Bedrock Velocity Research Articles

2D Near‐Surface Full‐Waveform Tomography Reveals Bedrock Controls on Critical Zone Architecture

AbstractFor decades, seismic imaging methods have been used to study the critical zone, Earth's thin, life‐supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near‐surface processes, therefore limiting progress in critical zone science. Full‐waveform tomography can overcome this limitation by leveraging more seismic data and enhancing the resolution of geophysical imaging. In this study, we apply 2D full‐waveform tomography to match the phases of observed seismograms and elucidate previously undetected heterogeneity in the critical zone at a well‐studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 m over lateral scales of just tens of meters and image steep low‐velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full‐waveform tomography results agree significantly better with velocity logs than previously published traveltime tomography models. Full‐waveform tomography thus appears unprecedentedly capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.

Deriving amplification factors from simple site parameters using generalized regression neural networks: implications for relevant site proxies

Most modern seismic codes account for site effects using an amplification factor (AF) that modifies the rock acceleration response spectra in relation to a “site condition proxy,” i.e., a parameter related to the velocity profile at the site under consideration. Therefore, for practical purposes, it is interesting to identify the site parameters that best control the frequency-dependent shape of the AF. The goal of the present study is to provide a quantitative assessment of the performance of various site condition proxies to predict the main AF features, including the often used short- and mid-period amplification factors, F_{a} and F_{v}, proposed by Borcherdt (in Earthq Spectra 10:617–653, 1994). In this context, the linear, viscoelastic responses of a set of 858 actual soil columns from Japan, the USA, and Europe are computed for a set of 14 real accelerograms with varying frequency contents. The correlation between the corresponding site-specific average amplification factors and several site proxies (considered alone or as multiple combinations) is analyzed using the generalized regression neural network (GRNN). The performance of each site proxy combination is assessed through the variance reduction with respect to the initial amplification factor variability of the 858 profiles. Both the whole period range and specific short- and mid-period ranges associated with the Borcherdt factors F_{a} and F_{v} are considered. The actual amplification factor of an arbitrary soil profile is found to be satisfactorily approximated with a limited number of site proxies (4–6). As the usual code practice implies a lower number of site proxies (generally one, sometimes two), a sensitivity analysis is conducted to identify the “best performing” site parameters. The best one is the overall velocity contrast between underlying bedrock and minimum velocity in the soil column. Because these are the most difficult and expensive parameters to measure, especially for thick deposits, other more convenient parameters are preferred, especially the couple left( {V_{{{text{s}}30}} ,f_{0} } right) that leads to a variance reduction in at least 60%. From a code perspective, equations and plots are provided describing the dependence of the short- and mid-period amplification factors F_{a} and F_{v} on these two parameters. The robustness of the results is analyzed by performing a similar analysis for two alternative sets of velocity profiles, for which the bedrock velocity is constrained to have the same value for all velocity profiles, which is not the case in the original set.Graphical abstractPerformance of various site proxies (velocity contrast Cv, fundamental frequency f0, harmonicvelocity average over the top 30 m VS30, total sediment thickness Depth, average correspondingvelocity VSm) to predict the short-period (top, Fa) and mid-period (bottom, Fv) amplification factors.Proxies may be considered alone, or in combination with several other proxies.

Rapid ice unloading in the Fleming Glacier region, southern Antarctic Peninsula, and its effect on bedrock uplift rates

