Seismic Attenuation Tomography Research Articles

Resolution and trade-offs in global anelastic full-waveform inversion

Summary Improving the resolution of seismic anelastic models is critical for a better understanding of the Earth’s subsurface structure and dynamics. Seismic attenuation plays a crucial role in estimating water content, partial melting, and temperature variations in the Earth’s crust and mantle. However, compared to seismic wavespeed models, seismic attenuation tomography models tend to be less resolved. This is due to the complexity of amplitude measurements and the challenge of isolating the effect of attenuation in the data from other parameters. Physical dispersion caused by attenuation also affects seismic wavespeeds, and neglecting scattering/defocusing effects in classical anelastic models can lead to biased results. To overcome these challenges, it is essential to account for the full 3D complexity of seismic wave propagation. Although various synthetic tests have been conducted to validate anelastic Full-Waveform Inversion (FWI), there is still a lack of understanding regarding the trade-off between elastic and anelastic parameters, as well as the variable influence of different parameter classes on the data. In this context, we present a synthetic study to explore different strategies for global anelastic inversions. To assess the resolution and sensitivity for different misfit functions, we first perform mono-parameter inversions by inverting only for attenuation. Then, to study trade-offs between parameters and resolution, we test two different inversion strategies (simultaneous and sequential) to jointly constrain the elastic and anelastic parameters. We found that a sequential inversion strategy performs better for imaging attenuation than a simultaneous inversion. We also demonstrate the dominance of seismic wavespeeds over attenuation, underscoring the importance of determining a good approximation of the Hessian matrix and suitable damping factors for each parameter class.

Seismic attenuation tomography of Eastern Europe from ambient seismic noise analysis

SUMMARY The Eastern-Europe region (EER), is a complex geotectonic area that captures part of the Alpine-Himalayan Orogen, the subduction of multiple NeoTethys Branches and part of the East European Craton. It is one of the most exciting geological areas in Europe due to a diversity of tectonic processes acting within it: extensional basin evolution, oceanic subduction, post-collisional volcanism, as well as active crustal deformation associated with the push of the Adria plate or the pull of the actively detaching Vrancea slab. This makes EER an excellent natural laboratory to study the behaviour of the lithosphere–asthenosphere system in a heterogeneous tectonic setting. To investigate the lateral heterogeneity and physical properties of the crust in the EER, we use ambient seismic noise data recorded by the vertical components of broad-band stations that have been operational between 1999 and 2020 in Eastern Europe and surrounding regions. We used this significant amount of data and the latest processing techniques of the ambient seismic noise field based on the continuous wavelet transform to compute cross-correlations between various station pairs, turning every available seismic station into a virtual source. The coda of the interstation cross-correlograms were used to determine coda quality factors (Qc) of Rayleigh waves in four different period ranges (3.0–5.0, 5.0–10.0, 10.0–20.0 and 20.0–30.0 s) and to invert them in the 2-D space, constructing the highest resolution attenuation tomography of the region. Our results reveal high attenuation features throughout the northeast Pannonian region, the Bohemian Massif, the East Carpathians and the Moesian Platform. Nevertheless, our findings do not emphasize a close correlation between the depth of sedimentary basins and attenuation features identified at longer periods. In addition, Qc variations are larger at short periods, indicating higher heterogeneity in the uppermost crust of Eastern Europe. Our findings demonstrate the higher efficiency of noise correlation approaches relative to earthquake data analyses investigating Qc at low frequencies.

