Teleseismic Delay Research Articles

Body Wave Tomography of the Cascadia Subduction Zone and Juan de Fuca Plate System: Identifying Challenges and Solutions for Shore‐Crossing Data

AbstractRecent seismic results from the Cascadia Initiative indicate that heterogeneity in the oceanic asthenosphere affects subduction dynamics. Accurate characterization of the oceanic upper mantle is thus necessary to fully understand subduction processes, including the behavioral segmentation of the megathrust. A key challenge is integrating onshore and offshore datasets, which span large variations in near‐surface features that teleseismic body wave tomography is ill‐posed to resolve. Here, we perform a series of P and S forward modeling predictions to better understand the relative contribution of elevation, crustal thickness, offshore sedimentation, and near‐surface velocity structure to teleseismic delay times. Crustal thickness and elevation variations dominate the signal, contributing ∼1 s of delay time difference for P‐waves (roughly double for S). We test several inversion strategies to account for near‐surface features, identifying potential artifacts and causes of imaging errors. Undamped station statics are found to absorb mantle structures and introduce low‐velocity artifacts beneath the forearc. Our preferred inversion strategy utilizes a three‐dimensional starting model (including elevation) of the upper 50 km and heavily damped station statics, which we find leads to better resolution of mantle structure, particularly at asthenospheric depths. These insights guide inversions of observed delay times from the Cascadia subduction zone and Juan de Fuca plate system. We present a new onshore‐offshore S model and an updated P model. Major features are common to both models, including localized subslab low‐velocity anomalies, along‐strike variations in slab structure, and offshore heterogeneity, while regional differences may reflect changes in Vp/Vs.

Buoyant Asthenosphere Beneath Cascadia Influences Megathrust Segmentation

AbstractGreat megathrust earthquakes (magnitude 8+) do not typically rupture an entire convergent margin but rather are limited to one or more along‐strike segments. A fundamental question of subduction zone dynamics and hazard assessment is what physical properties or dynamic processes govern megathrust segmentation. Here we use onshore‐offshore teleseismic delay time data to tomographically image the upper mantle seismic structure of the Cascadia subduction zone. Our results reveal along‐strike segmentation in the oceanic asthenosphere beneath the subducting plate with pronounced low‐velocity anomalies below regions of increased plate locking and greater occurrence of episodic tremor and slip. We attribute the anomalous asthenospheric velocities to independent mantle upwellings associated with hot spot‐derived material in northern Cascadia and fragmentation processes at a diffuse plate boundary in southern Cascadia. Based on these relations, we hypothesize that subslab buoyancy modulates the plate coupling force at the thrust interface, thereby contributing to the localization of subduction zone segmentation.

Deep structure of Pyrenees range (SW Europe) imaged by joint inversion of gravity and teleseismic delay time

We present a new model of the lithosphere and asthenosphere structure down to 300 km depth beneath the Pyrenees from the joint inversion of recent gravity and teleseismic data. Unlike previous studies, crustal correction was not applied on teleseismic data in order (i) to preserve the consistency between gravity data, which are mainly sensitive to the density structure of the crust lithosphere, and traveltime data, and (ii) to avoid the introduction of biases resulting from crustal reductions. The density model down to 100 km depth is preferentially used here to discuss the lithospheric structure of the Pyrenees, whereas the asthenospheric structure from 100 to 300 km depth is discussed from our velocity model. The absence of a high density anomaly in our model between 30 and 100 km depth (except the Labourd density anomaly) in the northern part of the Pyrenees seems to preclude eclogitization of the subducted Iberian crust at the scale of the entire Pyrenean range. Local eclogitization of the deep Pyrenean crust beneath the western part of the Axial Zone (west of Andorra) associated with the positive central density anomaly is proposed. The Pyrenean lithosphere in density and velocity models appears segmented from east to west. No clear relation between the along-strike segmentation and mapped major faults is visible in our models. The Pyrenees’ lithosphere segments are associated with different seismicity pattern in the Pyrenees suggesting a possible relation between the deep structure of the Pyrenees and its seismicity in the upper crust. The concentration of earthquakes localized just straight up the central density anomaly can reflect the subsidence and/or delamination of an eclogitized Pyrenean deep root. The velocity model in the asthenosphere is similar to previous studies. The absence of a high-velocity anomaly in the upper mantle and transition zone (i.e. 125 to 225 km depth) seems to preclude the presence of a detached oceanic lithosphere beneath the European lithosphere.

