Shallow Seismic Velocity Research Articles

Velocity Measurements of Powdered Rock at Low Confining Pressures and Comparison to Lunar Shallow Seismic Velocity

AbstractSeismic methods will be useful for future lunar near‐surface characterization, and high‐fidelity elastic models will be required to aid interpretation of seismic observations. To develop an elastic lunar near‐surface model, we performed ultrasonic velocity measurements of lunar regolith simulant at low confining pressure and developed a rock physics model calibrated to these measurements. Grain contact models based on Hertz‐Mindlin theory produce accurate results at high confining pressure (i.e., several hundred meters or more burial depth) but historically fail to predict observed velocities in unconsolidated media at low pressure. Therefore, we heuristically modified existing models to fit our measured data over a range of porosities and confining pressures. To compare with Apollo 14 and 16 active seismic experiments, we used our new heuristic rock physics model to produce lunar subsurface velocity profiles. We performed ray tracing through our velocity profiles to calculate seismic traveltime, which results in good agreement with first arrivals interpreted from the Apollo experiments. Our model suggests a slightly higher velocity‐pressure dependence than inferred from in situ measurements, which may be due to porosity reduction in the lunar regolith from impact‐induced and natural vibrations.

Processing Ambient Noise Data Using Phase Cross‐Correlation and Application Toward Understanding Spatiotemporal Environmental Effects

AbstractTypical use of ambient noise interferometry focuses on longer period (>1 s) waves for exploration of subsurface structure and other applications, while very shallow structure and some environmental seismology applications may benefit from use of shorter period (<1 s) waves. We explore the potential for short‐period ambient noise interferometry to determine shallow seismic velocity structures by comparing two methodologies, the conventional amplitude‐based cross‐correlation and linear stacking (TCC‐Lin) and a more recently developed phase cross‐correlation and time‐frequency phase‐weighted‐stacking (PCC‐PWS) method with both synthetic and real data collected in a heterogeneous karst aquifer system. Our results suggest that the PCC‐PWS method is more effective in extracting short‐period wave velocities than the TCC‐Lin method, especially when using data collected in regions containing complex shallow structures such as the karst aquifer system investigated here. In addition to the different methodologies for computing the cross correlation functions, we also examine the relative importance of signal‐to‐noise ratio and number of wavelengths propagating between station pairs to determine data/solution quality. We find that the lower number of wavelengths of 3 has the greatest impact on the network‐averaged group velocity curve. Lastly, we test the sensitivity of the number of stacks used to create the final empirical Green's function, and find that the PCC‐PWS method required about half the number of cross‐correlation functions to develop reliable velocity curves compared to the TCC‐Lin method. This is an important advantage of the PCC‐PWS method when available data collection time is limited.

Probing environmental and tectonic changes underneath Mexico City with the urban seismic field

Abstract. The sediments underneath Mexico City have unique mechanical properties that give rise to strong site effects. We investigated temporal changes in the seismic velocity at strong-motion and broadband seismic stations throughout Mexico City, including sites with different geologic characteristics ranging from city center locations situated on lacustrine clay to hillside locations on volcanic bedrock. We used autocorrelations of urban seismic noise, enhanced by waveform clustering, to extract subtle seismic velocity changes by coda wave interferometry. We observed and modeled seasonal, co- and post-seismic changes, as well as a long-term linear trend in seismic velocity. Seasonal variations can be explained by self-consistent models of thermoelastic and poroelastic changes in the subsurface shear wave velocity. Overall, sites on lacustrine clay-rich sediments appear to be more sensitive to seasonal surface temperature changes, whereas sites on alluvial and volcaniclastic sediments and on bedrock are sensitive to precipitation. The 2017 Mw 7.1 Puebla and 2020 Mw 7.4 Oaxaca earthquakes both caused a clear drop in seismic velocity, followed by a time-logarithmic recovery that may still be ongoing for the 2017 event at several sites or that may remain incomplete. The slope of the linear trend in seismic velocity is correlated with the downward vertical displacement of the ground measured by interferometric synthetic aperture radar, suggesting a causative relationship and supporting earlier studies on changes in the resonance frequency of sites in the Mexico City basin due to groundwater extraction. Our findings show how sensitively shallow seismic velocity and, in consequence, site effects react to environmental, tectonic and anthropogenic processes. They also demonstrate that urban strong-motion stations provide useful data for coda wave monitoring given sufficiently high-amplitude urban seismic noise.