Lateral Variations of Attenuation in the Crust of Alaska Using Lg Q Tomography

ABSTRACT We have conducted a crustal seismic (QLg) attenuation tomography study across Alaska using recordings from the EarthScope USArray from 2014 to 2019. The resolving power of the inversion is 150 × 150 km for most of Alaska, and it is 75 × 75 km in central and southern Alaska. Numerous fault systems and high mountain ranges are present across Alaska and accommodate compression in the north–south direction and shearing of southern Alaska toward the west. These mountain ranges include the Brooks range in the north, the Alaska range in central Alaska, and the Aleutian range in the southwest. The average LgQ for all of Alaska is significantly higher than in the western United States and Canada. This lower average attenuation impacts seismic hazard estimates for the region. According to the tomographic results, we see a significant variation of the QLg values from low to high across the southern part of the Brooks range. Also, we found higher attenuation in the southeast region of Alaska, where the Wrangell volcanoes are located. Moreover, we see an area of lower attenuation associated with weak frequency dependence in the south-central region of Alaska next to Anchorage. Another anomaly with lower attenuation can be seen extending from central Alaska to southeast Alaska, possibly associated with the Yukon–Tanana terrane. There are a few areas like southwest Alaska associated with the Togiak terrane and an area next to Fairbanks in Alaska’s interior that shows lower attenuation with lower frequency dependence and higher attenuation with higher frequency dependence, respectively, for low frequencies up to 3 Hz. Our model’s highest η zones (η≳95) are mostly confined to major tectonic terranes and other major tectonic elements such as faults and fractures. Regional variations in crustal attenuation can impact local seismic hazard estimates if incorporated into the hazard analysis.

Crustal attenuation structure of the Tianshan tectonic belt and its spatiotemporal variations

The quality factor value (Q) of the crustal medium, which can describe the anelasticity within the Earth’s interior, is a sensitive indicator of changes in the crystalline structure induced by temperature and phase transformations. Although the velocity structure of the Tianshan region in Central Asia has been extensively studied, studies regarding its Q values are limited. These studies focus mainly on the crustal attenuation structure of the Tianshan region; however, their results are limited to the qualitative analyses of the local areas or averages over large areas. Therefore, in this study, we conducted seismic attenuation tomography to create a Q map of the crust underneath the Tianshan tectonic belt (TTB) at a resolution of 0.8° × 0.8° using data from 24,273 near-source waveforms recorded by 51 observation stations of the Xinjiang regional seismic network from 2009 to 2020. The regional distribution of the static and sliding-average values (QS) was calculated. The average value (Q0) of TTB was approximately 523. Additionally, Qs exhibited considerable lateral variations that strongly correlate with the surface tectonics of the TTB region. Furthermore, the velocity and attenuation structures of the TTB were positively correlated. The main part of the TTB exhibited high velocities and Q (indicating low attenuation), whereas the areas adjoining the Tarim and Junggar basins (at the South and North of the TBB, respectively) and their margins exhibited low velocities and Q (indicating high attenuation). This suggested that the attenuation structure of the TTB was highly consistent with its velocity and density structures. Since 1900, most earthquakes in the TTB having magnitudes ≥6.0 earthquakes have occurred at the junctions of high- and low-Q-value areas, or in areas with low Q values. According to the Qs values in different periods, the average Qs of the entire TTB only varied between 500 and 540. However, the average Qs in the middle TTB region portrays an upward trended over time (from 494 in 2010 to 554 in 2020). The average Qs of the southwestern TTB region has been relatively stable, varying between 490 and 530. The Q0 of the southwestern TTB region was lower than that observed in the middle TTB region in most of the time. This observation is more consistent with the tectonic activity recorded in the southwestern TTB region (with greater intensity) than that observed in the middle Tianshan. In addition, the number of earthquakes with magnitudes ≥4.0 correlated positively with the regional average Qs in the middle and southwestern Tianshan. Notably, the higher the regional average Q value, the larger the number of moderate earthquakes. This correlation suggests that in earthquake-prone regions, the accumulation and release of stress influence the opening or closure of crustal fractures, resulting in noticeable changes in the Q values. The findings of our study provide novel insights into the mechanisms of earthquakes and their correlation with the structure of the Earth’s crust.

