Seismometer Array Research Articles

Comparison of 1D linear, equivalent-linear, and nonlinear site response models at six KiK-net validation sites

Vertical seismometer arrays represent a unique interaction between observed and predicted ground motions, and they are especially helpful for validating and comparing site response models. In this study, we perform comprehensive linear, equivalent-linear, and nonlinear site response analyses of 191 ground motions recorded at six validation sites in the Kiban–Kyoshin network (KiK-net) of vertical seismometer arrays in Japan. These sites, which span a range of geologic conditions, are selected because they meet the basic assumptions of one-dimensional (1D) wave propagation, and are therefore ideal for validating and calibrating 1D nonlinear soil models. We employ the equivalent-linear site response program SHAKE, the nonlinear site response program DEEPSOIL, and a nonlinear site response overlay model within the general finite element program Abaqus/Explicit. Using the results from this broad range of ground motions, we quantify the uncertainties of the alternative site response models, measure the strain levels at which the models break down, and provide general recommendations for performing site response analyses. Specifically, we find that at peak shear strains from 0.01% to 0.1%, linear site response models fail to accurately predict short-period ground motions; equivalent-linear and nonlinear models offer a significant improvement at strains beyond this level, with nonlinear models exhibiting a slight improvement over equivalent-linear models at strains greater than approximately 0.05%.

Seismic activity of the Nevados de Chillán volcanic complex after the 2010 Mw8.8 Maule, Chile, earthquake

Couplings between large magnitude subduction zone earthquakes and the subsequent response of their respective volcanic arcs are generally accepted, but the mechanisms driving this coupling are not known. The 2010 Maule earthquake (Mw8.8) ruptured approximately 500km along strike in the south-central part of Chile, and provides an opportunity to investigate earthquake–volcano interactions. In an exploratory campaign, we deployed four broadband seismometers atop and surrounding the Nevados de Chillán volcanic complex because it is located directly behind one of the two primary slip patches of the Maule Earthquake. The data recorded (from December 2011 to April 2012) shows significant seismic activity, characterized by numerous volcano tectonic events and tremor episodes occurring within the volcanic complex. We recorded two strong aftershocks of the Maule earthquake (Mw 6.1 in January 2012, and Mw 7.1 in April 2012) and investigated the response of the volcano to the incoming seismic energy. We find that volcanic tremor increased significantly upon arrival of the seismic waves from the Mw 6.1 event, which was then followed a few hours later by a significant increase in volcano-tectonic events when tremors subsided. This delay between tremor and volcano-tectonic events suggests that fluid and/or magma migration (manifested as tremor) readjusted the local stress state that then induced the volcano-tectonic events. The increased activity persisted during the subsequent two weeks. In contrast, the Mw 7.1 event, which occurred at a similar epicentral distance from the volcano, did not produce any significant seismic response of the volcanic complex. Analysis of the particle velocity records of the two events shows that the volcanic system was perturbed in different ways because of the incidence angle of the incoming energy, inducing a back-elliptical vertical and fault parallel motion for the Mw 6.1 and no clear directional dependence for the Mw 7.1. This suggests dilatation-induced fluid migration within the complex, and a kinematic mechanism of the perturbation rather than the perturbation amplitude. Our results demonstrate the importance for continued monitoring of the arc behind Maule, with an increased seismometer and GPS array density to determine the style of deformation currently occurring in the arc.

The shallow boreholes at The AltotiBerina near fault Observatory (TABOO; northern Apennines of Italy)

Abstract. As part of an interdisciplinary research project, funded by the European Research Council and addressing the mechanics of weak faults, we drilled three 200–250 m-deep boreholes and installed an array of seismometers. The array augments TABOO (The AltotiBerina near fault ObservatOry), a scientific infrastructure managed by the Italian National Institute of Geophysics and Volcanology. The observatory, which consists of a geophysical network equipped with multi-sensor stations, is located in the northern Apennines (Italy) and monitors a large and active low-angle normal fault. The drilling operations started at the end of 2011 and were completed by July 2012. We instrumented the boreholes with three-component short-period (2 Hz) passive instruments at different depths. The seismometers are now fully operational and collecting waveforms characterised by a very high signal to noise ratio that is ideal for studying microearthquakes. The resulting increase in the detection capability of the seismic network will allow for a broader range of transients to be identified.