Daily and Seasonal Variations of Shallow Seismic Velocities in Southern California From Joint Analysis of H/V Ratios and Autocorrelations of Seismic Waveforms

AbstractEstablishing a baseline of ongoing secular velocity variations at the subsurface can improve the accuracy of detecting and interpreting short‐term velocity changes, and advance the understanding of observed seismic motions and the behavior of subsurface materials. Toward these goals, we develop and apply a deconvolved autocorrelation (DA) method to estimate regional daily and seasonal changes of seismic velocities in southern California. The DA method combines advantages of traditional autocorrelation and Horizontal‐to‐Vertical Spectral Ratio, and is used to analyze over 10 years of data recorded by 50 stations. The results indicate widespread daily and seasonal changes of up to 10% and 4%, respectively, in the top tens of meters of the crust. The thickness of the surface layer, distance from the coast, and topographic variations are important factors controlling the amplitudes of the resolved velocity variations. The results suggest that changes of soil moisture and thermoelastic strain are likely dominant factors affecting the daily and seasonal variations, respectively. The developed DA method can improve the accuracy and robustness of estimated changes of subsurface materials at other locations.

Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field, California in response to large regional earthquakes and long-term geothermal activities

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field, California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014. We find clear co-seismic velocity reductions during the 2010 M 7.2 ​El Mayor–Cucapah, Mexico earthquake at ∼100 ​km further south, followed by long-term recoveries. The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands, indicating that material damage and healing process mostly occurred in the shallow depth. In addition, the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions. The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions, but subtle long-term velocity increases. We also build 3D transient water flow models based on monthly injection/extraction rates, and find correlations between several water flow parameters and co-seismic velocity reductions. We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production. The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.

Urban Seismic Site Characterization by Fiber‐Optic Seismology

AbstractAccurate ground motion prediction requires detailed site effect assessment, but in urban areas where such assessments are most important, geotechnical surveys are difficult to perform, limiting their availability. Distributed acoustic sensing (DAS) offers an appealing alternative by repurposing existing fiber‐optic cables, normally employed for telecommunication, as an array of seismic sensors. We present a proof‐of‐concept demonstration by using DAS to produce high‐resolution maps of the shallow subsurface with the Stanford DAS array, California. We describe new methods and their assumptions to assess H/V spectral ratio—a technique widely used to estimate the natural frequency of the soil—and to extract Rayleigh wave dispersion curves from ambient seismic field. These measurements are jointly inverted to provide models of shallow seismic velocities and sediment thicknesses above bedrock in central campus. The good agreement with an independent survey validates the methodology and demonstrates the power of DAS for microzonation.

Imaging and Monitoring Temporal Changes of Shallow Seismic Velocities at the Garner Valley Near Anza, California, Following the M7.2 2010 El Mayor‐Cucapah Earthquake