Rapid regional warming in the Antarctic Peninsula has led to the significant retreat and eventual collapse of several major ice shelves since the 1970s, triggering the subsequent acceleration and thinning of their feeding glaciers. The Wordie Ice Shelf, lying off the west coast of the Antarctic Peninsula, has undergone long-term disintegration since the 1960s with a substantial calving event occurring around 1989, followed by continuous steady retreat and its almost-complete disappearance. The dynamic response of the upstream glaciers to the ice shelf collapse and the response of the solid Earth to the associated mass loss are not fully understood. To quantify the mass loss from the system, we generated a digital elevation model (DEM) using airborne vertical and oblique imagery from 1966 and compared it to a DEM derived from 2008 SPOT data. This analysis reveals lowering over that time of approximately 60 m at the front of Fleming Glacier. Using IceBridge and ICESat-2/GLAS data spanning 2002–2014, we show an increased rate of mean ice-surface lowering, with rates post-2008 more than twice those of 2002–2008. We use these load change data as a basis for the simulation of viscoelastic solid Earth deformation. We subtract modeled elastic deformation rates, and a suite of modeled viscous rates, from GPS-derived three-dimensional bedrock velocities at sites to the south of Fleming Glacier to infer properties of Earth rheology. Assuming the pre-breakup bedrock uplift was positive due to post-Last Glacial Maximum (LGM) ice retreat, our viscoelastic-corrected GPS uplift rates suggest upper mantle viscosities are >2×1019 Pas and likely >1×1020 Pas in this region, 1–2 orders of magnitude greater than previously found for the northern Antarctic Peninsula. Horizontal velocities at the GPS site nearest the Fleming Glacier, after the application of elastic and plate tectonic corrections, point away from Marguerite Bay rather than the present glacier front. This suggests that horizontal motion in the region reflects the earlier retreat of the glacier system following the LGM, compatible with a relatively strong mantle in this region. These findings highlight the need for improved understanding of ice load changes in this region through the late Holocene in order to accurately model glacial isostatic adjustment.

Shear wave velocity profile estimation by integrated analysis of active and passive seismic data from small aperture arrays

We present an integrated approach for deriving the 1D shear wave velocity (Vs) information at few tens to hundreds of meters down to the first strong impedance contrast in typical sedimentary environments. We use multiple small aperture seismic arrays in 1D and 2D configuration to record active and passive seismic surface wave data at two selected geotechnical sites in Germany (Horstwalde & Löbnitz). Standard methods for data processing include the Multichannel Analysis of Surface Waves (MASW) method that exploits the high frequency content in the active data and the sliding window frequency–wavenumber (f–k) as well as the spatial autocorrelation (SPAC) methods that exploit the low frequency content in passive seismic data. Applied individually, each of the passive methods might be influenced by any source directivity in the noise wavefield. The advantages of active shot data (known source location) and passive microtremor (low frequency content) recording may be combined using a correlation based approach applied to the passive data in the so called Interferometric Multichannel Analysis of Surface Waves (IMASW).In this study, we apply those methods to jointly determine and interpret the dispersion characteristics of surface waves recorded at Horstwalde and Löbnitz. The reliability of the dispersion curves is controlled by applying strict limits on the interpretable range of wavelengths in the analysis and further avoiding potentially biased phase velocity estimates from the passive f–k method by comparing to those derived from the SPatial AutoCorrelation method (SPAC). From our investigation at these two sites, the joint analysis as proposed allows mode extraction in a wide frequency range (~0.6–35Hz at Horstwalde and ~1.5–25Hz at Löbnitz) and consequently improves the Vs profile inversion.To obtain the shear wave velocity profiles, we make use of a global inversion approach based on the neighborhood algorithm to invert the interpreted branches of the dispersion curves. Within the uncertainty given by the apparent spread of forward models we find that besides a well defined sediment velocity range also a reasonable minimum estimate of bedrock depth and bedrock velocity can be achieved. The Vs estimate for the best model in Horstwalde ranges from ~190m/s at the surface up to ~390m/s in the bottom of the soft sediment column. The bedrock starts earliest around 200m depth and bedrock velocities are higher than 1000m/s. In Löbnitz, we observe slightly lower velocities for the sediments (~165–375m/s for the best model) and a minimum thickness of 75m.

Menominee Crack: Bedrock Pop‐Up Event near Menominee, Michigan

ABSTRACT In the early morning of 4 October 2010, a loud noise was heard and shaking was felt in a rural area north of Menominee, Michigan. Within a day, residents discovered what we now identify as a “pop‐up” feature; due to the extensional crack along its crest, it has become known as the Menominee Crack. Within a few days, physical observations were made, and an additional geophysical survey was conducted in 2013 to further investigate the origin of the crack and associated ridge. An autoleveling and seismic refraction survey performed at the site revealed a bedrock layer approximately 2 m underneath the glacial till and also found a significant difference in bedrock velocities observed parallel and perpendicular to the crack. Modeling of a fractured limestone, assuming the seismic raypaths cross fractures in one direction but not in the other, is consistent with these velocities. The origin of the feature remains unknown. Glacial unloading was complete many thousands of years ago, and no significant removal of overburden occurred in recent years, although the occurrence and timing of the removal of a large tree may be more than coincidental.