Sensor placement for the analysis of seismic surface waves: sources of error, design criterion and array design algorithms

Seismic surface waves can be measured by deploying an array of seismometers on the surface of the earth. The goal of such measurement surveys is, usually, to estimate the velocity of propagation and the direction of arrival of the seismic waves. In this paper, we address the issue of sensor placement for the analysis of seismic surface waves from ambient vibration wavefields. First, we explain in detail how the array geometry affects the mean-squared estimation error of parameters of interest, such as the velocity and direction of propagation, both at low and high signal-to-noise ratios (SNRs). Secondly, we propose a cost function suitable for the design of the array geometry with particular focus on the estimation of the wavenumber of both Love and Rayleigh waves. Thirdly, we present and compare several computational approaches to minimize the proposed cost function. Numerical experiments verify the effectiveness of our cost function and resulting array geometry designs, leading to greatly improved estimation performance in comparison to arbitrary array geometries, both at low and high SNR levels.

Pn anisotropy in Mesozoic western Pacific lithosphere

AbstractPn is the high‐frequency, scattered P phase guided for great distances within the old oceanic lithosphere. Two arrays of ocean bottom seismometers were deployed on old (150–160 Ma) seafloor in the northwestern Pacific south of Shatsky Rise for the Pacific Lithosphere Anisotropy and Thickness Experiment. We use Pn phases from 403 earthquakes during the 1 year of deployment to measure apparent velocities across the arrays. Each array was deployed on a separate limb of a magnetic bight, formed near a fast‐spreading, ridge‐ridge‐ridge triple junction. Using high‐frequency waves (5–10 Hz), we look at variations of Pn velocities as a function of azimuth. In the western array, we find Pn anisotropy with velocities ranging from ~8.7 km/s in the back azimuth (θ) direction of 310° to ~7.7 km/s at ~350°. In the eastern array, the velocity ranges from ~8.5 km/s in back azimuth direction of ~210° to ~7.7 km/s at 260° and ~310°. We observe rapid velocity changes with azimuth in the both arrays requiring sinusoidal variations of roughly equal amplitude as a function of both 2θ and 4θ, which is not expected for the orthorhombic symmetry of olivine or orthopyroxene. The fastest directions on the two limbs are roughly orthogonal to each other suggesting the dominance of fossil anisotropy, but the fast directions of the 2θ components are skewed counterclockwise from the spreading directions. We speculate that the rapid azimuthal variations may be caused by vertical stratification with changing anisotropy with depth in the oceanic lithosphere.

Upper mantle Q structure beneath old seafloor in the western Pacific

AbstractThe Pacific Lithosphere Anisotropy and Thickness Experiment ocean bottom seismometer array south of the Shatsky Rise allows imaging the attenuation structure of the upper mantle in the oldest parts of the Pacific Ocean. The array consisted of eight seismometers with a lateral extent of 200 km by 600 km. Intermediate‐ and deep‐focus earthquake sources in the Mariana and Izu‐Bonin subduction zones provide paths that probe different depths beneath the seafloor. Acceleration spectra from the vertical component of P waves are inverted for the attenuation operator and corner frequency. Pn propagation indicates very high Qp values in the lithosphere, of order 1000, while path‐average Qp values in the upper mantle are on the order of 400. The variety of source epicentral distances and depths enables us to deduce the vertical Qp−1 structure of the upper mantle. Using the amplitude spectra directly in a weighted least squares problem that accounts for the effects of triplications in the traveltime curves, we invert for Qp−1 as a function of depth. We demonstrate that Qp is frequency dependent and adopt a power law dependence with coefficient α = 0.27. We resolve three distinct layers using a reference frequency of 1 Hz with Qp on the order of 145 at depths 100–250 km, Qp on the order of 350 at depths 250–410 km, and very high Qp with attenuation indistinguishable from zero for depths below 410 km. The P waves have a component of scattering, and thus, Qp−1 represents the maximum intrinsic attenuation beneath normal undisturbed oceanic lithosphere.