AbstractSeismograms from ~700 local earthquakes recorded at various depths (0, 6, 15, 22, 50, and 150 m) by sensors of the Garner Valley Downhole Array in Southern California are used to analyze the shallow velocity structure and temporal changes of seismic velocities after the 2010 M7.2 El Mayor‐Cucapah (EMC) earthquake. The direct P and S wave travel times between surface and borehole stations reveal very low shear wave velocities (178–259 m/s) and very high Vp/Vs ratios (6.2) in the top 22 m. Temporal changes of seismic velocities after the EMC earthquake are estimated using autocorrelations of data in moving time windows at two borehole stations (22 and 50 m) and seismic interferometry between multiple station pairs of the Garner Valley Downhole Array. The S wave velocity in the top 6 m drops abruptly by 14.3 ± 3.3%, during the passage of surface waves from the EMC event with a peak ground acceleration of 39 Gal, and recovers in ~236 s. The average velocity reductions decrease with depth and are 10.9 ± 3.1%, 8.5 ± 2.1%, 6.3 ± 2.1%, and 4.5 ± 2.0% in the top 15, 22, 50, and 150 m, respectively. Comparisons of seismic interferometry results between sensor pairs at 0–22 and 22–150 m indicate that statistically significant velocity changes are limited at the site to the top 22 m. Pore pressure data are in phase with the surface displacement and reach maxima when the highest velocity drop occurs, suggesting fluid effects contribute to the observed velocity reductions.

Imaging and monitoring of the shallow subsurface using spatially windowed surface-wave analysis with a single permanent seismic source

Development of shallow subsurface monitoring systems is important for monitoring the ground stability of shallow formation, and also for conventional deep seismic monitoring because with current techniques, temporal changes in shallow seismic velocities can influence monitoring results for the deep subsurface. We have developed an effective shallow seismic imaging and monitoring system with high spatiotemporal resolution and accuracy using a continuous and controlled source system, the accurately controlled routinely operated signal system (ACROSS). The method applies surface-wave analysis to characterize and monitor the shallow subsurface from the spatiotemporal variation of phase velocities. Because the number of available ACROSS units is usually limited, estimating a shallow subsurface with high spatial resolution is a challenging issue in ACROSS-based monitoring. To overcome this problem, we introduced a 2D spatial window into multichannel analysis of surface waves. We analyzed continuous ACROSS data acquired during seven different data periods from 2014 to 2016 at the Aquistore [Formula: see text] storage site in Canada. As a result, we clearly estimated spatial variations of phase velocities using only a single ACROSS unit. The numerical experiments of our method suggested that the spatial variations could be associated with shallow geologic boundaries in the study area. We identified clear seasonal variations of phase velocities in winter, possibly related to ground freezing in shallow sediments, and we showed the high temporal stability of our monitoring approach in warmer seasons. These results indicated that our approach would have the potential to identify spatiotemporal change in shallow subsurface associated with natural phenomena or fluid leakage.

A Shallow Seismic Velocity Model for the Groningen Area in the Netherlands

AbstractThe province of Groningen in the Netherlands holds one of the world's largest natural gas fields, and it has been an important source of energy for Western Europe for many decades. The seismicity in recent years called for a better understanding of the local subsurface, and therefore a dense network of 70 boreholes was installed in early 2015. Each borehole is equipped with four geophones and a surface accelerometer. In this study, data from this network are used to determine the shallow velocity structure that is important for the quantification of the seismic hazard and accurate source localizations. Compressional and shear wave velocity profiles with uncertainties are derived for each of the 200 m deep boreholes using passive seismic interferometry applied to local event data. The resulting seismic velocity distributions are presented as contour maps for 50 m depth intervals. The maps show strong lateral variations, where areas of low VP/VS ratio correspond to regions of sedimentary infill. The shear wave velocities were derived using the transverse component seismograms. Because the sensor orientations of the borehole geophones were unknown, they had to be determined first. This was done using a novel method based on cross correlations between the geophones and their colocated surface accelerometer. In addition, by extensive cross‐correlation analysis over the network, several installation inconsistencies were identified and resolved.