Gaps, tears and seismic anisotropy around the subducting slabs of the Antilles

Seismic anisotropy in and beneath the subducting slabs of the Antilles is investigated using observations of shear-wave splitting. We use a combination of teleseismic and local events recorded at three-component broadband seismic stations on every major island in the area to map anisotropy in the crust, the mantle wedge and the slab/sub-slab mantle. To date this is the most comprehensive study of anisotropy in this region, involving 52 stations from 8 seismic networks. Local event delay times (0.21±0.12s) do not increase with depth, indicating a crustal origin in anisotropy and an isotropic mantle wedge. Teleseismic delay times are much larger (1.34±0.47s), with fast shear-wave polarisations that are predominantly parallel to trend of the arc. These observations can be interpreted three ways: (1) the presence of pre-existing anisotropy in the subducting slab; (2) anisotropy due to sub-slab mantle flow around the eastern margin of the nearly stationary Caribbean plate; (3) some combination of both mechanisms. However, there are two notable variations in the trench-parallel pattern of anisotropy — trench-perpendicular alignment is observed in narrow regions east of Puerto Rico and south of Martinique. These observations support previously proposed ideas of eastward sublithospheric mantle flow through gaps in the slab. Furthermore, the pattern of anisotropy south of Martinique, near Saint Lucia is consistent with a previously proposed location for the boundary between the North and South American plates.

Observed and predicted North American teleseismic delay times

Using a newly developed cross-correlation-based computer tool we measured absolute delay times of teleseismic P and S waves recorded by about 2000 broadband seismic stations from EarthScope's USArray, previous PASSCAL arrays, and additional permanent networks in North America. We estimate contributions to the delays from crustal structure, using various prior crustal models, and from event-side heterogeneity, using the delay time distribution itself. We then subtract these contributions from our measurements and map the average delay at each station location. We analyze these average delay times to investigate the structure of the North American mantle and formation of the North American continent. Mantle S delay times from stations west of the Rocky Mountains are 4.2 s larger than delay times from stations within the US portion of stable North America. Locally, S delays at Yellowstone are another 4 s larger than those west of Rocky Mountains. Decreasingly sharp delay time contrasts occur across the Rocky Mountains, the Appalachian Mountains, and on the southern edge of the North American Craton, respectively. These delay time gradients of various steepness broadly coincide with surface geological boundaries, but are offset to either side of these boundaries. Predictions of teleseismic S delays from twelve three-dimensional tomographic mantle S velocity models agree generally with observed delay time patterns, but underestimate the delays and advances to varying degrees. Tomographic models derived from different data than teleseismic arrival times predict similarly valid delays as models derived from teleseismic arrivals, while overestimating the smoothness of delay time patterns to varying degrees. We further utilize these tomographic models to predict and study the size and distribution of delay contributions from modeled heterogeneity in different depth ranges. This study shows that the 80–240 km depth range is the dominant contributor to delay time contrasts and variance. This depth range generally corresponds to asthenosphere in the western US and lithosphere in the central and eastern US. The average depth to which the observed delays require the central US lithosphere to extend is likely shallower than 240 km and consistent with the seismic bottom of lithosphere imaged in seismic-velocity models not derived from teleseismic delay times.

A new P-velocity model for the Tethyan margin from a scaled S-velocity model and the inversion of P- and PKP-delay times

We estimate a 3D P-velocity model for the Tethyan margin by inverting P- and PKP-delay times. The inversion is relative to a 3D reference model, which is a scaled S-velocity model for the same region. This S-velocity model was derived by jointly inverting regional S and Rayleigh waveform fits, teleseismic arrival times, Rayleigh-wave group velocities, and independent Moho constraints. Thus, our 3D reference model includes structures resolved over a larger depth range as well as more information on aseismic regions with few stations than is typically obtained from traditional teleseismic delay time inversions. We then inverted P- and PKP-delay times to obtain perturbations relative to the scaled 3D reference model. Comparing our P-velocity model (EAPV11) with P-velocity models derived from P data only, we find a model with more uniform and better depth resolution, including velocity anomalies for aseismic regions with few stations such as North Africa, southeastern Arabia, and the East European platform. Using EAPV11 to predict arrival times for relatively accurately located events that were not used in the inversion shows that our model produces significant variance reductions for these data as well. Therefore, our approach to build P-velocity models based on 3D reference S-velocity models may provide a practical way to better estimate P-velocity anomalies in the uppermost mantle and beneath aseismic regions with few stations.

