed our attention to methods aimed at locating discontinuities by identifying anomalies in SW propagation and attenuation. The considered methods are the autospectrum computation and the attenuation analysis of Rayleigh waves (AARW). These methods were developed for purposes and/or scales of analysis that are different from those of this work, which aims at detecting and characterizing sharp subvertical discontinuities in the shallow subsurface. We applied both methods to two data sets, synthetic data from a finite-element method simulation and a field data set acquired over a fault system, both presenting an abrupt lateral variation perpendicularly crossing the acquisition line. We also extended the AARW method to the detection of sharp discontinuities from large and multifold data sets and we tested these novel procedures on the field case. The two methods are proven to be effective for the detection of the discontinuity, by portraying propagation phenomena linked to the presence of the heterogeneity, such as the interference between incident and reflected wavetrains, and energy concentration as well as subsequent decay at the fault location. The procedures we developed for the processing of multifold seismic data set showed to be reliable tools in locating and characterizing subvertical sharp heterogeneities.

Geophysical assessment of migration and storage conditions of fluids in subduction zones

By enhancing mass transfer and energy release, the cycle of volatiles and melt is a major component of subduction. Investigating this fluid cycle is therefore critical to understand the past and current activity of subduction zones. Fluids can significantly affect rock electrical conductivity and elastic parameters that are measured using electromagnetic and seismic methods, respectively. This letter emphasizes how these geophysical methods complement each other to provide information about the storage of fluids in subduction systems. By compiling electromagnetic and seismic results from various subduction zones, a possible correlation between electrical conductivity and seismic wave attenuation anomalies in the mantle wedge is observed, consistent with fluid accumulation. A possible relationship between geophysical properties and the slab age is also suggested, whereas no significant trend is observed between electrical conductivity or seismic wave attenuation and estimates of water flux in the mantle wedge. These field-based relationships require further constrains, emphasizing the need for new measurements in the laboratory.

Ultrasonic Behavior Near Phase Transitions in (PbySn1−y)2P2S6 Ferroelectric Materials

The ultrasonic investigations were performed in Sn2P2S6 family layered crystals, where tin was substituted by lead. We have shown that in mixed crystals the anomalies of ultrasonic velocity and attenuation appear in the vicinity of the phase transitions. The critical ultrasonic anomalies shifted to lower temperatures when Pb concentration increased from 0.1 to 0.3 in material. The relative change of longitudinal ultrasonic velocity along z-axis at the phase transition temperature gradually decreased from 30% in pure Sn2P2S6 to 15% for sample with y = 0.3 content of Pb. Such large velocity change causes the large ultrasonic attenuation anomaly. The ultrasonic velocity and attenuation behaviour in the ferroelectric phase was described using Landau theory implying increase of order parameter relaxation time: τ = τ0/(TC–T) and free energy expansion including sixth order terms. It was shown that prefactor τ0 increases with Pb content.

High‐resolution 3‐D P wave attenuation structure of the New Madrid Seismic Zone using local earthquake tomography

AbstractA three‐dimensional (3‐D), high‐resolution P wave seismic attenuation model for the New Madrid Seismic Zone (NMSZ) is determined using P wave path attenuation (t*) values of small‐magnitude earthquakes (MD < 3.9). Events were recorded at 89 broadband and short‐period seismometers of the Cooperative New Madrid Seismic Zone Network and 40 short‐period seismometers of the Portable Array for Numerical Data Acquisition experiment. The amplitude spectra of all the earthquakes are simultaneously inverted for source, path (t*), and site parameters. The t* values are inverted for QP using local earthquake tomography methods and a known 3‐D P wave velocity model for the region. The four major seismicity arms of the NMSZ exhibit reduced QP (higher attenuation) than the surrounding crust. The highest attenuation anomalies coincide with areas of previously reported high swarm activity attributed to fluid‐rich fractures along the southeast extension of the Reelfoot fault. The QP results are consistent with previous attenuation studies in the region, which showed that active fault zones and fractured crust in the NMSZ are highly attenuating.