Pronounced zonation of seismic anisotropy in the Western Hellenic subduction zone and its geodynamic significance

Many subduction zones exhibit significant retrograde motion of their arc and trench. The observation of fast shear-wave velocities parallel to the trench in such settings has been inferred to represent trench-parallel mantle flow beneath a retreating slab. Here, we investigate this process by measuring seismic anisotropy in the shallow Aegean mantle. We carry out shear-wave splitting analysis on a dense array of seismometers across the Western Hellenic Subduction Zone, and find a pronounced zonation of anisotropy at the scale of the subduction zone. Fast SKS splitting directions subparallel to the trench-retreat direction dominate the region nearest to the trench. Fast splitting directions abruptly transition to trench-parallel above the corner of the mantle wedge, and rotate back to trench-normal over the back-arc. We argue that the trench-normal anisotropy near the trench is explained by entrainment of an asthenospheric layer beneath the shallow-dipping portion of the slab. Toward the volcanic arc this signature is overprinted by trench-parallel anisotropy in the mantle wedge, likely caused by a layer of strained serpentine immediately above the slab. Arcward steepening of the slab and horizontal divergence of mantle flow due to rollback may generate an additional component of sub-slab trench-parallel anisotropy in this region. Poloidal flow above the retreating slab is likely the dominant source of back-arc trench-normal anisotropy. We hypothesize that trench-normal anisotropy associated with significant entrainment of the asthenospheric mantle near the trench may be widespread but only observable at shallow-dipping subduction zones where stations nearest the trench do not overlie the mantle wedge.

Depth to basement and seismic velocity structure from passive seismic soundings in central Australia

SUMMARY We constrain the depth and seismic structure of stiff sediment cover overlying a prospective basement terrane using a passive seismic technique which uses surface wave energy from microtremor (also known as ambient seismic energy or seismic noise). This may be applied to mineral exploration under cover to decrease the inherent ambiguity in modelling potential field data for exploration targeting. Data from arrays of portable broadband seismometers are used to produce vertical profiles of seismic velocity structure using Both the Multimode Spatially Averaged Coherency (MMSPAC) method which measures the azimuthal average of the coherency between sensor pairs with a common separation, and the Horizontal to Vertical Spectral Ratio (HVSR) method, to estimate the seismic velocity structure and cover thickness respectively. We have developed field protocols to ensure consistent acquisition of high quality data in a variety of ground conditions. A wavefield approaching the theoretical ideal for MMSPAC processing is created by combining the energy content of an off-road vehicle, driven around the seismometer array, and ambient sources. We find that this combination results in significantly higher quality MMSPAC waveforms in comparison to that obtained using ambient energy alone. Under ideal conditions a theoretical maximum depth of investigation of 600m can be achieved with a hexagonal sensor array with 50m radius and both MMSPAC and HVSR, although the maximum thickness of sedimentary cover in the study area limits the depth of investigation to approximately 180m. The modelling procedure we employ is sensitive to layer thicknesses of ± 5 %.

Applying distance sampling to fin whale calls recorded by single seismic instruments in the northeast Atlantic

Automated methods were developed to detect fin whale calls recorded by an array of ocean bottom seismometers (OBSs) deployed off the Portuguese coast between 2007 and 2008. Using recordings collected on a single day in January 2008, a standard seismological method for estimating earthquake location from single instruments, the three-component analysis, was used to estimate the relative azimuth, incidence angle, and horizontal range between each OBS and detected calls. A validation study using airgun shots, performed prior to the call analysis, indicated that the accuracy of the three-component analysis was satisfactory for this preliminary study. Point transect sampling using cue counts, a form of distance sampling, was then used to estimate the average probability of detecting a call via the array during the chosen day. This is a key step to estimating density or abundance of animals using passive acoustic data. The average probability of detection was estimated to be 0.313 (standard error: 0.033). However, fin whale density could not be estimated due to a lack of an appropriate estimate of cue (i.e., vocalization) rate. This study demonstrates the potential for using a sparse array of widely spaced, independently operating acoustic sensors, such as OBSs, for estimating cetacean density.