Imaging lithospheric interfaces and 3D structures using receiver functions, gravity, and tomography in a common inversion scheme

Joint inversions are now commonly used in the earth sciences. They have been developed to better understand the structure of the earth, since they provide more constraints on the inverted parameters. We propose a new process to simultaneously invert several data sets in order to better image 3D crustal and upper mantle structures. Our inversion uses three kinds of data that present good complementarity: (1) P-wave receiver functions to provide Moho depth variations, (2) teleseismic delay times of P-waves to retrieve velocity anomalies in the crust and the upper mantle, and (3) gravity anomalies to image density variations at the lithospheric scale. We use a stochastic scheme, where receiver functions are first inverted. The interpolated resulting Moho depths are incorporated as a priori information into the joint inversion of teleseismic delay times and gravity anomalies process. Moreover, velocity and density can be linked by empirical relationships, which justifies the joint inversion of those parameters. In our stochastic approach, we perform a model space search for Moho variations, P-velocity, and density structure to find an acceptable fit to the three data sets. In order to preferentially sample the good data fit regions, we chose the neighborhood algorithm of Sambridge to optimistically survey the model space. We model the delay times with 3D raytracing using evenly spaced velocity–density nodes. We present here the first results given by this method on synthetic tests.

Two-dimensional and three-dimensional finite element modelling of mantle processes beneath central South Island, New Zealand

SUMMARY Numerical models that include pre-existing faults, elasto‐visco‐plastic rheology, thermal structure, realistic plate boundary conditions, and surface erosion are built to investigate alternative hypotheses of mantle flow geometry beneath central South Island, New Zealand. Bouguer gravity anomalies, teleseismic traveltime delays and seismic velocity structures are used to constrain models. Model predictions are compared with topography, uplift, magnetotelluric resistivity structure, isostatic gravity anomalies (IGAs), and seismicity. We find that the wedge model, in which the mantle lithosphere of the Australia plate is wedged between crust and mantle lithosphere of the Pacific plate, gives reasonable predictions of topography, uplift rate, IGAs, and stress and strain state in the lithosphere. Coupled erosional-tectonic models suggest that tectonic compression primarily determines the asymmetric surface relief, with erosion influencing its detail and having small effects on shallow stresses. Both 2-D and 3-D models suggest that the neotectonics of central South Island is dominated by weak faults. 3-D model results show that the Alpine fault and mantle shear zone accommodate most convergent and strike-slip plate motion, confirming non-partitioned plate motion in this region. Adding the regional strike-slip component has little effect on the convergent velocity field, and only changes the stress field by rotating the horizontal principal stresses out of the plane of the cross-section.

Basin Structure Influences on the Propagation of Teleseismic Waves in the Santa Clara Valley, California

We have investigated ground-motion amplification in the Santa Clara Valley (scv) using teleseismic P waves observed during the 1998 deployment of 41 short-period seismometers. The Santa Clara Valley Seismic Experiment (scvse) (Lindh et al. , 1999; Fletcher et al. , 2003) recorded many local and regional earthquakes and seven large ( M w > 6.4) teleseisms. Measured teleseismic P -wave arrival-time delays, relative P -wave amplification, and P -wave energy were used in the analysis. The relative P -wave amplification is found to correlate strongly with the arrival-time delays. In addition, the P -wave energy is found to correlate with the observed teleseismic delays. We also compared observed P -wave arrival-time delays and P -wave amplification with synthetics computed by using 3D finite-difference simulations of the teleseismic wave field to model these parameters using both the University of California, Berkeley (ucb) (Stidham et al. , 1999; Stidham, 1999) and the U.S. Geological Survey (usgs) (Brocher et al. , 1997; Jachens et al. , 1997) 3D velocity models. The results indicate that arrival-time delays on the order of ±0.25 sec correlate strongly with the reported basin depths in the two models. We find that the correlation between the arrival-time delays and basin depth is strongest for the usgs model. However, the ucb velocity model yields wave amplification that better matches the data. The finite-difference simulations indicate that, in general, the observations may be reproduced by either of the 3D velocity models, although refinements to the proposed 3D structure for the scv are needed.