The Central United States Seismic Observatory: Site Characterization, Instrumentation, and Recordings

ABSTRACT The Central United States Seismic Observatory is a vertical seismic array located in southwestern Kentucky within the New Madrid seismic zone. It is intended to describe the effects of local geology, including thick sediment overburden, on seismic‐wave propagation, particularly strong motion. The three‐borehole array is composed of seismic sensors placed on the surface, in the bedrock, and at various depths within the 585‐m‐thick sediment overburden. The array’s deep borehole also provided a unique opportunity in the northern Mississippi embayment for the direct geologic description and geophysical measurement of the complete Late Cretaceous–Quaternary sediment column. A seven‐layer intrasediment velocity model is interpreted from the complex, inhomogeneous stratigraphy. The S ‐ and P ‐wave sediment velocities range between 160 and 875 m/s and between 1000 and 2300 m/s, respectively, and their bedrock velocities range between 1452 and 3775 m/s, respectively. Seismometers and accelerometers operate both at the surface and 2 m into bedrock, with strong‐motion accelerometers at depths of 30, 259, and 526 m. The array operation has been frequently interrupted by the large hydrostatic pressures on the deeper instrumentation; however, the full array has recorded weak‐motion response from 95 earthquakes at local, regional, and teleseismic distances. Initial observations reveal a complex spectral mix of amplification and de‐amplification across the array, indicating the site effect in this deep‐sediment setting is not simply generated by the shallowest layers. Preliminary horizontal‐to‐vertical spectral ratio (H/V) experiments show the bedrock vertical and horizontal amplitudes are not equal, a required assumption for site characterization. Further, there are marked differences between spectral ratios from the directly measured transfer function (H/H) and H/V for particular earthquakes. On average, however, the H/H and H/V methods are coincident within a narrow band of frequencies ranging between 0.35 and 1.1 Hz.

Joint inversion of spatial autocorrelation curves with HVSR for site characterization in Newcastle, Australia

In order to investigate site characterization of Newcastle affected by the 1989 Newcastle Earthquake (ML = 5.6), we conducted microtremor array measurements. The spatial autocorrelation (SPAC) method was applied to estimate the S-wave velocity structure from observed microtremor data. Although the inversion of spatial autocorrelation curves is effective for estimating shallow S-wave velocity structures within the sedimentary layer, it is usually difficult to estimate the boundary between the sedimentary layer and bedrock due to a lack of amplitude of vertical components of microtremors at low frequencies. On the other hands, it is well known that horizontal to vertical particle motion spectral ratios (HVSR) have information which assists in resolving the bedrock depth and velocity. Thus, we have applied joint inversion of the spatial autocorrelation curves with HVSR to observed microtremors. Since observed HVSR curves are subject to fluctuations due to unknown Love and body wave contributions or other noise effects, there is difficulty in fitting absolute values of observed HVSR. Therefore, we evaluate observed HVSR and theoretical H/V spectra by zero-lag cross-correlation to fit the shapes of HVSR. The observed HVSR curve has accurate information for the deeper velocity structure and therefore, the estimated velocity model by joint inversion differs in velocity estimations at depth. It is concluded that the joint inversion of the spatial autocorrelation curves with HVSR is useful in practice for obtaining improved estimates of S-wave velocity models down to bedrock.