Hydrothermal Fluids and Where to Find Them: Using Seismic Attenuation and Anisotropy to Map Fluids Beneath Uturuncu Volcano, Bolivia

AbstractMapping fluid accumulation in the crust is pertinent for numerous applications including volcanic hazard assessment, geothermal energy generation, and mineral exploration. Here, we use seismic attenuation tomography to map the distribution of fluids in the crust below Uturuncu volcano, Bolivia. Seismic P wave and S wave attenuation, as well as their ratio (QP/QS), constrain where the crust is partially and fully fluid‐saturated. Seismic anisotropy observations further constrain the mechanism by which the fluids accumulate, predominantly along aligned faults and fractures in this case. Furthermore, subsurface pressure‐temperature profiles and conductivity data allow us to identify the most likely fluid composition. We identify shallow regions of both dry and H2O/brine‐saturated crust, as well as a deeper supercritical H2O/brine column directly beneath Uturuncu. Our observations provide a greater understanding of Uturuncu's transcrustal hydrothermal system, and act as an example of how such methods could be applied to map crustal fluid pathways and hydrothermal/geothermal systems elsewhere.

Warm versus cold crust in the Tien Shan orogenic belt revealed by seismic Lg attenuation tomography

SUMMERY Due to the far-field effect of the India–Eurasia collision, the Tien Shan orogenic belt has been undergoing reactivation and modification. Two end-member models of the geodynamic mechanisms are (1) surface uplift due to crustal shortening caused by lithospheric compression and (2) mountain formation resulting from thermal upwelling of asthenospheric mantle materials generated by lithospheric subduction. However, the topography along the Tien Shan orogenic belt changes significantly, and the deep structure and dynamic process are quite different beneath the Tien Shan orogenic belt from both geological and geophysical observations. Therefore, the reactivation and modification of the Tien Shan orogenic belt are likely influenced by both geodynamic mechanisms, which also generate various thermal anomalies in the crust. Seismic Lg-wave attenuation is very sensitive to crustal material composition and status and can point to the presence of partial melting within the crust resulting from mantle upwelling. In this study, we develop a high-resolution Lg-wave attenuation model between 0.05 and 10.0 Hz in Northwest China and use lateral attenuation variations to infer thermal structures in the crust. The central Tien Shan is characterized by prominent low-QLg anomalies, whereas relatively high-QLg distributions are imaged beneath the eastern and western Tien Shan. The surface uplift and crustal deformation are mostly related to the upwelling of hot mantle materials in the central Tien Shan and are likely induced by lithospheric compression in the eastern and western Tien Shan. However, low-Q anomalies are observed in the junction between the Pamir Plateau and western Tien Shan, indicating that the uplift in the south of the western Tien Shan is related to thermal subduction-induced upwelling and intrusion into the crust due to the collision between the Indian and Eurasian plates. The Kazakh Shield, characterized by pronounced high-QLg values, is likely a cold and hard terrane, and hence blocks the far-field effect of the India–Eurasia collision.

Cross-well seismic attenuation tomography with correction for coupling effects at source and receiver positions in the Yutsubo geothermal field, Japan

Seismic attenuation is an important property in the geothermal field because it is related to fluid content and reservoir fractures. However, seismic attenuation analysis is easily affected by various factors that must be carefully treated. We investigated cross-well tomography data acquired in the Yutsubo geothermal field and revealed factors affecting the frequency response of data for attenuation tomography. The amplitude of the first arrival waveforms suggests the presence of effects from source coupling, and properties of the surrounding medium affect the radiation pattern and absolute amplitude values. Also, the amplitude and centroid frequencies of the first arrival waveforms show highly variable values along traces, and analysis using an autocorrelation function suggests that there are also effects due to receiver coupling. In this study, we developed a method to correct a coupling effect by assuming that wavefields inside the first Fresnel zone have the same amplitude and frequency character. Our approach uses neighboring traces inside the first Fresnel zone to estimate correction functions. By applying correction functions, the final attenuation tomography results showed a decrease in standard deviation, which indicates that applying a coupling correction increased the inversion stability. In addition, we investigated the interference of guided waves inside a low-velocity layer using finite-difference modeling. A comparison of synthetic and real data suggests the possibility of the phenomena in our data, which is not widely recognized in the geothermal field. We adopted an approach that excluded data having the potential of interference from those waves. Our attenuation tomography results extracted a high-attenuation zone where open fractures are thought to be distributed.