Delay times and shear wave splitting in the Mediterranean region

SUMMARY We measure SKS splitting parameters and P and S delay times in the Mediterranean region using broad-band recordings from the temporary MIDSEA array and permanent networks. The MIDSEA seismic array has substantially improved data coverage along the European‐ Mediterranean margins and in northern Africa. Shear wave splitting is observed for 95 per cent of the stations, with splitting times up to 2 s. We propose five anisotropic domains, based on the characteristics of the observations. In the western part of the region we observe an alignment with plate motion, suggesting an asthenospheric origin for anisotropy. In the central and near eastern part, the fast directions are less coherent laterally and influenced by extension and subduction processes. Along the Dead Sea fault the fast directions are parallel to the relative motion between Arabia and Africa, which we attribute in part to coherent vertical deformation. Our measurements of teleseismic, relative P- and S-wave delay times within the Mediterranean region reach peak-to-peak values of 2.5 and 8 s, respectively. Consistent with International Seismological Centre (ISC) delay times, we observe the earliest arrivals along the Hellenic subduction zone and some of the latest along the Dead Sea fault. We find that S-wave delays are roughly three times as large as those of P waves. From a slight frequency dependence of our cross-correlation based delay time measurements, we estimate Mediterranean lateral heterogeneity to have a dominant size of one or more hundred kilometres. We further determine that the anisotropy implied by our SKS splitting measurements does not significantly bias the pattern of teleseismic delay times.

Depths to density discontinuities beneath the Adamawa Plateau region, Central Africa, from spectral analyses of new and existing gravity data

New gravity data from the Adamawa Uplift region of Cameroon have been integrated with existing gravity data from central and western Africa to examine variations in crustal structure throughout the region. The new data reveal steep northeast-trending gradients in the Bouguer gravity anomalies that coincide with the Sanaga Fault Zone and the Foumban Shear Zone, both part of the Central African Shear Zone lying between the Adamawa Plateau and the Congo Craton. Four major density discontinuities in the lithosphere have been determined within the lithosphere beneath the Adamawa Uplift in central Cameroon using spectral analysis of gravity data: (1) 7–13 km; (2) 19–25 km; (3) 30–37 km; and (4) 75–149 km. The deepest density discontinuities determined at 75–149 km depth range agree with the presence of an anomalous low velocity upper mantle structure at these depths deduced from earlier teleseismic delay time studies and gravity forward modelling. The 30–37 km depths agree with the Moho depth of 33 km obtained from a seismic refraction experiment in the region. The intermediate depth of 20 km obtained within region D may correspond to shallower Moho depth beneath parts of the Benue and Yola Rifts where seismic refraction data indicate a crustal thickness of ∼ 23 km. The 19–20 km depths and 8–12 km depths estimated in boxes encompassing the Adamawa Plateau and Cameroon Volcanic Line may may correspond to mid-crustal density contrasts associated with volcanic intrusions, as these depths are less than depths of 25 and 13 km, respectively, in the stable Congo Craton to the south.

On ACH, or how reliable is regional teleseismic delay time tomography?

ACH (named after Aki et al., 1976, Bull. Seismol. Soc. Am., 66: 501–524; Aki et al., 1977, J. Geophys. Res., 82: 277–296) is a standard, widely used, method for three-dimensional seismic imaging of the Earth. The fundamental hypothesis which underlies the method is that the time residuals generated outside the given target volume (from the seismic source to the bottom of the modelled zone) are approximately constant across the seismic array. The main purpose of this study is to check this assumption. We computed travel times for a given station and event distribution using a three-dimensional global Earth model taken from seismic tomography. We found that the relative residuals generated outside the target volume are not negligible and that the fundamental hypothesis underlying ACH is thus not verified. These deviations are generated in the lower and/or upper mantle and the corresponding proportions are entirely dependent on the raypaths. The bias in the inverted model is statistically similar to the input model outside the target volume. We thus recommend caution in any interpretations involving ACH-generated models. A secondary, somewhat independent, outcome of this study is that Fermat's principle, used to linearise the inverse problem in ray theory based tomography, seems to be valid without any restrictions (given our input model is representative for the true Earth) for rays with turning points in the lower mantle. For rays with turning points in the upper mantle, the constant raypath approximation is probably not true. This applies to global tomography as well.