Mode misidentification in Rayleigh waves: Ellipticity as a cause and a cure

The surface wave method is a popular tool for geotechnical characterization because it supplies a cost-effective testing procedure capable of retrieving the shear wave velocity structure of the near-surface. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted. Typically, in active testing, single-component vertical receivers are used. In most cases, the inversion is carried out assuming that the experimental dispersion curve corresponds to a single mode, mostly the fundamental Rayleigh mode, unless clear evidence dictates the existence of a more complex response, e.g., in presence of low-velocity layers and inversely dispersive sites. A correct identification of the modes is essential to avoid serious errors. Here we consider the typical case of higher-mode misidentification known as “osculation” (“kissing”), where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs, with a continuous smooth transition, around a well-defined frequency where the two modes get very close to each other. Osculation happens generally in presence of strong velocity contrasts, typically with a fast bedrock underlying loose sediments. The practical limitations of the acquired active data affect the spectral and modal resolution, making it often impossible to identify the presence of two modes. In some cases, modes have a very close root and cannot be separated at the osculation point. In such cases, mode misidentification can create a large overestimation of the bedrock velocity and a large error on its depth. We examine the subsoil conditions that can generate this unwanted condition, and the common field acquisition procedures that can contribute to producing data having such deceptive Rayleigh dispersion characteristics. This mode misidentification depends strongly on the usual approach of measuring only the vertical component of ground motion, as the mode osculation is linked to the Rayleigh wave ellipticity polarization, and therefore we conclude that multicomponent data, using also horizontal receivers, can help discern the multimodal nature of surface waves. Finally, we introduce a priori detectors of subsoil conditions, based on passive microtremor measurements, that can act as warnings against the presence of mode osculation, and relate these detectors to the frequencies at which dispersion curves can be misidentified. Theoretical results are confirmed by real data acquisition tests.

Seismic reflection imaging of the mafic-ultramafic Windimurra Igneous Complex, Yilgarn Craton, Western Australia

A deep seismic reflection survey was carried out over the Meso-Neoarchean Youanmi Terrane, Yilgarn Craton in Western Australia, including 3 seismic lines across the Windimurra Igneous Complex. This gabbroic intrusion is the largest exposed single mafic-ultramafic intrusion in Australia, currently mined for magnetite-hosted vanadium in its upper zone. Acquisition was carried out with 3 Hemi 60 vibrators, 80 or 40 m VP interval, 40 m group interval, and a 300 channel symmetric split spread, resulting in 75 to 150 fold high quality data. Key processing steps included refraction and residual automatic statics, spectral equalisation, detailed stacking velocity analysis, dip moveout (DMO) correction and post stack Kirchhoff migration. The Windimurra Igneous Complex exhibits high seismic velocity (approximately 6.5 km/s), both in interval velocity from stacking velocity and bedrock velocity from refraction statics analysis. Seismic attenuation is low, resulting in high frequency and good resolution to the base of the complex, seen at a maximum two way time of around 2.5 seconds. Gross structure on the seismic sections is also consistent with the gravity and magnetic data. An inward dipping conical structure is evident and consistent with surface geological observations. The base of the complex is marked by distinctive high amplitude multi-layered reflectors, which may represent significant thicknesses of ultramafic zone material unexposed at the surface. This zone has been a long sought after target for Cr-PGE exploration. The complex internal structure of the intrusion, possibly including discordant igneous layering features, has been well imaged up to the base of regolith.

2D reflection seismic investigations at the Kevitsa Ni-Cu-PGE deposit, northern Finland

In 2007, a 2D reflection seismic survey was conducted at the Kevitsa Ni-Cu-PGE (platinum group elements) deposit in northern Finland. The aims of the survey were to delineate the overall extent of the ore-bearing Kevitsa ultramafic intrusive complex, to study the seismic response of the disseminated ore deposit, to potentially find indications for new ore deposits, and to extract structural information at depth that may be associated with mineralization. In the processing sequence, specific focus was given to finding optimal CDP-line geometries for the crooked-line survey profiles and, due to highly variable bedrock velocities, to detailed velocity analysis. Our conventional processing sequence, involving prestack DMO corrections followed by poststack migration, resulted in high-quality images of the subsurface. First, the data were used to establish the shape and extent of the Kevitsa intrusion, thus providing an overall framework for future exploration in the area. In particular, the data suggest deeper, up to about 1.5 km depth, continuation of the intrusion than previously thought. Furthermore, the images reveal variable reflectivity characteristics within the intrusion from nonreflective to internally reflective. The Kevitsa deposit is located within a part of the intrusion which is associated with distinct, gently dipping reflectivity fabric down to a depth of about 1 km, spatially constrained within a restricted zone internal to the intrusion. This zone can be used as a guideline for the near-mine exploration efforts, and the reflectivity is dominantly associated with magmatic layering controlling the extent of the bulk of economic mineralization. The seismic data also reveal a complex pattern of faults, in particular a series of major fault and shear zones bracketing and crosscutting the Kevitsa intrusion as a whole. Additionally, our interpretation of the data indicates a possible shared origin of the Kevitsa intrusion and the nearby Satovaara intrusion.