Seismicity and structure of the 85°E volcanic complex at the ultraslow spreading Gakkel Ridge from local earthquake tomography

Accretion mechanisms at ultraslow spreading ridges are still only poorly understood due to difficult survey conditions for seismic experiments at these ridges. Melts gets focused in distinct magmatic centres, with thin crust in between. At the 85°E/85°N volcanic complex at Gakkel ridge, Arctic Ocean, where a recent spreading episode has been observed, perennial sea ice cover challenges traditional investigations of seismic structure and microseismicity. We used an unusual survey set-up to gather seismological data during 16 days in July 2007, using seismometer arrays mounted on ice floes. Despite only 12 stations, the drift of the ice floes over the survey area resulted in a multitude of crossing rays in the rift valley. The data included 303 microearthquakes of which 248 events could be confidently located. We compiled a 1D velocity model by localizing a subset of these earthquakes with a suite of randomly created velocity models. In this model, the Moho is placed at 7 km depth below seafloor, inferring a thick, basaltic crust. Using 124 events which were recorded by at least two arrays, we inverted for P-wave velocity structure in a local earthquake tomography. Resolution tests indicate reliable results in the central rift valley, illuminating the thermal structure underneath the Asgard volcanic chain in the aftermath of its most recent spreading episode. Our results show microearthquake activity down to 16 km beneath seafloor, inferring a cold lithosphere. Most hypocentres cluster at the centre of the rift valley at the site the Asgard volcanic chain. This may mean that existing thermal models for this class of ridges have to be refined. An area of decreased seismic velocities crosses the rift valley at this location and microearthquake activity is located at its eastern fringe where the velocity gradient is highest. We therefore speculate that the reduced velocities may be caused by warm intruded material and that the observed seismicity predominatly reflects the relaxation of thermal stresses following recent intrusion.

Constraining depth to basement for mineral exploration using microtremor: A demonstration study from remote inland Australia

We constrain the depth and seismic structure of stiff sediment cover overlying a prospective basement terrane using a passive seismic technique which uses surface wave energy from microtremor (also known as ambient seismic energy or seismic noise). This may be applied to mineral exploration under cover to decrease the inherent ambiguity in modeling potential field data for exploration targeting. We use data from arrays of portable broadband seismometers, processed using both the multimode spatially averaged coherency (MMSPAC) method and the horizontal to vertical spectral ratio (HVSR) method, to produce profiles of seismic velocity structure along a 12-km transect. We have developed field protocols to ensure consistent acquisition of high-quality data in near-mine and remote locations and a variety of ground conditions. A wavefield approaching the theoretical ideal for MMSPAC processing is created by combining the energy content of an off-road vehicle, driven around the seismometer array, and ambient sources. We found that this combination results in significantly higher-quality MMSPAC waveforms in comparison with that obtained using ambient energy alone. Under ideal conditions, a theoretical maximum depth of investigation of 600 m can be achieved with a hexagonal sensor array with 50-m radius and MMSPAC and HVSR. The modeling procedure we employ is sensitive to layer thicknesses of [Formula: see text]. A high-velocity layer in the sediment package reduces the sensitivity to deeper structure. This can limit the modeling of underlying layers but may be addressed by detailed analysis of the HVSR peaks. Microtremor recordings including off-road vehicle noise, combined with the MMSPAC and HVSR processing techniques, may therefore be used to constrain sediment structure and depth to basement in a cost-effective and efficient method that could contribute greatly to future mineral exploration under cover.

