Near-surface Low-velocity Layers Research Articles

Near-surface characterization using shear-wave resonances: A case study from offshore Svalbard, Norway

Shear-wave (S-wave) resonances are typically observed when the surficial marine sediments over a rock substrate have relatively low S-wave velocities. We observe these phenomena using ocean-bottom fiber-optic distributed acoustic sensing (DAS) in two subsea fiber-optic telecommunication cables in Svalbard, Norway. Strong seismic energy from sufficiently large earthquakes is required to trigger and enhance the multiple-order modes of S-wave resonances. Here, we use the interpreted S-wave resonance frequencies of the first two modes to determine the thickness and the S-wave velocity of the near-surface low-velocity layer (LVL) beneath the seafloor. In addition, we use existing active P-wave seismic reflection data to determine the LVL thickness and to help build a more accurate S-wave velocity model from the S-wave resonance frequencies. The estimated S-wave velocity varies laterally within the LVL formation. Here, we find that the sediments or deposits with high S-wave velocity presented in the estimated LVL model agree with the distribution of some glacigenic sediments and landforms deposited in the survey area. Therefore, S-wave resonances measured by ocean-bottom DAS can be used to characterize the corresponding near-surface LVLs.

Topographic Response to Simulated Mw 6.5–7.0 Earthquakes on the Seattle Fault

ABSTRACTWe explore the response of ground motions to topography during large crustal fault earthquakes by simulating several magnitude 6.5–7.0 rupture scenarios on the Seattle fault, Washington State. Kinematic simulations are run using a 3D spectral element code and a detailed seismic velocity model for the Puget Sound region. This model includes realistic surface topography and a near-surface low-velocity layer; a mesh spacing of ∼30 m at the surface allows modeling of ground motions up to 3 Hz. We simulate 20 earthquake scenarios using different slip distributions and hypocenter locations on a planar fault surface. Results indicate that average ground motions in simulations with and without topography are similar. However, shaking amplification is common at topographic highs, and more than a quarter of all sites experience short-period (≤2 s) ground-motion amplification greater than 25%–35%, compared with models without topography. Comparisons of peak ground velocity at the top and bottom of topographic features demonstrate that amplification is sensitive to period, with the greatest amplifications typically manifesting near a topographic feature’s estimated resonance frequency and along azimuths perpendicular to its primary axis of elongation. However, interevent variability in topographic response can be significant, particularly at shorter periods (<1 s). We do not observe a clear relationship between source centroid-to-site azimuths and the strength of topographic amplification. Overall, our results suggest that although topographic resonance does influence the average ground motions, other processes (e.g., localized focusing and scattering) also play a significant role in determining topographic response. However, the amount of consistent, significant amplification due to topography suggests that topographic effects should likely be considered in some capacity during seismic hazard studies.

Continental like crust beneath the Andaman Island through joint inversion of receiver function and surface wave from ambient seismic noise

We study shear wave velocity structure of the crust beneath the Andaman Island through joint inversion of the teleseismic receiver function and Rayleigh wave group velocity measurements from 10 broadband seismographs over the Island. The group velocities in the periods from 5 to 21s are obtained using cross-correlation of six month's ambient seismic noise data recorded by these seismic stations. Joint inversion results show ~2 to 6km thick subsurface low shear velocity (Vs ~1.3–2.5km/s) layer followed by a 12–14km thick layer of silicic material (average Vs ~3.5km/s). The lower crust is mapped as an 8–12km thick mafic layer with Vs ~4.0km/s. Uppermost mantle shear wave velocity is ~4.55km/s. The near-surface low-velocity layer is interpreted as the Andaman flysch sediments. The crustal thickness beneath the Andaman Island varies from ~24km in the north to ~32km in the south. The shear wave velocity-depth results show that the crustal structure beneath the Andaman Island is akin to continental crust, possibly the Burma continental crust. The subducting Indian plate may lie down below this overriding plate.