Derivation of a Reference Shear-Wave Velocity Model from Empirical Site Amplification

Abstract The definition of a reference bedrock condition representative of a region of interest is of great significance in seismic-hazard assessment. It is highly beneficial when ground-motion prediction equations are referenced to a specific site condition, particularly in the case of site-specific seismic-hazard analyses. When known, the effect of any given site with respect to the reference can then be applied to the predicted ground motion. However, the choice of a reference velocity profile is not straightforward, mainly due to the high variability of the velocity structure in the shallower layers. A new method to define the regional reference rock profile is proposed. The method relates quarter-wavelength average velocity at a site to frequency-dependent amplification. A reference bedrock velocity profile can then be directly defined in relation to expected amplification characteristics over a number of sites. We compare 27 quarter-wavelength velocity profiles from seismic station locations in Switzerland with empirical amplification functions derived from spectral modeling. From this comparison, a set of frequency-dependent calibration relationships is established. Assuming that the reference profile is defined by a lack of any relative amplification, the quarter-wavelength velocity profile that corresponds to unitary spectral amplification can be extracted. The reference velocity profile can then be obtained through an inversion procedure and defines the reference for the ground-motion prediction equation (GMPE). The proposed reference velocity profile is compared with previous reference velocity profiles. A good agreement is found between the different methods. Additionally, an estimation of the transfer function for the Swiss reference rock condition is provided. This can be used to correct recorded or estimated spectral amplitudes for the local response of the reference site. Finally, it is shown that the coefficients from the aforementioned correlations can be used to estimate a generic amplification function at any site with a known quarter-wavelength velocity profile.

Traveltime tomographic inversion with simultaneous static corrections — Well worth the effort

We have developed a first-arrival traveltime inversion scheme that jointly solves for seismic velocities and source and receiver static-time terms. The static-time terms are included to compensate for varying time delays introduced by the near-surface low-velocity layer that is too thin to be resolved by tomography. Results on a real data set consisting of picked first-arrival times from a seismic-reflection 2D/3D experiment in a crystalline environment show that the tomography static-time terms are very similar in values and distribution to refraction-static corrections computed using standard refraction-statics software. When applied to 3D seismic-reflection data, tomography static-time terms produce similar or more coherent seismic-reflection images compared to the images using corrections from standard refraction-static software. Furthermore, the method provides a much more detailed model of the near-surface bedrock velocity than standard software when the static-time terms are included in the inversion. Low-velocity zones in this model correlate with other geologic and geophysical data, suggesting that our method results in a reliable model. In addition to generally being required in seismic-reflection imaging, static corrections are also necessary in traveltime tomography to obtain high-fidelity velocity images of the subsurface.

Geodetic measurements of vertical crustal velocity in West Antarctica and the implications for ice mass balance

We present preliminary geodetic estimates for vertical bedrock velocity at twelve survey GPS stations in the West Antarctic GPS Network, an additional survey station in the northern Antarctic Peninsula, and eleven continuous GPS stations distributed across the continent. The spatial pattern of these velocities is not consistent with any postglacial rebound (PGR) model known to us. Four leading PGR models appear to be overpredicting uplift rates in the Transantarctic Mountains and West Antarctica and underpredicting them in the peninsula north of 65°. This discrepancy cannot be explained in terms of an elastic response to modern ice loss (except, perhaps, in part of the peninsula). Therefore, our initial geodetic results suggest that most GRACE ice mass rate estimates, which are critically dependent on a PGR correction, are systematically biased and are overpredicting ice loss for the continent as a whole.

Site Effects at Long Periods from Digital Strong Motion Records of the KiK-net, Japan

The high-quality digital records of the Japanese KiK-net were examined, with the aim of studying the influence of local site conditions on the displacement spectral ordinates at long periods. The attention was limited to those records for which the velocity profiles up to 100–200 m depth were known, and corresponding surface and borehole accelerograms were available. Based on the available records and with the support of 1D numerical simulations, different aspects that may have an influence on the amplification of long period spectral ordinates were studied, including the bedrock velocity profile, the site classification using Vs,30, and the earthquake magnitude. Small amplification factors at long periods were found, ranging from 1 to 1.3, with median value from 1.05 to 1.1, for Eurocode 8 site classes B and C, respectively. Only for two records on soft soils (at the boundary between Eurocode 8 classes C and D), from small magnitude earthquakes, large amplification factors were obtained, up to about 4. A good correlation was found of the amplification levels with the response spectral ratio D(T0)/D(10), where D(T0) and D(10) are displacement spectral ordinates of the input signal at bedrock, at the fundamental period T0 of the soil profile and at T = 10 s, respectively.