Statistically-based approach for monitoring of micro-seismic events

We consider the problem of optimal statistical estimation of micro-seismic source parameters using multichannel data from surface arrays of seismometers affected by a strong seismic noise. The problem is treated as a statistical task of parameter estimation for a general type multidimensional linear model with random or completely unknown input time functions. The maximum-likelihood generic estimators are derived and their relationship with the well-known seismic emission tomography (SET) algorithm is established. The proposed estimation algorithms perform processing of multichannel discrete observations in the frequency domain and can be implemented in on-line mode. Using the method of successive independent trials (Monte-Carlo), we demonstrate that a proposed statistical estimation algorithm provides much higher accuracy of the micro-seismic event source location in real noise conditions than the widely used SET algorithm and, as a consequence, is more reliable in practical applications.

Extraction of Moho-Generated Phases from Vertical and Radial Receiver Functions of a Seismic Array

Abstract Receiver‐function analysis is an effective tool for investigating crustal seismological structure. Here, we present the extraction of the Moho‐reflected PpPp that emerges in teleseismic P coda via a deconvolution process. Using nonlinear waveform analysis (an approach using simulated annealing technique) we estimate the source wavelet of a teleseismic P wave from records of the vertical component observed at an array of seismometers. PpPp recorded on the vertical component can be extracted by deconvolving individual vertical components by the resulting source wavelet. By employing this technique in a case study in southwestern Japan, seismic images from PpPp , as well as from Ps and PpPs , successfully image the continental Moho, the oceanic Moho, and the top surface of the Philippine Sea slab. In addition, we found that the amplitude of PpPp is useful in precisely determining crustal properties, such as vertically averaged V P / V S and the crustal thickness, by grid‐search techniques. It is also important to take into account the variations of the conversion/reflection coefficients for decreasing errors of the parameters in the grid‐search technique. Moreover, we demonstrate that improved seismic images of horizontal discontinuities can be obtained by using a stacking technique.

Estimation of Deep Shear-Wave Velocity Profiles in Lima, Peru, Using Seismometers Arrays

The estimation of the shear-wave velocity profile in Lima, Peru, was originally performed through surface wave observation in microtremor arrays. In the observation of these surface waves, a low signal problem for long periods was identified that resulted in a poorly done correlation between signals recorded by sensors and, consequently, in difficulty obtaining deep velocity profiles with good resolution. As an alternative, surface wave observation from seismic records was proposed. To confirm the feasibility of this methodology, seismometers were installed in an approximately circular configuration on the campus of the National University of Engineering in Lima. The procedures used to carry out analysis are similar to those used when analyzing microtremor arrays, with the exception that only the coda of seismic records is used for analysis. Results show that the dispersion curve obtained from seismometer arrays agree well with dispersion curve obtained from microtremor arrays and are predominant in a large period range. Finally, the estimated profile is verified using the observed H/V spectrum.

Slow slip and tremor search at Kilauea Volcano, Hawaii

AbstractKilauea Volcano, Hawaii, has hosted a long series of slow slip events observed since the installation of the continuous GPS network in 1996. Kilauea's slow slip events are inferred to occur on the decollement fault at 8 km depth beneath its south flank, with a location updip of the epicenters of large, regular earthquakes. Fault slip typically lasts about two days, and the events have magnitudes equivalent to Mw 5.3–6.0. While slow slip events in subduction zones are commonly accompanied by tectonic tremor (also called nonvolcanic tremor), no tremor has yet been reported in association with Kilauea's slow slip events. Instead, there are swarms of small triggered earthquakes, which is a characteristic only seen at select subduction zones (e.g., Boso and Hikurangi). A temporary array of seismometers was installed at Kilauea in 2007 in anticipation of a slow slip event. Here we use several established methods to perform a systematic search for tectonic tremor during geodetically defined slow slip events, as well as searching for tremor triggered by teleseismic surface waves. We do not detect tectonic tremor using any of these methods, although we are able to detect episodes of previously identified deep offshore volcanic tremor at 15–20 km depth and volcanic tremor from Kilauea. Although Kilauea's seismic network may not be adequate to observe tectonic tremor because Hawaii is seismically noisy and its crust is highly attenuating, it is also possible that the specific fault conditions on Kilauea's decollement are not conducive to such tremor generation.