Analysis of complicated structure seismic wave fields

In western China seismic wave fields are very complicated and have low signal to noise ratio. In this paper, we focus on complex wave field research by forward modeling and indicate that density should not be ignored in wave field simulation if the subsurface physical properties are quite different. We use the acoustic wave equation with density in the staggered finite-difference method to simulate the wave fields. For this purpose a complicated geologic structural model with rugged surfaces, near-surface low-velocity layers, and high-velocity outcropping layers was designed. Based on the instantaneous wave field distribution, we analyzed the mechanism forming complex wave fields. The influence of low velocity layers on the wave field is very strong. A strong waveguide occurs between the top and base of a low velocity layer, producing multiples which penetrate into the earth and form strong complex wave fields in addition to reflections from subsurface interfaces. For verifying the correctness of the simulated wave fields, prestack depth migration was performed using different algorithms from the forward modeling. The structure revealed by the stacked migration profile is same as the known structure.

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.

The importance of the spread system geometry on the image reconstruction of seismic tomography

The influence of spread and shot systems is investigated using common measuring geometries like conventional crosshole and multi-offset VSP. Both synthetic and field data are used to understand better the importance of commonly used spread and shot systems in the image reconstruction process. In the synthetic case, a faulted structure under a near-surface low velocity layer is used. The results of the seismic images compared with the original model and the similarities in the structure and in the velocities are investigated. Tomograms derived from the real data are compared with ground truth. Our study shows that realistic spread systems can produce nearly the same quality images as in the ideal ones.

Implications for intraplate volcanism and back-arc deformation in northwestern New Zealand, from joint inversion of receiver functions and surface waves

SUMMARY We employ a joint inversion of teleseismic receiver functions and surface wave phase velocities to determine the shear wave velocity structure in the crust and upper mantle beneath northwestern New Zealand. Receiver functions primarily contain information on velocity contrasts, while surface waves are sensitive to the average shear velocity with depth. By performing a joint inversion we reduce the limitations of each method, resulting in a more robust shear wave model. Inversion results reveal regions of low shear wave velocity of ∼2.8 km s −1 in the mid-crust (10‐19 km depth) and ∼4.0 km s −1 in the upper mantle (70‐90 km depth) beneath Quaternary intraplate basalt fields. We infer that the mid-crustal low-velocity zones (LVZs) are bodies of partial melt, most likely rhyolite intrusions. We suggest that the upper mantle LVZ is caused by the melt-producing regions of the upper mantle and is a source for the basalts of the Auckland Volcanic Field. This is in agreement with models for a shallow upper mantle source rather than a deep-seated mantle plume for the Auckland volcanism. Average shear velocities for the upper crust are 3.4‐3.6 km s −1 , increasing to 3.6‐4.0 km s −1 in the lower crust. The Moho is interpreted to be 29 ± 1 km deep in the southern end of the array, shallowing to 26 ± 1 km towards the edge of the continental shelf of northern New Zealand. Low upper mantle shear velocities of 4.2 ± 0.1 km s −1 are observed, and are thought to represent a small percentage of partial melt in the upper mantle beneath this back-arc region. These low velocities extend to greater depth (∼90 km) beneath the southern stations, indicating that mantle deformation associated with the mantle wedge to the south may extend as far north as the southern end of our array. Near-surface low-velocity layers extend to 6 km depth beneath a station at the northern tip of New Zealand, likely representing the expression of a thick sequence of sediments from the Northland Allochthon and the more recent Northland Basin.