Kaveri Crater – An Impact Structure in the Precambrian Terrain of Southern India

ABSTRACT The region to the east of Palghat gap is of low elevation and nearly circular in shape. It forms a part of the Kaveri river basin. The predominantly gneissic terrain is surrounded by the charnockitic hill ranges, prominent among which are Nilgiris and Biligirirangan to the north; and Anaimalai and Kodaikanal to the south. The charnockite massifs have a steep slope facing the circular feature and a gentler slope in the opposite direction. Fractures/faults/shear zones are noticed in many parts. The Bouguer anomaly in the gneissic terrain is elliptical in shape and positive, relative to the surrounding elevated region. The magnetic contours are also elliptical and the magnetic basement is deeper by about one km compared to regions in the periphery. The shallow seismic velocity picture from Chennimalai to Palani indicates a graben structure. The velocity structure also depicts a 4-5 km Moho upwarp near Chennimalai. Junction between the gneissic and charnockitic terrain and even beyond, is marked by the presence of pseudotachylites and breccia. Field and petrographic studies indicate presence of suevite, Planar Deformation Features (PDF), Planar Fractures, diaplectic glass of quartz and plagioclase and spherical inclusion in suevite. These evidences taken together point to an extra-terrestrial impact which created a crater of approximately 120 km in diameter. Several lines of indirect evidences point to Neoproterozoic age for the impact. Kaveri Crater is the fourth largest crater on the surface of the Earth; other larger craters being Vredefort (160 km), Chicxulub (150 km) and Sudbury (130 km).

Temporal variation of the shallow subsurface at the Aquistore CO2 storage site associated with environmental influences using a continuous and controlled seismic source

AbstractThe development of reliable systems for monitoring injected CO2 is essential in carbon capture and storage projects. We applied time‐lapse surface wave analysis to measure temporal variations of the shallow subsurface among 11 periods (0.2–21.6 days) of continuous seismic data acquired from 2014 to 2016 at the Aquistore CO2 storage site in Canada. We focused on monitoring environmental influences on shallow seismic velocity, which are unrelated to CO2 injection into deep reservoirs. A continuous, controlled seismic source system called Accurately Controlled Routinely Operated Signal System was used to enhance the temporal resolution and source repeatability. Observed phase velocities were clearly higher in winter than in warmer seasons. The seasonal variation could be reproduced by an increase in the shallow S wave velocity during winter associated with the greater extent of freezing of partially saturated rock. We also observed gradual increases or decreases in phase velocities as the seasons changed, which could be related to gradual freezing or melting of ice. Our monitoring system thus could be effective for monitoring temporal variations of the shallow subsurface associated with the degree of freezing. Furthermore, the high temporal stability of our monitoring approach in warm seasons may make it possible to immediately identify CO2 leakage in the shallow subsurface.

Deep crustal structure of the northeastern Gulf of Mexico: Implications for rift evolution and seafloor spreading

AbstractWe image deep crustal structure using marine seismic refraction data recorded by a linear array of ocean‐bottom seismometers in the Gulf of Mexico Basin Opening project (GUMBO Line 3) in order to provide new constraints on the nature of continental and oceanic crust in the northeastern Gulf of Mexico. GUMBO Line 3 extends ~524 km from the continental shelf offshore Pensacola, Florida, across the De Soto Canyon and into the central Gulf basin. Travel times from long offset, wide angle reflections and refractions resolve compressional seismic velocities and layer boundaries for sediment, crystalline crust, and upper mantle. We compare our results with coincident multichannel seismic reflection data. Our velocity model recovers shallow seismic velocities (~2.0–4.5 km/s) that we interpret as evaporites and clastic sediments. A Cretaceous carbonate platform is interpreted beneath the De Soto Canyon with seismic velocities >5.0 km/s. Crystalline continental crust thins seaward along GUMBO Line 3 from 23–10 km across the De Soto Canyon. High seismic velocity lower crust (>7.2 km/s) is interpreted as extensive syn‐rift magmatism and possibly mafic underplating, common features at volcanic rift margins with high mantle potential temperatures. In the central Gulf basin we interpret thick oceanic crust (>8 km) emplaced at a slow full‐spreading rate (~24 mm/yr). We suggest a sustained thermal anomaly during slow seafloor‐spreading conditions led to voluminous basalt flows from a spreading ridge that overprinted seafloor magnetic anomalies in the northeastern Gulf of Mexico.