Increase in melt fraction along a south-north traverse below the Tibetan Plateau: evidence from seismology

A mid-crustal low-velocity zone and crustal melt fraction contrast the Lhasa block in the Maxiang Yangbajain zone with the Tethyan Himalayas in the south. Evidence from wide-angle reflections is extended by vertical reflection and differential S to P wave teleseismic delays. The temperature and rheology implied allow phase transitions which may change crustal buoyancy and elevation (Le Pichon et al., 1997), and may allow lateral mass flow. In the uppermost mantle, particular path geometries to a tight temporary teleseismic array allow detection of a decrease in velocity northward through the Lhasa block. A larger relative variation in late teleseismic S with respect to P indicates an increase in Poisson's ratio, hence of partial melt fraction. This may be seen as evidence of a higher position of the mantle asthenosphere. Level Tibet hence appears to be underlain by variation in the structure within the crust across the Indus-Tsangpo suture, and in the structure within the mantle, further northward of the middle Lhasa block. These variations are most readily attributable to spatial variation in partial melt fraction, and hence temperature, which also induces phase transformations. Mode and amount of deformation and control of buoyancy on elevation may vary accordingly. The dynamics of the system is evidenced by seismic anisotropy which we relate to ductile flow.

Teleseismic Delay Times In A 3-D Earth and A New Look At theSDiscrepancy

SUMMARY When the earliest measurements of the Earth's normal modes became available, it was soon noticed that V, models satisfying this new data set were generally slower than models based on S traveltimes. This S discrepancy has since then been attributed to the influence of dispersion that accompanies attenuation, but it ignores a competing process that may have the same effect: in a heterogeneous earth, seismic rays will bend to take advantage of local high-velocity anomalies. This effect is difficult to quantify because of the computational problems associated with the determination of minimum traveltime paths in heterogeneous earth models in three dimensions. We have developed a new method to find absolute minimum traveltimes even in very complicated earth models and applied this to a large number of rays generated in different random earth models. The S discrepancy is a function of correlation length and amplitude of V, heterogeneity. A standard deviation u of 0.15 km sC', with a correlation length of 200 km in the whole mantle, would give a discrepancy even larger than observed at A > 60 and must be discarded. A u of 0.15 km s-' in the upper mantle and 0.05 km s-' elsewhere explains a discrepancy of 1-2 s, depending on the correlation length. Since the total discrepancy is estimated to be about 4 s, this still allows for effects of attenuation. We conclude that the S-wave discrepancy can be used to set limits to the heterogeneity in the lower mantle at short correlation lengths. With our present-day knowledge of the magnitude of the discrepancy a heterogeneity larger than 1 per cent with correlation length 200 km is unlikely.

Exploration of the lower lithosphere; northeastern United States

In this study we report on the modeling of broadband seismograms from moderate-sized earthquakes at regional distances. We demonstrate that the longer-period motions (World Wide Standardized Seismograph Network; WWSSN) preceding the direct S arrival can be modeled reasonably well with a crustal model consisting of a layer over a half-space. Although a generic model, assumed to be the same everywhere, can be used to model the P nl waveshape, some adjustments are required to fit the P n and S n timing for specific paths. Inversion of three-component seismograms assuming such simple models proves effective in source retrieval using a single station. Shorter periods can then be modeled by adding more detail to the velocity structure, holding the source fixed and repeating the process. Application of this modeling procedure to earthquakes occurring in the New England region proves interesting. In particular, paths on opposite sides of the Appalachian Thrust Belt produce distinct models. Paths along the western side of the belt can be modeled with an average crustal thickness of 35 km, the compressional and shear velocities of the crust are 6.5 km s −1 and 3.6–3.7 km s −1, and those of the upper mantle 8.4–8.5 km s −1 and 4.7–4.8 km s −1, respectively. The results are in agreement with the normal shield parameters. Paths along the eastern side of the belt indicate more variability but yield consistently lower lithospheric velocities by 3–4% for P waves. These models predict P-wave travel time delays across this zone of about 0.3 s, in agreement with teleseismic delays.