Propagator and wave-equation inversion for near-receiver material properties

SUMMARY Near-receiver material properties are required for the separation of the recorded wavefield into upgoing and downgoing P and S waves, and are also important for static time-shift corrections. However, it is difficult, especially in land seismics, to obtain reliable estimates for these local material properties using conventional techniques. We compare three methods for estimating these material properties using a 3-D geophone configuration. The first two methods are based on inversion of the wave equation and can be used on almost all of the recorded wavefield. However, they require that the wavefield is recorded by a dense 3-D receiver group to allow the computation of either spatial wavefield derivatives or interpolants. The third approach is based on the inversion of the vertical wavefield propagator. We present a procedure for estimating this propagator using only two multicomponent geophones, one buried and one positioned at the surface. Propagator estimation and inversion avoids the explicit computation of wavefield derivatives, and is therefore less sensitive to measurement errors than both wave-equation inversion schemes. However, in the form presented it requires the identification of arrivals of incoming waves that are isolated in time, and can only be applied to such data. Noise tests demonstrate that the propagator inversion provides accurate estimates for P- and S-wave velocities of a near-surface low-velocity layer, and is robust with respect to signal-generated near-surface reverberations. In case of a near-surface velocity gradient, velocities are obtained which are consistent with effective medium velocities.

Local Site Effects in the Town of Benevento (Italy) from Noise Measurements

— The study of ground motion amplification produced by surface geology is extremely interesting in the Benevento area, Southern Italy, as it is characterized by high seismic hazard. The present moderate-to-low seismicity makes the noise method appropriate to estimate the seismic site response in the area. The three components of seismic noise have been recorded in five sites in the Benevento metropolitan area characterized by different surface geology, in order to estimate the seismic site response. In evaluating site amplification effects we used the direct interpretation of amplitude spectra and standard spectral ratio techniques, evaluating sediment-to-bedrock, sediment-to-average and H/V spectral ratios. The temporal evolution of the noise spectra is analysed within one day, in order to assess the stationarity of the noise signal. The noise wavefield properties have been studied through polarization analyses in selected bands of frequency, where spectral peaks are observed to dominate, to better understand the real nature of those peaks. Results give evidence of low amplification levels, missing any correlation between spectral amplitudes and sediment thickness over the basement. We interpret this result as due to the poor impedance contrast between sediments and basement, which is characterized by low values of shear waves velocity. Moreover, sharp amplitude peaks are observed in the raw spectra of the sediment-sites, in the 2–4 Hz frequency band; a numerical simulation interprets this effect as possibly associated with a wide-scale structure, invoking the presence of a sharper impedance contrast at greater depth. At high frequencies the action of ambient noise sources, mainly active on horizontal components of motion, is retained dominant to generate the prominent peaks observed in the H/V spectral ratios; in some cases the presence of a near-surface low-velocity layer can contribute to amplify the seismic motion generated at these frequencies.

LgwaveQtomography in Central America

SUMMARY The lateral variation of Lg wave spectral attenuation in Central America was studied through tomographic inversion at various frequencies between 0.5 and 5 Hz, separately for each frequency. The input data set consisted of 558 travel paths recorded on short-period and broadband seismic stations in the region. The size and quality of the data set were sufficient to resolve significant lateral variation in the Lg wave quality factor Q Lg. The average dependence of Q Lg with frequency was found to be Q Lg( f ) = 182 f 0.84 , corresponding to high attenuation. Low Q Lg values in the Nicaraguan Depression presented the most significant variation from the average. The strong attenuation of Lg waves in the Nicaraguan depression was also observed in the visual analysis of the seismograms. This observation is possibly explained by the nearsurface low-velocity layers in the depression. Low Q Lg values were also found along the chain of volcanos due to increased scattering and partial melting beneath the volcanic belt. This study is the first attempt to determine Q Lg for Central America and to provide knowledge on attenuation of Lg waves needed for the prediction of ground motion during future earthquakes.