Improvements to shallow seismic velocity tomography method

The errors in seismic velocity recontructions in straight rays tomography method are unsatisfactory for the incompletely displayed seismic array around the panel. To improve resolution in these approaches, we propose a processing procedure consisting in re-applying the standard procedure after the obtained velocity model has changed. This modification is carried out based on the statistical analysis of the results obtained after the standard procedure was applyed for the first time. The procedure was tested on theoretical models and real data sets.

Ambient Noise Rayleigh Wave Shallow Tomography in the Tehran Region, Central Alborz, Iran

The Tehran region is one of the most densely concentrated areas of seismic activity in northern Iran. This region is dominated by the central Alborz Mountains belt and a large composite cone, the Damavand volcano. Located in the foothills of the Alborz Mountains, the city of Tehran, with a population of ∼15 million, is subject to high seismic risk (Jackson 2006; Berberian 2005). It is also surrounded by many active faults that have caused destructive earthquakes in the past (Berberian and Yeats 2001). Recent seismicity studies ( e.g. , Mottaghi et al. 2010) indicate that this region is under t|he threat of some major and yet unresolved active faults. Strong ground motion due to amplification of seismic energy by shallow velocity structure can be responsible for serious property damage (German and Beroza 2008). In order to accurately predict ground motion through waveform modeling, it is necessary to accurately image the shallow crustal seismic velocity. The local tomographic images available in the Tehran area come from body-wave earthquake tomography ( e.g. , Mostafanejad et al. 2011) and do not allow precise control of the near-surface (depth < 5 km) velocity structure. Furthermore, this model is affected by a variety of factors such as the effect of uneven source (or station) distribution and potential errors in identifiable seismic sources. We address these limitations by using the information extracted from ambient seismic noise. The present study uses continuous data recorded at a variety of sensors to investigate shallow crustal structure surrounding Tehran. We extract inter-station Green’s functions (GFs), with Rayleigh waves as the largest component, from beams of ambient noise cross-correlations at stations in and around the city of Tehran. We first estimate group velocity dispersion curves in the period band between 2 and 4 sec for each station-pair to invert for the 2D …

Shallow lithospheric contribution to mantle plumes revealed by integrating seismic and geochemical data

Using an integrated geophysical and geological approach based on a comparative analysis of 27 different parameters, we investigate both the location and origin of the source material for hot spot volcanoes. We report a significant correlation between extreme geochemical compositions of lavas with both shallow seismic velocities and the depth extent to which a hot spot source can be traced with seismic tomography. These relationships provide the first integrated evidence for buoyantly rising plumes of mantle material (plume depth extent correlation), incorporating thermally eroded continental material in the shallow mantle (anti‐correlation with slow shallow seismic velocities). The extreme geochemical composition of the lavas is successfully modeled with this process, placing constraints on dynamical processes in the mantle and explaining the origin of a significantly debated mantle compositional end‐member (enriched mantle; EM1).

Bedrock fracturing, threshold hillslopes, and limits to the magnitude of bedrock landslides