Characteristics of steady state fluvial topography above fault‐bend folds

In steady state convergent orogens, erosion balances lateral as well as vertical bedrock motions. For simple geometrical reasons, the difference between the total steady state erosion flux and its vertical component is up to 30% for typical fluvial slopes and bedrock streamline inclinations, suggesting that lateral advection is also likely to be expressed topographically. In order to understand these geomorphologic consequences, we focus on steady state topography developed on active fault‐bend folds. First, we derive an analytical solution for the slopes of detachment‐limited streams that incorporates lateral advection. Next, we conduct experiments using a numerical two‐dimensional landscape evolution model (Channel‐Hillslope Integrated Landscape Development model (CHILD)) incorporating linear diffusion on hillslopes and detachment‐limited stream channel incision above a fault‐bend fold. The concavity and steepness indices of steady state long profiles are functions of bedrock velocity magnitude and direction, streamflow direction, and fluvial erosivity. Asymmetry of mountain range profiles varies as a function of fluvial erosivity or bedrock velocity only if we account for the lateral velocity component. This asymmetry is equally sensitive to this lateral component, fluvial incision, and hillslope diffusion. However, the effect of diffusion on drainage divide position is significant only at high diffusivities, short length scales, low bedrock advection rates, or relatively low fluvial erosivity. Thus in most mountain ranges and fault blocks, drainage divide migration is expected to be dictated by stream channel erosion. Model results are shown to be consistent with topography in the Siwalik Hills, Nepal, which overlie fault‐bend folds produced above the frontal fault systems in the Himalayan foreland.

Simulated Strong Ground Motions for the GreatM9.3 Sumatra–Andaman Earthquake of 26 December 2004

AbstractOn 26 December 2004, a devastating earthquake of M 9.3 occurred offshore northern Sumatra. Due to the size of this earthquake and the accompanying tsunami wave, disastrous consequences have been observed in several countries around the Indian Ocean. The tectonics in the region are characterized by the oblique, north-northeast-oriented subduction of the Indian–Australian plate under the Sunda microplate with a rate of 6–6.5 cm/yr. This oblique convergence results in strain partitioning, where the trench-perpendicular thrust faulting along the subducting slab accommodates the east–west component of the motion, whereas the north–south component of the motion is probably accommodated by the right-lateral strike-slip faulting along the Great Sumatran fault and the Mentawi fault. Source parameters of the 26 December 2004 event have been used for modeling the resulting ground motions in the nearby affected regions. Results give an insight on the importance of ground shaking in the total destruction of places like Banda Aceh, northern Sumatra, Indonesia. The modeling is performed for a multiasperity finite fault using a hybrid procedure combining deterministic modeling at low frequencies and semistochastic modeling at high frequencies. Results show that strong shaking was distributed over a large area including northwestern Sumatra and its offshore islands. In Banda Aceh, which experienced significant damage, bedrock velocities reached 60 cm/sec with duration of the shaking of ca. 150 sec. The largest ground motions occurred near the strongest asperities of the fault plane, where velocities of 200 cm/sec are modeled for bedrock conditions.

Rayleigh wave dispersion curve inversion via genetic algorithms and Marginal Posterior Probability Density estimation