Inside Mt. Vesuvius: a new method to look at the seismic (velocity and attenuation) tomographic imaging

<p>New velocity and attenuation images of the geological structures below Mt. Vesuvius have been obtained using the programming facilities as well as the enhanced graphical power of Mathematica<span><sup>8TM</sup></span>. The velocity and attenuation space distributions, already calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes, are first optimally interpolated and then graphically represented in a new Mathematica<span><sup>8TM</sup></span> code notebook (a powerful computational document with more facilities than a simple code) developed by the present authors. The notebook aims at interactively and friendly representing 3D volume distributions of velocity and attenuation parameters. The user can easily obtain vertical sections (N-S, E-W, NE-SW and NW-SE oriented) and define color scales to represent velocity or attenuation variations or prefer iso-surface plots to represent the pattern of peculiar geological structures. The use of dynamic graphical representation, allowing the sliding of any (horizontal and/or vertical) slice through the volume under study, gives an unusual and powerful vision of any small velocity or attenuation anomaly. The (open source) code, coupled with the friendly use of internal routines of Mathematica, allows to adapt the graphical representation to any user necessity. The method appears to be particularly adapt to represent attenuation images, where the space variations of the parameters are strong with respect to their average. The 3-D plots of the interpolated velocity and attenuation fields enhance the image of Mt. Vesuvius structure, evidencing low-velocity associated with high attenuation anomalies which appeared unfocused in the plots reported by Scarpa et al. [2002] and De Siena et al. [2009].</p>

New insights into Mt. Vesuvius hydrothermal system and its dynamic based on a critical review of seismic tomography and geochemical features

<p>The seismic velocity and attenuation tomography images, calculated inverting respectively P-wave travel times and amplitude spectra of local VT quakes at Mt. Vesuvius have been reviewed and graphically represented using a new software recently developed using Mathematica<span><sup>8TM</sup></span>. The 3-D plots of the interpolated velocity and attenuation fields obtained through this software evidence low-velocity volumes associated with high attenuation anomalies in the depth range from about 1 km to 3 km below the sea level. The heterogeneity in the distribution of the velocity and attenuation values increases in the volume centred around the crater axis and laterally extended about 4 km, where the geochemical interpretation of the data from fumarole emissions reveals the presence of a hydrothermal system with temperatures as high as 400-450°C roughly in the same depth range (1.5 km to 4 km). The zone where the hydrothermal system is space-confined possibly hosted the residual magma erupted by Mt. Vesuvius during the recent eruptions, and is the site where most of the seismic energy release has occurred since the last 1944 eruption.</p>

High resolution regional seismic attenuation tomography in eastern Tibetan Plateau and adjacent regions

[1] The Q of regional seismic phases Lg and Pg within the crust is assumed as a proxy for crustal Qβ and Qα, which is used as a constraint of crustal rheology. We measure regional-phase Q of the eastern Tibetan Plateau and adjacent areas. This method eliminates contributions from source and site responses and is an improvement on the Two-Station Method (TSM). We have generated tomographic images of crustal attenuation anomalies with resolution as high as 1°. In general we observe low Q in the northernmost portions of the Tibetan Plateau and high Q in the more tectonically stable regions such as the interior of the Qaidam basin. The calculated site responses appear to correlate with topography or sediment thickness. Furthermore the relationship between earthquake magnitudes and calculated source terms suggest that the RTM method effectively removes the source response and may be used as an alternative to source magnitude.

On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La1− x Sr x MnO3 perovskite manganite materials with different compositions (x = 0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.

Seismic wave attenuation beneath the Australasian region

The attenuation of seismic waves is strongly influenced by temperature and rheological changes, and so provides an important supplement to information from seismic wavespeeds. Differential attenuation measurements are made for seismic waves refracted through the mantle beneath the Australasian region and are then used to construct 3D images of the attenuation of shear and compressional waves. The differential attenuation between different seismic phases is estimated using a spectral ratio method in the frequency band 0.25–1.0 Hz using a multi-taper method. Over this band the attenuation is nearly independent of frequency so that the logarithmic spectral ratio is a linear function of frequency. Differential measurements are made either between P and S phases on the same record or between stations for P and S waves separately. Images of seismic attenuation for the Australasian region are produced using a tomographic inversion, with the fast marching method employed to trace ray paths in an initial 3-D model derived from surface wave tomography. There is a deep seated horizontal contrast between central Australia and the eastern seaboard. The crustal and lithospheric mantle beneath the Archean and Proterozoic rocks in the west and in the middle of the continent have low seismic attenuation, whereas the Phanerozoic material in the east is more attenuative. Regions with recent volcanism, most likely associated with hot spots such as near Bass Strait and the Coral Sea, display high seismic attenuation anomalies. There is a strong contrast in attenuation between the relatively low loss lithosphere and the high loss asthenosphere beneath with a change by a factor of 6–10. Anisotropy in attenuation is slight even though some would be expected from the radial anisotropy seen in shear wavespeed in the lithosphere.