Ambient Noise Attenuation Tomography Reveals an Asymmetric Damage Zone Across San Jacinto Fault Near Anza, California

AbstractWe perform seismic attenuation tomography of the shallow structure for the San Jacinto Fault in the Ramona Reservation of southern California. The study uses ambient seismic noise recorded by a linear array of 65 3‐C sensors across the fault. We extract amplitude decay information, with uncertainty, from noise interferometry functions. To account for strong heterogeneities in the complex shallow fault zone structure, we apply a frequency‐dependent amplitude correction for focusing/defocusing effects using inverted phase velocity maps obtained by solving the transport equation. We then estimate the attenuation structure based on the linear station‐triplet method for both Love and Rayleigh waves. The attenuation tomography indicates strong attenuation correlated with known San Jacinto Fault surface traces. The Love wave attenuation tomography reveals an asymmetric damage zone that exists primarily on the fault side with faster seismic velocity, consistent with earthquake ruptures on a fault bimaterial interface with preferred propagation direction to the northwest.

Seismic attenuation tomography of the Sn phase beneath the Turkish-Iranian Plateau and the Zagros mountain belt

AbstractThe Turkish-Iranian Plateau and the Zagros highlands are among the most prominent physiographic features in the Middle East and were formed as a result of continental collision between the Arabian and Eurasian plates. To better understand the nature of the lithospheric mantle and the origin of the observed seismic anomalies in this region, we investigated seismic attenuation of the uppermost mantle by detailed measurements of the quality factor of the Sn seismic phase (Sn Q). To that end, we collected a large data set consisting of 30 years (1990–2020) of waveforms recorded by 1266 permanent and temporary seismic stations, applying both the two-station method (TSM) and reverse two-station method (RTM) to measure path-averaged Sn Q. Finally, we performed a tomographic inversion on the path-averaged Sn Q to map the lateral variations of the upper-mantle attenuation across the northern Middle East. Our Sn attenuation maps show moderately low Q (<250) values beneath the Turkish-Iranian Plateau and high Q values (>350) beneath the Zagros and northern edge of the Arabian plate. Furthermore, our Sn Q model is broadly consistent with seismic velocity models in the region suggesting that most of the seismic anomalies are the result of thermal rather than compositional effects.

Temporal Changes in Seismic Velocity and Attenuation at The Geysers Geothermal Field, California, From Double‐Difference Tomography

AbstractWater injection and Enhanced Geothermal System (EGS) technologies have been used to exploit heat resources from geothermal reservoirs. Detecting spatial and temporal changes in reservoir physical properties is important for monitoring reservoir condition changes due to water injection and EGS. Here, we determine high‐resolution models of the temporal changes in the three‐dimensional P wave velocity and attenuation (Vp and Qp) structures between the years 2005 and 2011 in the northwestern part of The Geysers geothermal field, California, using double‐difference seismic velocity and attenuation tomography. The northwest Geysers has a shallow normal temperature reservoir (NTR) underlain by a high temperature reservoir (HTR) that has substantial underutilized heat resources but may be more fully utilized in the future through EGS. In the southeastern part of the northwest Geysers, however, EGS has been successfully but unintentionally applied for at least 50 years because the waters injected into the NTR have been flowing into the HTR. Our models are well resolved in this area and show that the NTR and HTR have different seismic responses (seismicity, Vp, and Qp) to water injection, which can be explained by the injection‐induced differences in fracturing and saturation that are likely related to their geological properties. Our results indicate that the joint analysis of changes in seismicity, velocity, and attenuation is valuable for characterizing changes in reservoir fracturing and saturation conditions. Our results suggest that high‐permeability zones and/or pre‐existing permeable fault zones are important for the success of EGS at The Geysers and potentially other geothermal systems.