A multiple-filter F detector method for medium-aperture seismometer arrays

S U M M A R Y The generalized F method, developed for teleseismic signal detection at small-aperture arrays, is extended to medium-aperture arrays and regional-distance signals using a multiple-filter technique. The technique allows the continuous estimate of the instantaneous amplitude and phase of the array seismograms, while at the same time allowing time-domain beamforming, making the method applicable to situations where the transit time of the signal across an array is much greater than the signal duration. The method is tested by application to waveform data from 22 seismometer arrays of the International Monitoring System, being set up to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty. A comparison with the results of the traditional signal detection method used by the International Data Centre (IDC) shows that the F detector increases candidate first P associations with IDC Reviewed Event Bulletin events, whereas at the same time halving the overall number of detections. The F method increases the number of associations at 21 of the 22 arrays, and for all signal slownesses.

Inverting ground motion from a seismometer array to obtain the vertical component of rotation: A test using data from explosions

To study factors that affect reliability of the derived vertical rotational ground motions (about vertical axis) from translational array waveforms, we have conducted a field test using explosion. The dataset was collected on March 4, 2008 from an active source experiment of the Taiwan Geodynamics Research project (TAIGER, 2008), in which 750kg of explosives were set off about 500m away from a specially designed seismic array in free field consisting of 8 tri-axial rotational sensors, 13 tri-axial accelerometers, and 12 six-channel, 24-bit dataloggers with GPS receivers. Our results show that the waveform fits were greatly improved when the explosion-induced crustal strains were less than 2×10−5 for our station configuration. As a result, our modeling for the large amplitude part of the mainshock was not entirely successful. However, we have successfully recovered the first order features of the later part of the vertical rotational ground motions using translational velocity waveforms in the 0.5–20Hz bandwidth using the Spudich and Fletcher’s procedure (2009). Moreover, we used a modified Jaeger’s (1969) formula and derived similar rotational synthetics. We found that the best rotational rate waveform fits were derived using a station configuration of a dense, small-aperture, translational array combined with a few large-offset stations. A small-aperture array is needed to fulfill the uniform rotation assumption. However, inverting data from an array of small spatial dimension requires waveforms of high signal to noise ratio and high sampling rates. Surprisingly, large-offset translation stations provided additional constraints in time and space, helping to fit the low frequency waveforms. Using the inverted result, we forward predicted the vertical translational waveforms not used in the inversion and got excellent fits. This suggests that the inverted results are of good quality for predicting the complete translational ground motions, even though only horizontal components were used in the inversion. Array-derived or directly measured rotational ground motions can be used to predict translational ground motions in the near-by region. Lessons learned from this study could be helpful for future studies using translational ground motions to derive dynamic ground strains, tilts, and torsions.

The Unique Contribution of the International Data Centre Reviewed Event Bulletin to Global Seismicity Catalogs