Bedrock fracturing and rock strength are widely believed to influence landscape morphology and erosional resistance. Yet, understanding of the quantitative relationship between rock-mass strength and landscape evolution remains limited. Here we present a new application of seismic refraction surveys that uses variations in seismic velocity to interpret differences in bedrock fracture density with depth. We use a comparative study of Fiordland and the western Southern Alps of New Zealand to examine how differences in rock type and bedrock fracturing influence landscape morphology and landslide response to rock uplift. In both regions, slopes appear invariant with differential rock-uplift rates and slope distributions reveal modal hillslope angles of ~ 32°. The majority of landslides initiate on slopes steeper than the modal hillslope angle, however, landslide magnitude–frequency distributions reveal order-of-magnitude differences between the regions, with Fiordland experiencing considerably smaller and less frequent landsliding events. Landslide-driven denudation rates of ~ 9 mm/yr in the western Southern Alps and between ~ 0.1 and 0.3 mm/yr in Fiordland approximate estimates of long-term rock-uplift rates for each region. The invariance of hillslope angles, near-normal slope distributions, predominance of landslide initiation on slopes steeper than modal values, and the apparent balance between rates of uplift and landslide-driven erosion suggest that hillslopes in both regions are at threshold angles. Their similar modal slopes further suggest that both ranges are characterized by equivalent rock-mass strength, despite striking differences in lithology. Additionally, our seismic analysis reveals nearly identical surface p-wave velocities. The unexpected equivalence of both modal slopes and surface velocities between these lithologically distinct ranges is attributed to contrasting degrees of surface fracturing that have differentially affected the intact rock properties, such that they now yield equivalent surface velocities and hillslope-scale strengths. Given that surface fractures help regulate threshold angles by modulating hillslope strength; we propose that shallow seismic velocities may provide a quantitative proxy for rock-mass strength. We define two contrasting fracture and landsliding environments. In Fiordland, dense geomorphic fracturing that is focused within the shallow subsurface appears to limit the depth and magnitude of most bedrock landslides. Conversely, in the western Southern Alps, tectonic forces produce pervasive fracturing with depth that results in larger, and deeper landslides. Our data suggest that bedrock fracturing at the Earth's surface modulates threshold hillslope angles, whereas the depth of bedrock fracturing influences the magnitude and frequency of landslide response to tectonic rock uplift.

Heterogeneous seismic velocity structure of the upper lithosphere at Kane oceanic core complex, Mid‐Atlantic Ridge

The Kane oceanic core complex (OCC) is a large, corrugated megamullion that was formed by a long‐lived detachment fault at the axis of the Mid‐Atlantic Ridge adjacent to Kane Fracture Zone between 2.1 and 3.3 Ma. We use refracted arrivals recorded along a 6‐km‐long hydrophone streamer during a multichannel seismic survey to constrain the shallow seismic velocity structure of the OCC. Results are presented in high‐resolution traveltime seismic tomographic models along six lines that cover all of the main morphological features of the megamullion. The models show large lateral variability in P wave velocity within the upper ∼0.5–2.0 km of the lithosphere, and these variations correlate to first order with observed variations in lithology, documented by in situ basement samples and seafloor morphology. Lithological interpretation of the velocity models indicates that there is marked lateral variability in distribution of gabbroic intrusions, serpentinized peridotites, and basalts at scales of a few kilometers to ∼10 km. Serpentinized peridotites appear to dominate the central and older parts of the OCC. High‐velocity gabbros are consistently (but not exclusively) present closer to the termination of the Kane detachment fault and toward the ends of the OCC. The structure of the lithosphere exhumed by the Kane detachment fault is far from the standard ophiolite‐based Penrose model, and it does not show segment‐centered magmatism that is commonly interpreted at slow spreading ridges. If the gabbros exhumed toward the termination of the OCC were emplaced deep (∼10 km) beneath the spreading axis, they may have constituted a weak zone that focused initiation of the Kane detachment fault. Alternately, as the OCC footwall was being exhumed the gabbros may have been emplaced because of dynamic changes in melt supply, changes in mantle fertility, or decompression melting. Late stage volcanism is clearly associated with a major high‐angle normal fault that cuts the detachment surface; this volcanism may have been stimulated or enhanced by bending stresses in the bending footwall. The shape of the large‐scale corrugated morphology of the OCC is nearly invariant in the dip direction across major changes in basement lithology, indicating that once established, the form of the Kane detachment fault was highly resistant to modification.