Crustal Seismic Attenuation of the Central United States and Intermountain West

AbstractSeismic attenuation is generally greater in the western United States (WUS) than the central and eastern United States (CEUS), but the nature of this transition or location of this boundary is poorly constrained. We conduct crustal seismic (Lg) attenuation tomography across a region that stretches from the CEUS across the Rocky Mountains to the Basin and Range using a total of 115,870 amplitude measurements from 106 earthquakes recorded on 544 stations across five frequency bands spanning 0.5–16 Hz. Similar to previous studies, we find higher attenuation in the WUS (Q0 ∼ 190) than the nominally CEUS (Q0 ∼ 250) and comparatively high attenuation on the Gulf Coast (Q0 ∼ 175). Our models defy simple east versus west regionalization, however. Heterogeneity within the Rocky Mountain region—low attenuation in the Colorado Plateau interior and Wyoming Craton (Q0 ∼ 230) compared to high attenuation in the southern Rockies (Q0 ∼ 110)—exceeds the gross differences between the CEUS and western United States. These province‐scale patterns are readily interpreted in terms of intrinsic attenuation. The boundary between the Colorado Plateau and Basin and Range hosts the highest attenuation imaged in the study area (Q0 ∼ 90), consistent with localized scattering across contrasting crustal structure. Focused high attenuation in the southern Rockies may represent the effects of represent in situ partial crustal melt. Within the CEUS, second‐order bands of comparatively high attenuation align with the Proterozoic Yavapai‐Mazatzal suture zone and Midcontinent Rift. This complex attenuation structure defies broad regionalization and suggests a need for path‐specific models near these boundaries and for critical infrastructure.

Anelasticity and Lateral Heterogeneities in Earth's Upper Mantle: Impact on Surface Displacements, Self‐Attraction and Loading, and Ocean Tide Dynamics

AbstractSurface displacement and self‐attraction and loading (SAL) elevation induced by ocean tides are known to be affected by material properties of the solid Earth. Recent studies have shown that, in addition to elasticity, anelasticity considerably impacts surface displacements due to ocean tide loading (OTL). We employ consistent 3D seismic elastic and attenuation tomography models to construct 3D elastic and anelastic earth models, and derive corresponding averaged 1D elastic/anelastic models. We apply these models to systematically study the impact of anelasticity and lateral heterogeneity on M2 OTL displacements and SAL elevation. We find that neglecting lateral heterogeneities highly underestimates displacements and SAL elevation in mid‐ocean‐ridge regions and in some coastal areas of North and Central America. In comparison to PREM, 3D anelastic models can increase the predicted amplitudes of the vertical displacement and SAL elevation by up to 1.5 mm. The increased amplitudes reduce the discrepancy between GPS‐observed OTL displacements and their predictions based on PREM in places like Cornwall (England), Brittany (France), and the Ryukyu Islands (Japan). Applying our results to ocean tides, we discover that the impact on ocean tide dynamics exceeds the predicted SAL elevation correction with an RMS of about 1 mm, reaching an RMS of more than 5 mm in areas like North Atlantic or East Pacific. Due to the fact that such a value reaches the accuracy of modern data‐constrained tidal models, we regard the impact of anelastic shear relaxation as significant in tidal modeling.

Ambient noise Love wave attenuation tomography for the LASSIE array across the Los Angeles basin.

The Los Angeles basin is located within the North America-Pacific plate boundary and contains multiple earthquake faults that threaten greater Los Angeles. Seismic attenuation tomography has the potential to provide important constraints on wave propagation in the basin and to provide supplementary information on structure in the form of the distribution of anelastic properties. On the basis of the amplitude information from seismic interferometry from the linear LASSIE array in the Los Angeles basin, we apply station-triplet attenuation tomography to obtain a 2D depth profile for the attenuation structure of the uppermost 0.6 km. The array crosses four Quaternary faults, three of which are blind. The attenuation tomography resolves strong attenuation (shear attenuation Qs ~ 20) for the fault zones and is consistent with sharp boundaries across them.