Magnetotelluric transect of Long Valley caldera: Resistivity cross‐section, structural implications, and the limits of a 2-D analysis

Twenty‐four magnetotelluric (MT) soundings have been collected in an east‐west profile across the center of Long Valley caldera. The average station spacing is approximately 1 km and appears adequate to sample the important features of the upper crustal and deeper resistivity structures. Additional control on the shallowest resistivity is provided by a continuous profile of time domain electromagnetic soundings coincident with the western portion of the MT line. Our MT data set reveals numerous resistivity structures which illuminate the evolution and present state of the Long Valley system. Many of these have been quantified through two‐dimensional (2-D) finite element modeling emphasizing the transverse magnetic (TM) mode. Important structural components include low‐resistivity layers 0.5–1.5 km deep under the eastern half of the caldera, beneath the axial graben of the resurgent dome, and under the west caldera moat. Most of this layering appears to lie in post‐caldera Early Rhyolite tuffs, and the uppermost unwelded Bishop Tuff. These rhyolite units have been observed to be porous and highly altered and to commonly contain Pleistocene intercalated lacustrine clays. The remainder and majority of the Bishop Tuff appears highly resistive. A low resistivity layer also occurs below the axial graben near the base of the Bishop Tuff (1.5 km). Hydrothermal fluids or alteration in precaldera volcanic strata or, less likely, carbonaceous metasediments may be the cause of this. Resistive, probably crystalline basement at high levels is apparent beneath the center of resurgence. Low resistivities are modeled at a depth around 5 km below the entire west moat and central graben and may represent a zone of hydrothermal fluids released from magma crystallization, with potential magmatic contributions at greater depths. The correspondence between this low resistivity and teleseismic delay and low density zones found in other studies is quite striking. A subtle anomaly in the transverse electric (TE) mode impedance is weakly suggestive of a midcrustal conductive axis centered beneath the central graben and resurgent dome. However, it cannot be simulated by two‐dimensional transverse electric calculations and requires a full three‐dimensional evaluation to ensure that the anomaly does not represent resistivity complexity in just the upper few kilometers. A fundamental, caldera‐wide 3-D effect is documented by comparison of observed and computed TE impedance and vertical magnetic field data. The abrupt termination of conductive caldera sediments less than 10 km north and south of our profile greatly depresses the observed TE apparent resistivity and vertical magnetic field relative to the model calculations for periods greater than 0.3 s for the central and eastern caldera. Analysis of the TE mode data also suggests that a similar finite‐strike effect lies in the response at periods greater than 3 s due to the mid‐crustal west moat conductor. The TM mode measurements are judged to also contain some large‐scale departure from the 2-D assumption related to horizontal current gathering from the north and south. This inflates the apparent resistivity and decreases the phase somewhat around 10 s over the central portion of the caldera relative to the 2-D model response. The regional profile of resistivity for the data at hand can be modeled with a 40 ohm‐m basal half‐space beneath 30 km of crust of 1000 ohm‐m or more. Although stations outside the caldera are very desirable to constrain this deep profile better, there is no evidence for a discrete low‐resistivity layer deep below Long Valley in contrast to our interpretation in the northeastern Basin and Range.

A teleseismic delay time study across the Central African Shear Zone in the Adamawa Region of Cameroon, West Africa

Summary. P-wave relative teleseismic residuals were measured for a network of seismological stations along a 300 km profile across the Adamawa Plateau and the Central African Shear Zone of central Cameroon, to determine the variation in crust and upper mantle velocity associated with these structures. A plot of the mean relative residuals for the stations shows a long wavelength (> 300 km) variation of amplitude 0.45 s. the slowest arrivals are located over and just to the north, of the faulted northern margin of the Adamawa Plateau. the residuals do not correlate with topography, surface geology or the previously determined crustal structure, in any simple way. The Aki inversion technique has been used to invert the relative residuals into a 3-D model of velocity perturbations from a mean earth model. the results show the region is divided roughly into three blocks by two subvertical boundaries, striking ENE and traversing both the crust and upper mantle down to depths greater than 190km. the central block, which is 2 per cent slower than the adjacent blocks, roughly corresponds to the Central African Shear Zone. the Adamawa Plateau, as an individual uplifted area, is explained by the interaction of a regional anomalous upper mantle associated with the West African Rift System, and the Central African Shear Zone, which provided a conduit for heat flow to the surface.