Shallow seismic structure of Kunlun fault zone in northern Tibetan Plateau, China: implications for the 2001Ms8.1 Kunlun earthquake

SUMMARY The shallow seismic velocity structure of the Kunlun fault zone (KLFZ) was jointly deduced from seismic refraction profiling and the records of trapped waves that were excited by five explosions. The data were collected after the 2001 Kunlun Ms8.1 earthquake in the northern Tibetan Plateau. Seismic phases for the in-line record sections (26 records up to a distance of 15 km) along the fault zone were analysed, and 1-D P- and S-wave velocity models of shallow crust within the fault zone were determined by using the seismic refraction method. Sixteen seismic stations were deployed along the off-line profile perpendicular to the fault zone. Fault-zone trapped waves appear clearly on the record sections, which were simulated with a 3-D finite difference algorithm. Quantitative analysis of the correlation coefficients of the synthetic and observed trapped waveforms indicates that the Kunlun fault-zone width is 300 m, and S-wave quality factor Q within the fault zone is 15. Significantly, S-wave velocities within the fault zone are reduced by 30–45 per cent from surrounding rocks to a depth of at least 1–2 km, while P-wave velocities are reduced by 7–20 per cent. A fault-zone with such Pand S-low velocities is an indication of high fluid pressure because Vs is affected more than Vp. The low-velocity and low-Q zone in the KLFZ model is the effect of multiple ruptures along the fault trace of the 2001 Ms8.1 Kunlun earthquake.

Upper mantle and crustal seismic structure beneath the Northwestern Pacific Basin using a seafloor borehole broadband seismometer and ocean bottom seismometers

In August 2000 a seafloor borehole broadband seismometer, WP-2, was installed in the northwestern Pacific basin, and seismic experiments with ocean bottom seismometers (OBSs), a single-channel seismic streamer, and airguns were performed. The WP-2 continued to record data autonomously, and a total of 436 days of broadband seismic records were eventually retrieved. Reflecting the low noise environment, many teleseismic events were recorded. Shallow seismic velocity models just below the OBSs were derived by the tau–p (τ–p) method for the airgun-OBS data and the single-channel seismic data. The crustal structure was estimated from the OBSs and WP-2 data by forward modeling, using a two-dimensional ray tracing method. The results of the seismic surveys show that the crustal seismic structure around WP-2 is laterally homogeneous and corresponds to typical oceanic crust. The uppermost mantle exhibits seismic anisotropy. The velocity variations are about 5% for P-waves and about 3.5% for S-waves, and the fast direction appears to be perpendicular to the magnetic lineations. Travel times of earthquakes recorded by the WP-2 and the previous seismological studies suggest that the lower part of the lithosphere has greater anisotropy than the uppermost mantle. To explain late first arrivals from the earthquakes that occurred in the slow direction with epicentral distances between 1600 and 2200 km, a low velocity zone below a depth of 30 km and a rapid increase of velocity at a depth of 210 km are inferred. Receiver function analysis of 16 events with a high signal-to-noise (S/N) ratio from the WP-2 data was performed, and discontinuities were estimated at depths of 416 and 666 km. These discontinuity depths are consistent with those of the average beneath the ocean.

Lithosphere structure across the Dead Sea Transform as constrained by Rayleigh waves observed during the DESERT experiment

SUMMARY The interdisciplinary Dead Sea Rift Transect (DESERT) project that was conducted in Israel, the Palestine Territories and Jordan has provided a rich palette of data sets to examine the crust and uppermost mantle beneath one of Earth’s most prominent fault systems, the Dead Sea Transform (DST). As part of the passive seismic component, thirty broad-band sensors were deployed in 2000 across the DST for roughly one year. During this deployment, we recorded 115 teleseismic earthquakes that are suitable for a fundamental mode Rayleigh wave analysis at intermediate periods (35‐150 s). Our initial analysis reveals overall shear velocities that are reduced by up to 4 per cent with respect to reference Earth model PREM. To the west of the DST, we find a seismically relatively fast but thin lid that is about 80 km thick. Towards the east, shallow seismic velocities are low while a deeper low velocity zone is not detected. This contradicts the currently favoured thermomechanical model for the DST that predicts lithospheric thinning through mechanical erosion by an intruding plume from the Red Sea. On the other hand, our current results are somewhat inconclusive regarding asthenosphere velocities east of the DST due to the band limitation of the recording equipment in Jordan.