Characterization of subsurface polycyclic aromatic hydrocarbons at the Deepwater Horizon site

Here, we report the initial observations of distributions of polycyclic aromatic hydrocarbons (PAH) in subsurface waters near the Deepwater Horizon oil well site (also referred to as the Macondo, Mississippi Canyon Block 252 or MC252 well). Profiles of in situ fluorescence and beam attenuation conducted during 9‐16 May 2010 were characterized by distinct peaks at depths greater than 1000 m, with highest intensities close to the wellhead and decreasing intensities with increasing distance from the wellhead. Gas chromatography/mass spectrometry (GC/MS) analyses of water samples coinciding with the deep fluorescence and beam attenuation anomalies confirmed the presence of polycyclic aromatic hydrocarbons (PAH) at concentrations reaching 189 μg L−1 (ppb). Subsurface exposure to PAH at levels considered to be toxic to marine organisms would have occurred in discrete depth layers between 1000 and 1400 m in the region southwest of the wellhead site and extending at least as far as 13 km.

Improved 2-D attenuation analysis for Northern Italy using a merged dataset from selected regional seismic networks

A merged, high-quality waveform dataset from different seismic networks has been used to improve our understanding of lateral seismic attenuation for Northern Italy. In a previous study on the same region, Morasca et al. (Bull Seismol Soc Am 98:1936–1946, 2008) were able to resolve only a small area due to limited data coverage. For this reason, the interpretation of the attenuation anomalies was difficult given the complexity of the region and the poor resolution of the available data. In order to better understand the lateral changes in the crustal structure and thickness of this region, we selected 770 earthquakes recorded by 54 stations for a total of almost 16,000 waveforms derived from seismic networks operating totally or partially in Northern Italy. Direct S-wave and coda attenuation images were obtained using an amplitude ratio technique that eliminates source terms from the formulation. Both direct and early-coda amplitudes are used as input for the inversions, and the results are compared. Results were obtained for various frequency bands ranging between 0.3 and 25.0 Hz and in all cases show significant improvement with respect to the previous study since the resolved area has been extended and more crossing paths have been used to image smaller scale anomalies. Quality-factor estimates are consistent with the regional tectonic structure exhibiting a general trend of low attenuation under the Po Plain basin and higher values for the Western Alps and Northern Apennines. The interpretation of the results for the Eastern Alps is not simple, possibly because our resolution for this area is still not adequate to resolve small-scale structures.

Ultrasonic and Piezoelectric Investigations of Phase Transitions in Ferroelastic Cu6PS5(I,Br) Mixed Crystals

The ultrasonic velocity and attenuation anomalies were observed near the elastic phase transitions (PT) in the vicinity of 270 K in Cu 6 PS 5 (I,Br) mixed superionic crystals, where I was partially substituted by Br. We have shown that elastic anomalies at phase transition gradually decrease with increasing of iodine concentration in the material. The critical anomalies were described by relaxational Landau theory, which implies the increase of order parameter relaxation time: τ = τ o /(T c − T). The prefactor τ o increases with increasing iodine content in crystal. The ultrasonic anomalies near low temperature superionic phase transitions were weak. These transitions were observed using piezoelectric detection of ultrasonic waves by surface barrier. The temperature dependencies of detected by surface barrier signal were measured and it was shown that this method is very sensitive for indication of phase transitions in such superionic crystals.

Phase transition in sodium bismuth tungstate NaBi(WO4)2 – acoustic studies

AbstractAcoustic studies of sodium bismuth tungstate, NaBi(WO4)2, single crystal were carried out in the temperature range from 290 K to 340 K. Strong anomalies in ultrasound velocity and attenuation were observed above room temperature for both longitudinal and transverse waves. The results obtained suggested a phase transition at 319.2 K. The order parameter responsible for the phase transition was found to be coupled linearly with strains in transverse acoustic waves and nonlinearly with those in longitudinal waves. It was suggested that the phase transition is related with substitutional ordering of the Na+ and Bi3+ ions at the cation sites. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)