Magnitude bias in the Great Basin and its implications for explosion magnitude versus yield estimates

Comparison of teleseismic P‐wave delays in the vicinity of the Central Nevada Test Site in Hot Creek Valley, Nevada, with P‐delay data over a wider region in the Great Basin, shows that upper mantle speeds under Hot Creek Valley stations are higher than the average for the Great Basin as a whole, but lower than those beneath Pahute Mesa. These observations indicate that the caldera complex in Hot Creek Valley may have a high speed root similar to that proposed to exist beneath the Silent Canyon caldera at Pahute Mesa, however the Hot Creek Valley anomaly is not as strong as the Pahute Mesa anomaly. These high‐speed upper mantle structures cause defocusing for outgoing rays resulting in a shadow zone at the distance range corresponding to many European stations. The shadow zone caused by the Pahute Mesa structure is much more pronounced and consequently magnitudes of Pahute Mesa explosions can be underestimated relative to the magnitude of Hot Creek Valley explosions of similar yield. When magnitude versus yield relationships developed using Pahute Mesa data are applied to other areas, such as Hot Creek Valley, they can result in a biased estimate of yield.

Lithospheric structure beneath the Kenya Dome

Summary. The existence of an anomalously low-velocity, low-density zone within the upper mantle beneath the Kenya Dome has been deduced on the basis of previous gravity and seismic studies. This paper describes an experiment to measure teleseismic delay times across the Gregory Rift near the equator and along a SE radius of the Kenya Dome. The delay times have been determined with good relative accuracy and provide further independent evidence for the existence of the anomalous zone. The pattern of delay times along the two profiles and at other stations indicates that the zone thins rapidly to the SE away from the rift axis, mirroring the attenuation observed, from Kaptagat, for the same zone to the NW. The trend is for the thinning to become very much less rapid with distance, but there is also clear evidence for localized thickening of the zone under the Kilimanjaro–Chyulu volcanic area. Significantly smaller delay times are measured at the centre of the rift than at the edges. This is shown to indicate that the anomalous zone penetrates the crust to form an intrusion of relatively high-velocity material along the rift axis. The clear correlation of the delay time low with the axial Bouguer high indicates that they are both manifestations of the same underlying structure. Thus the delay time results provide independent confirmation of the existence of the axial intrusion previously inferred from gravity data. The width of this intrusion at the normal base of the crust is well defined by the data as 30 km.

Rheological properties of parts of the African lithosphere and their geodynamic significance

Earthquakes, volcanic activities and other manifestations of the restlessness of the Earth's interior are not uniformly spread across the African continent, and a basic problem of geodynamics is to explain the occurrence and distribution of these deep-seated processes. A study of the rheological conditions of the materials of the lithosphere under different parts of the continent has been undertaken in order to explain the variations in geological structure across large sections of Africa. Since these rheological properties are intimately bound to the thermal structure of the lithosphere, the temperature-depth relationship for four different zones of the continent has been established, using the surface heat flow value among other factors as a constraint in solving the heat conduction equation. Viscosity/depth profiles have been derived for parts of the African lithosphere from an extrapolation of laboratory data for both linear and non-linear creep laws. Estimates of viscosity range from about 10 17 Pa s −1 (i.e. 10 18 poise) under the East African Rift Valley system to 10 24 Pa s −1 (or 10 25 poise) below the West African shield in Niger Republic and Sierra Leone. High temperatures, low densities, low viscosities and high stress gradients characterize the East African Rift Valley areas, the Afar triangle, and some of the ‘swells’, e.g. Tibesti and Jebel Mara. The flow patterns in the upper mantle beneath these areas are grossly similar. The concept of a standard African lithosphere is examined and found to be wanting in view of the degree of heterogeneity in rheological properties. Data from electromagnetic deep soundings, gravity and teleseismic delay time studies are incorporated to present representative models of the lithosphere for the various tectonic regimes of the African continent.