Double-difference seismic attenuation tomography method and its application to The Geysers geothermal field, California

SUMMARY Knowledge of attenuation structure is important for understanding subsurface material properties. We have developed a double-difference seismic attenuation (DDQ) tomography method for high-resolution imaging of 3-D attenuation structure. Our method includes two main elements, the inversion of event-pair differential ${t^*}$ ($d{t^*}$) data and 3-D attenuation tomography with the $d{t^*}$ data. We developed a new spectral ratio method that jointly inverts spectral ratio data from pairs of events observed at a common set of stations to determine the $d{t^*}$ data. The spectral ratio method cancels out instrument and site response terms, resulting in more accurate $d{t^*}$ data compared to absolute ${t^*}$ from traditional methods using individual spectra. Synthetic tests show that the inversion of $d{t^*}$ data using our spectral ratio method is robust to the choice of source model and a moderate degree of noise. We modified an existing velocity tomography code so that it can invert $d{t^*}$ data for 3-D attenuation structure. We applied the new method to The Geyser geothermal field, California, which has vapour-dominated reservoirs and a long history of water injection. A new Qp model at The Geysers is determined using P-wave data of earthquakes in 2011, using our updated earthquake locations and Vp model. By taking advantage of more accurate $d{t^*}$ data and the cancellation of model uncertainties along the common paths outside of the source region, the DDQ tomography method achieves higher resolution, especially in the earthquake source regions, compared to the standard tomography method using ${t^*}$ data. This is validated by both the real and synthetic data tests. Our Qp and Vp models show consistent variations in a normal temperature reservoir that can be explained by variations in fracturing, permeability and fluid saturation and/or steam pressure. A prominent low-Qp and Vp zone associated with very active seismicity is imaged within a high temperature reservoir at depths below 2 km. This anomalous zone is likely partially saturated with injected fluids.

Seismic attenuation tomography in southwestern China: Insight into the evolution of crustal flow in the Tibetan Plateau

Crustal flow in southwestern China is commonly attributed to the continental collision between the Indian and the Eurasian plates. Therefore, studying crustal flow in this area can provide valuable insight into the collisional process. The quality factor (Q) is an indicator of seismic wave attenuation that provides constraints on lithology, rheology, and geodynamic processes, which are vital for interpreting the nature of the crustal flow in a given region. In this study, we used local seismic waveforms to obtain regional attenuation models for P- (QP) and S- (QS) waves in the Sichuan-Yunnan region (SYR). The Emeishan Large Igneous Province (ELIP) is characterized by high QP and QS values in the Middle Yunnan block (MYB) at depths greater than 5 km. The QP and QS values in most of the fault zones surrounding the MYB were lower than 300, indicating that the material within crustal faults causes high waveform attenuation. Our results suggest that the crustal flow was diverted into two channels in the northern MYB due to resistance from the ELIP. To illuminate the tectonic dynamics of the SYR, we propose a simple model that explains both the evolution of the crustal flow and the variation in the seismic wave attenuation.

Upper Crustal Weak Zone in Central Tibet: An Implication From Three‐Dimensional Seismic Velocity and Attenuation Tomography Results

AbstractA series of conjugate strike‐slip faults is the most prominent geologic feature in central Tibet and is considered to accommodate east‐west extension and coeval north‐south contraction. The development mechanism of the conjugate strike‐slip fault system is under debate because of unclear crustal physical properties and compositional variations. P and S wave arrivals from 414 local earthquakes recorded by the temporary Seismic Array Integrated Detection for a Window of Indian Continental Head array and the permanent China National Seismic Network were used for the velocity tomography, with additional P and S wave arrivals from 12 shots of the International Deep Profiling of Tibet and the Himalaya III reflection/refraction profile. The local earthquakes were simultaneously relocated with the updated velocity models. We also inverted for a three‐dimensional upper crustal Qp model with the same earthquake data set. The Vp structure near the surface shows that low‐Vp anomalies generally correspond to sedimentary basins and high‐Vp anomalies are related to exhumed metamorphic blocks in the study area. Relatively low Vp/Vs ratios in the upper crust indicate widely distributed quartz‐rich rocks. The low‐Vp zone from 0‐ to 10‐km depth (resolving depth limit) is spatially correlated with the Bangong‐Nujiang suture, possibly reflecting the compositional difference along the ophiolitic mélange belt accompanied by twin volcanic arcs from a double‐sided subduction. This interpretation is supported by relatively heterogeneous Qp values. This low‐velocity zone also implies relatively uniform stress and continuous deformation in the upper crust of central Tibet. The relatively weak materials in at least the upper crust would result in strain concentration and help the development of the conjugate strike‐slip fault system along the Bangong‐Nujiang suture.