Detection of fault structure under a near-surface low velocity layer by seismic tomography: synthetics studies

We have developed a new method to detect a fault structure under a near-surface low velocity layer (LVL) by seismic tomography. The field study showed that the tomography image reconstructed using borehole-surface configuration had a different result from that of using a crosshole configuration. The image reconstructed by using a borehole-surface configuration showed a decrease in seismic velocities along boreholes, and also the tomogram result using both configurations can not detect the subsurface fault structure. These phenomena are caused by the low velocity layer (LVL) at the top of investigation area. The basic idea hard is based on a downward continuation principle. By knowing the thickness of the LVL and the top of bedrock enables us to place ‘virtual receiver’ and/or ‘virtual source’ below the LVL. In this way, we can reconstruct the image by various tomographic methodologies. As an advantage, this method is easy to be use with the aid of ray tracing methodology. It can also reduce the effect of the near-surface LVL and can maximize the reconstructed image. The final result of our synthetic images by ILST, SIRT, and modified SIRT shows high accuracy and resolution for detection of fault structure under the low velocity layer.

An analysis of simulated and observed blast records in the Salt Lake Basin

Abstract We have simulated 0.2 to 1.2-Hz 3D elastic wave propagation in the Salt Lake Basin from a blast at a nearby open-pit mine. A fourth-order staggered-grid finite-difference method was used to simulate the blast in a two-layer basin model (58 × 43 × 9 km) consisting of semiconsolidated sediments up to 1.3-km thick surrounded by bedrock. Data from four blasts in the mine pit, recorded by a network of 10 digital three-component instruments, were compared to the results of the simulation. The simulation reproduces the overall pattern of ground-motion amplification at basin sites relative to a rock site, as measured by ratios of peak particle velocities, cumulative kinetic energies, and spectral magnitudes. Considering the simple two-layer basin model used in the 3D simulation, this finding suggests that the deep 3D basin structure significantly contributes to low-frequency ground-motion amplification in the Salt Lake Basin. Compared to the records from the simple two-layer 3D simulation, the records from a 2D P/SV-wave simulation that includes processes (1) through (3) provide a better match to the blast data—especially the observed durations of shaking. At five of the six stations along the profile, the 2D simulation reproduces the normalized radial and vertical peak particle velocities to within a factor of 2 and the normalized cumulative kinetic energies and spectral amplitudes on these components to within generally a factor of 3. Our results suggest that deep-basin resonance, reverberations in the near-surface low-velocity layer, attenuation, and topographic scattering significantly influence site amplification in the Salt Lake Basin. Future studies of site amplification in the Salt Lake Basin should include the effects of all of these mechanisms.

Tomographic imaging of lithospheric structures under Taiwan

Tomographic images of the crustal and mantle velocity structures under Taiwan are obtained by simultaneous inversion of local earthquake P-wave arrival times for hypocenters and P-wave velocity structures. In northern Taiwan, a high-velocity zone, coinciding with the Wadati-Benioff zone, can readily be identified as the subducted Philippine Sea plate. The imaged zone dips toward the north at an angle of 40° from a depth range of 20–55 to 100–130 km. An upper-mantle low-velocity wedge, ranging from 40 to 80 km in depth, exists above the subducted slab. Above this wedge is the Ilan Plain of northern Taiwan which lies at the west end of the Okinawa Trough, a well recognized back-arc basin; the crustal velocities under the Plain are also relatively low. The well-defined high-velocity zone and the low-velocity wedge provide some constraints on the thermal structures of the subduction system under northern Taiwan. In tomographic images across the central section of Taiwan, thickening of the crust and up-arching of the lower-crustal materials under the Central Range are commonly observed; the crust under the Western Foothills region is clearly thinner and the near-surface low-velocity layers are well developed. The structures under the Central Range show that although the Taiwan orogeny is quite young, a root, deeper in the north and shallowing to the south, has formed. The results of our tomography show that a significant portion of the lithosphere is involved in the Taiwan orogeny.

Determination of shallow refractor properties by 3D-CMP refraction seismic techniques

The determination of shallow refractor properties is a very important task in processing of land seismic reftection data, because inhomogeneities in the nearsurface low-velocity layers can severely distort the seismic image of the subsurface. Static corrections aim at compensating for these anomalies in order to obtain a correct structural image of deeper reftectors.