Surface wave dispersion curve inversion is a challenging problem for linear inversion procedures due to its highly non-linear nature and to the large numbers of local minima and maxima of the objective function (multi-modality). In order to improve the reliability of the inversion results, we implemented and tested a two-step inversion scheme based on Genetic Algorithms (GAs). The proposed scheme performs several preliminary “parallel” runs (first step) and a final global run using the previously-determined fittest models as starting population. In this work we focus on the inversion of shear-wave velocity and layer thickness while fixing compressional-wave velocity and density according to user-defined Poisson's ratios and velocity–density relationship respectively. The procedure can nonetheless perform the inversion under different degrees of regularization, depending on the a priori information and the desired degree of freedom of the system. Thanks to the large number of considered models, in addition to the fittest model, a mean model and its accuracy are evaluated by means of a statistical approach based on the estimation of the Marginal Posterior Probability Density (MPPD). We tested the proposed GA-based inversion scheme on three synthetic models reproducing a complex structure with low-to-moderate velocity cover (also including a low-velocity channel) lying over hard bedrock. For all the considered cases the bedrock velocity and depth were properly identified, and velocity inversion was reconstructed with minor uncertainties. The performed tests also investigate the influence of the first higher mode, the reduction of the frequency range of the considered dispersion curve as well as the use of different number of strata. While a limited frequency range of the dispersion curve (maximum frequency reduced from 80 to 40 Hz) does not seem to significantly limit the accuracy of the retrieved model, the adoption of the correct number of strata and the addition of the first higher mode help better focus the final solution. In conclusion, the proposed approach represents an improvement of a purely GA-based optimization scheme and the MPPD-based mean model typically offers a more significant and precise solution than the fittest one. Results of the inversion performed on a field data set were validated by borehole stratigraphy.

Topographic advection on fault-bend folds: Inheritance of valley positions and the formation of wind gaps

Drainage basins formed along the flanks of any individual linear mountain range or fault block are commonly observed to be self-similar in planform, uniformly spaced, and in some cases aligned with drainage basins on the opposing flank. Data from the Siwalik Hills, Nepal, illustrate this organization where drainage basins and valleys are aligned across the main divide. We suggest that valley alignment is a consequence of advection of topography across the divide. To explore this hypothesis, numerical experiments were conducted using a landscape evolution model of a fault-bend fold that simulates detachment-limited stream incision and linear hillslope diffusion. Results show that the presence of incised valleys and a lateral component of bedrock motion are necessary and sufficient conditions for advection of relief across the divide, a mechanism by which wind gaps form and valley spacing on the far side of a ridge is inherited from that on the near side. This topographic inheritance is promoted by low rock erodibility, low precipitation rates, fast bedrock velocities, and/or intermediate fault dips.

Seismic tomography studies of cover thickness and near-surface bedrock velocities

Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break traveltimes. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified methodalso solves for 3D velocity structure and static correctionssimultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, we test our tomography method on synthetic data withthe shot points in the bedrock below the cover. Next, we developa strategy for the case when the sources are within the cover. Themethod is then applied to field data from five crooked-line,high-resolution reflection seismic profiles ranging in lengthfrom 2 to [Formula: see text]. The crooked-line profiles make the study 2.5dimensional regarding bedrock velocities. The cover thicknessalong the profiles varies from 0 to [Formula: see text]. Estimated thickness ofthe cover agrees well with data from boreholes drilled near theprofiles. Low-velocity zones in the uppermost bedrock generallycorrelate with locations where reflections from the stackedsections project to the surface. Thus, the method is functional,both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.

The velocity structure of the Cariaco sedimentary basin, northeastern Venezuela, from refraction seismic data and possible relationship to earthquake hazard

The main damage from the July 9, 1997, Cariaco earthquake (Ms=6.8) was concentrated in the town of Cariaco and surrounding villages, which are located in the western part of the Cariaco sedimentary basin, close to the Gulf of Cariaco. Casanay, located at the eastern end of the sedimentary basin, suffered considerably less damage. The El Pilar fault, a right-lateral strike-slip fault that generated the earthquake, runs parallel to the southern border of the valley and crosses both towns. The determination of the velocity structure of the basin is the main objective of this study. Seismic refraction data were recorded along three lines, one of them along-strike and two perpendicular to the valley axis in the northern and southern bedrocks. Beneath Cariaco, approximately 1 km thick Quaternary sediments with seismic velocities of 1.9–2.1 km/s and bedrock velocities of more than 4 km/s were observed. The thickness of the Quaternary sediments varies within the basin, and Pleistocene sediments outcrop beneath Casanay. The increased thickness of the unconsolidated, water-saturated Quaternary sediments, together with the difference in the quality of buildings prior to the earthquake, probably is responsible for the damage pattern of the Cariaco earthquake.