New evidence for the serpentinization of the Palaeozoic basement of southeastern Sicily from joint 3-D seismic velocity and attenuation tomography

New evidence for the serpentinization of the Palaeozoic basement of southeastern Sicily from joint 3-D seismic velocity and attenuation tomography

Non-invasive characterization of a burial tumulus with use of seismic P-wave velocity and attenuation tomography

Knowledge of the engineering quality of soil material making up ancient earthen structures such as tumuli (burial mounds) is essential for planning and protecting their structural integrity during excavation and for designing preservation measures. A non-invasive method for assessing the engineering quality of a tumulus would be of benefit before archaeological exploration and during excavation, and in cases where excavation is not a choice. The relation of seismic velocity and attenuation to the engineering quality of geological materials is well established. We use seismic P-wave velocity and attenuation tomography to explore non-invasively soil differentiations and assess the bulk soil quality of a tumulus' lower embankment and foundations. The method is applied to a tumulus located in Northern Greece. Our results show that P-wave velocity patterns can image important structural and lithological differentiations by determining boundaries of soil conditions. Attenuation provides additional details on the spatial heterogeneity of soil quality within the embankment, as it appears to be sensitive to different factors related to soil quality. The combined results allowed us to outline the bedrock morphology of the tumulus foundations and identify areas of reduced soil strength in the embankment that we discuss with respect to the construction and usage of the tumulus. With use of the tomographic velocities we obtained parameter values for the soils of a tumulus body for the first time, that allowed us to assess the combined effect of grain size, porosity, saturation and consolidation on the bulk soil quality of the tumulus. We suggest that seismic tomography can be an efficient tool for spatially continuous and whole-scale non-invasive investigation of the soil quality and structural integrity of tumuli and similarly shaped structures, effectively contributing to the understanding of the structure's history and assisting excavation and protection design.

Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics

We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous–Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.

Seismic travel-time and attenuation tomography to characterize the excavation damaged zone and the surrounding rock mass of a newly excavated ramp and chamber

Seismic travel-time and attenuation tomography were applied to characterize the excavation damaged zone and the adjacent rock mass in the GFZ-Underground-Lab within the research and education mine Reiche Zeche of the Technical University Bergakademie Freiberg (Germany). The lab is situated in gneiss rocks at 150m depth and comprises three galleries which enclose an area of approximately 50m×100m. Along these galleries two seismic surveys were performed before and after the excavation of a new ramp and chamber. For both measurements, travel-time and attenuation tomographies for P-waves were performed with the ray-based inversion algorithm SIMULPS14. The seismic velocities were calculated from first-arrival travel times whereas a logarithmic-spectral-ratio approach was used to calculate the corresponding quality factors (Q) for attenuation tomography. A comparison of the tomograms reveals a decrease of average P-wave velocity values from 5.64km/s to 5.54km/s and of average Q-values from 29.8 to 26.5 in the whole area after the excavation of the new cavities. The maximum changes are located at already weakened zones either at the conjunction of two galleries or of a major fracture zone with a gallery. The attenuation tomography shows a higher sensitivity to rock mass changes than the travel-time tomography. However, the calculation of the Q-values demands a higher signal quality than the determination of the seismic travel times.