To monitor compliance with the Comprehensive Nuclear‐Test‐Ban Treaty (CTBT), the Preparatory Commission of the Comprehensive Nuclear‐Test‐Ban Treaty Organization (CTBTO) is setting up a network of 321 stations in the four verification technologies of seismology, hydroacoustics, infrasound, and radionuclides. Within this International Monitoring System (IMS) network are 170 seismic stations. The IMS seismic network is teleseismic in nature and designed to achieve approximately uniform coverage of the Earth for moderate sized events. A significant number of the IMS seismic stations consist of arrays of seismometers, adding to the network sensitivity and accuracy of the phase readings. At the end of the time period considered in this paper about 75% of the planned total of 170 seismic stations were certified and operational (www.ctbto.org/map). The data collected at the IMS network are transmitted through the Global Communications Infrastructure (GCI) in near real‐time or on request to the International Data Centre (IDC) in Vienna, Austria. Besides being archived and made accessible to Member States, these data are processed at the IDC and automatic and interactively reviewed bulletins are built. The moderate‐sized events detected by the IMS network are compiled in the IDC Reviewed Event Bulletin (REB), the final product of data analysis and interpretation. This bulletin is routinely produced and released to Member States since February 2000 on a daily basis with the exception of some 30 days (Koch, 2012) and contains nearly 30,000 events in the most recent years considered here. The International Seismological Centre (ISC) based at Thatcham, United Kingdom, prepares a global seismicity catalog that is a comprehensive summary of hypocenters and phase readings for all sufficiently large earthquakes. In this context the ISC aims to include all earthquakes in its bulletins that were recorded by one or more independently operated networks, to consistently re‐determine earthquake locations and magnitudes of events …

Seismic Evaluation of a Multipurpose Tower in Yellowknife, Northwest Territories, Canada

Locating scientific instruments in remote regions of the planet poses many common problems beyond just logistics, in particular when these instruments are to function autonomously or semiautonomously. In seismology, this is particularly prevalent for seismic stations located in the polar regions of the planet. Extremely cold, but also variable temperature, wind, and sunlight conditions as well as general inaccessibility hamper both operation and maintenance of equipment in such regions (Gill, 1974; Holcomb, 1982). The search for a reliable source of electrical power for the continuous operation of sensors and digitizing equipment continues to be one of these concerns. Many technologies have been investigated including radioisotope thermoelectric generation (e.g., Lamp, 1994), thermal electric generators (TEGs) (e.g., Holcomb, 1982), diesel engines, chemical batteries, wind and solar power generation (Anandakrishnan et al. , 2000), and more recently, fuel‐cell technology (e.g., Stehle et al. , 2011). Each technology has its own pros and cons, and in general, the technology that is chosen is based on which best suits the application, its requirements, and stresses of the station and its operators. For seismic instruments, this often requires that any noise generated by this choice does not adversely affect the operation or observations of ground motion the instrument is trying to measure. Such is the case at the Yellowknife teleseismic array in Canada’s Northwest Territories. Commissioned in 1962, the role of the Yellowknife seismic array (YKA) was to investigate the capabilities of distributed arrays of seismometers to monitor for the tell‐tale ground motions from explosions of nuclear weapons testing by the then nuclear‐capable countries of the United States, Great Britain, France, and the former Soviet Union. In one of the eventually five similar arrays constructed by the United Kingdom Atomic Energy Authority (UKAEA), the research was to determine if small numbers of seismic arrays could be …

Crustal structure of central Norway and Sweden from integrated modelling of teleseismic receiver functions and the gravity anomaly

SUMMARY Receiver functions have been calculated from teleseismic events recorded by an array of seismometers deployed on an E-W transect between the coasts of central Norway and Sweden. Forward and inverse modelling and migration of the receiver functions yields models for the subsurface velocity structure along the profile which have the crust thickening from c. 32 km at the Norwegian coast to c. 43 km beneath the central Scandinavian mountain range and then remaining constant beneath Sweden. There is some evidence for a low-velocity layer in the upper 10 km of the crust beneath parts of Norway and western Sweden and good evidence for a high-velocity lower crust underlying much of Sweden which thins to the west beneath Norway. Inverting the seismic velocities to density results in a very good correspondence between calculated and observed gravity anomalies. The results of this study do not support the presence of a significant crustal root providing buoyant support for the mountain range. Low topography and thick crust beneath Sweden are maintained by the high-velocity, high-density lower crustal layer. The upper crustal low-velocity layer is consistent with models based on existing refraction profiling and known geology and physical properties of the crust. There is no direct correlation between properties of the crust and topography suggesting that recent epeirogenic uplift has not resulted from modification of the crust.