Analyses of wavefields by the common excitation array (CEA) method

There is a number of problems associated with the processing of seismic data acquired over crystalline rocks where velocities vary as much in the horizontal direction as in the vertical direction and near-surface low-velocity layers may distort the signals recorded on the surface. In addition, waves arriving at the same time but at different dips will interfere with one another thereby masking reflections of interest. Traditional CDP processing has some difficulties associated with it in separating out the interfering events, as well as in imaging steeply dipping events. By employing a series of closely spaced shotpoints in an array, quasi-planar waves emanating at different angles may be simulated through slant stacking. Slant stacking is preferably done in the common receiver domain where statics and trace signal variations are less of a problem since near-surface velocity variations are less of a problem. After slant stacking to illuminate interfaces of various dips, the major reflecting events may be picked automatically and the data quantity reduced to the time, slowness and amplitude of the picked events. These picked events can then be migrated onto a depth section image where the interfering waves have been separated from one another and steeply dipping events are imaged. We apply the method to seismic data from the Urals near the SG4 deep borehole and compare the results with CDP processing of the same data. Results show that the common excitation array method matches the geological interpretations in the area better and that it images steeply dipping reflectors better.

Modeling of wave propagation in northern Los Angeles basin

AbstractThe Los Angeles basin is characterized by deep structural complexity and is the site of numerous earthquakes. We analyze the amplitude and waveform data of earthquakes recorded in the Los Angeles basin in order to identify the structurally controlled path effects. This has been done in order to determine which features of the structure affect the observations and to generate improved models in different parts of the basin increasing our capabilities of predicting seismic hazard. Several structural models of the L.A. basin that are based on regional subsurface studies, exposed sections, scattered oil wells, and other limited geophysical data have recently become available. In this study, we have used seismic modeling techniques together with geologic models to attempt to explain site variations in some important data bases. We have modeled waveforms of (1) the 19 January 1989 Malibu earthquake data recorded in Pasadena, (2) an M = 2.8 earthquake recorded by the USC downhole array in the Baldwin Hills, southern California, and (3) two aftershocks of the Whittier Narrows earthquake recorded at five stations along a profile.The modeling of the Malibu earthquake results in a layered one-dimensional model representing 6.5 km of layered sediment underlain by a high-velocity basement. Direct arrivals, basin reflected phases, and multiple reverberations are modeled fairly well by this one-dimensional model. The downhole array data in the Baldwin Hills show a significant near-surface amplification that can be modeled by very low-velocity near-surface materials. A series of aftershocks of the Whittier Narrows earthquake recorded at close distances at temporary recording sites shows a wide variation in peak accelerations due both to radiation pattern and propagation effects. The effect of lateral heterogeneity is evident in the travel time, where stations within the basin show later arrival times than those away from the basin for the same epicentral distance. Seismograms recorded at different sites show variation in waveforms due to multi-pathing of rays. We use such triplicated arrivals to constrain the structure of different interfaces. Using these criteria and the synthetic seismograms calculated by ray theory and finite difference methods, we have been able to model the tangential seismograms due to two of these events recorded at five stations along a two-dimensional profile.All three modeling exercises helped us understand wave propagation in the basin environment. A basin-reflected phase was identified in the seismogram from the Malibu earthquake, which was used to derive an effective depth of the basement. Near-surface amplification of the waves observed in the downhole data could be explained by constructive interference of the multiply reflected waves through the thin near-surface low-velocity layer. Similarly, site-dependent variation of waveform and travel time shown by the seismograms from the aftershocks of the Whittier Narrows, California, earthquake could be accounted for when a laterally varying structure was used.

Determination of focal mechanism solutions for four earthquakes from Monticello, South Carolina, and crustal structure by waveform modelling

Summary. We investigate the effect of thin near-surface crustal layers on seismic waveforms by modelling digital seismograms available from Monticello, South Carolina, a site for reservoir-induced seismicity. The epicentral distances to all stations used in this study are less than 5.0 km and the depths of the earthquakes are less than 2.0 km. These near-source seismograms are rich in frequencies as high as 50 Hz and are sensitive to crustal velocity layers as thin as 0.1 km. Velocity profiles from two boreholes (Secor et al.) were used as the starting models and modified as required by the comparison of synthetic seismograms with the observed data. The frequency-wavenumber (F-K) integration technique of Bouchon was used to calculate the synthetic response of these crustal models. Inclusion of some near-surface low-velocity layers over a crustal model produces many reverberations observed after the direct S arrival. The final model differs from the crustal model of Fletcher and Moos & Zoback by about 20-30 per cent and produces small travel time residuals. Having determined the velocity model we then determine the focal mechanisms of four small earthquakes whose magnitudes are less than 2.5 using both the search and the least squares inversion algorithms of Saikia & Herrmann. The amplitudes of direct P, SV, SH and some reverberating S waves from all components were used to analyse the focal mechanisms. The inferred mechanisms suggest that the area is characterized by reverse faulting. The P-axes are consistent with ENE compressional stress. The synthetic seismograms predicted for these events match the observed seismograms remarkably well in both the peak amplitudes of P and S waves and various amplitude ratios among different components.

Some Experiments in the Suppression of Coherent Noise on Land 3-D Seismic shot Records

In conventional land seismic reflection surveys, a source is triggered at or near to the surface of the earth. Vibrations from the source are reflected from the subsurface, and these are recorded by geophones or. groups of geophones normally placed in a line. Traces recorded by each group make up a shot record and comprise subcritical reflections or data, and linear noise trains such as refractions and high amplitude source generated noise commonly called ground roll (Fig. 1). This ground roll travels within the near-surface low velocity layer, and as such cuts across and obscures the flatter reflection events which have a higher apparent horizontal velocity in a shot record.

Evaluation of the relation between near-surface geological units and ground response in the vicinity of Long Beach, California

abstract Simulataneous recordings of Nevada Test Site nuclear events were made at sites underlain by alluvium in the Long Beach, California, area and at sites underlain by rock in the Palos Verdes and Pasadena areas. These data show peak-ground-velocity alluvium-to-rock ratios as large as 7 and spectral ratios as high as 11 in the period band from 0.2 to 6 sec. Comparison of the low-strain nuclear-explosion data and the San Fernando earthquake strong-motion data at three sites indicates that the alluvium-to-rock spectral ratios derived from the nuclear explosions are similar to those derived from the earthquake. Significant trends exist in the short-period data, indicating higher ground response at sites underlain at the near-surface by materials that have high void ratios and lower ground response with increasing thickness of Quaternary deposits. These results suggest that the short-period response is primarily controlled both by near-surface low-velocity layers and by attenuation in the Quaternary sediments. Comparison of the data of this study with data collected in other areas indicates that the long-period response increase with either increasing depth to basement or with alluvium thickness, when this thickness is greater than 400 m. From previous theoretical studies and these results, ground response in the long-period band is related to those underlying geological structures and major velocity contrasts that control the development of surface waves.

Amplification of seismic body waves by low-velocity surface layers

abstract Frequency-dependent amplification of seismic waves by near-surface low-velocity layers is a well-known phenomenon. This phenomenon was examined from both the analytic and experimental viewpoints for body waves (P, SV, SH). Groundmotion data, recorded in conjunction with the underground nuclear testing program at the Nevada Test Site, are used to provide experimental validation of the analytic models. Experimental amplification factors are derived from these data for a variety of recording-site near-surface geological configurations (alluvium, mine tailings, fill) and a wide dynamic range of ground-motion intensity (10−5 to 100 g). The variability in the mean amplification observed at a site for repeated measurements is described statistically. This analysis shows that, although the amplification at a given site varies on the average by a factor of about 1.4 across the frequency band of interest, from detonation to detonation, the frequency and magnitude of the dominant amplification are fairly consistent. The quality of the comparisons of the observed and calculated amplification indicates that the available linear analytic models are capable of describing the major features of the frequency-dependent amplification observed for this wide range of groundmotion intensity and recording-site geology.