Stacked Receiver Functions Research Articles

Prevalent thickening and local thinning of the mantle transition zone beneath the Baikal rift zone and its dynamic implications

The Baikal rift is the most seismically active continental rift in the world and is significant for studying the dynamics of continental rifts, although its precise dynamic mechanisms remain controversial. We calculated receiver functions (1748) from Global Seismographic Network seismic stations TLY and ULN and stacked receiver functions in different bins. Here we present discontinuities at depths of 410 km and 660 km and thickness of the mantle transition zone (MTZ) beneath the study area. The MTZ structure shows an obvious thickening (292 km) in the Baikal rift zone except for an area of limited thinning (230 km), whereas it is basically normal (250 km) beneath the Mongolian area, to the southeast of the Baikal rift. Combining these results with previous findings, we propose that the large-scale thickening beneath the Baikal rift zone is likely to be caused by the Mesozoic collision between the Siberian Platform and the Mongolia-North China Block or magmatic intrusion into the lower crust, which would result in crust and lithosphere thickening. Thus, the lower crust becomes eclogitized and consequently detached into the deep mantle because of negative buoyancy. The detachment not only induces asthenosphere upwelling but also accelerates mantle convection of water detached from the subducted slab, which would increase mantle melting, while both processes promote the development of the rift. Our preliminary results indicate that the detachment and the consequent hot upwelling have an important influence on the development of the Baikal rift, and a small-scale mantle upwelling indicated by the located thinning may have destroyed the lithosphere and promoted this development.

Structure of the subduction system in southern Peru from seismic array data

The subduction zone in southern Peru is imaged using converted phases from teleseismic P, PP, and PKP waves and Pwave tomography using local and teleseismic events with a linear array of 50 broadband seismic stations spanning 300 km from the coast to near Lake Titicaca. The slab dips at 30° and can be observed to a depth of over 200 km. The Moho is seen as a continuous interface along the profile, and the crustal thickness in the back‐arc region (the Altiplano) is 75 km thick, which is sufficient to isostatically support the Andes, as evidenced by the gravity. The shallow crust has zones of negative impedance at a depth of 20 km, which is likely the result of volcanism. At the midcrustal level of 40 km, there is a continuous structure with a positive impedance contrast, which we interpret as the western extent of the Brazilian Craton as it underthrusts to the west.Vp/Vs ratios estimated from receiver function stacks show average values for this region with a few areas of elevated Vp/Vs near the volcanic arc and at a few points in the Altiplano. The results support a model of crustal thickening in which the margin crust is underthrust by the Brazilian Shield.

Mantle transition zone shear velocity gradients beneath USArray

Broadband P-to-s scattering isolated by teleseismic receiver function analysis is used to investigate shear velocity (VS) gradients in the mantle transition zone beneath USArray. Receiver functions from 2244 stations were filtered in multiple frequency bands and migrated to depth through P and S tomography models. The depth-migrated receiver functions were stacked along their local 410 and 660km discontinuity depths to reduce stack incoherence and more accurately recover the frequency-dependent amplitudes of P410s and P660s. The stacked waveforms were inverted for one-dimensional VS between 320 and 840km depth. First, a gradient-based inversion was used to find a least-squares solution and a subsequent Monte Carlo search about that solution constrained the range of VS profiles that provide an acceptable fit to the receiver function stacks. Relative to standard references models, all the acceptable models have diminished VS gradients surrounding the 410, a local VS gradient maximum at 490–500km depth, and an enhanced VS gradient above the 660. The total 410 VS increase of 6.3% is greater than in reference models, and it occurs over a thickness of 20km. However, 60% of this VS increase occurs over only 6km. The 20km total thickness of the 410 and diminished VS gradients surrounding the 410 are potential indications of high water content in the regional transition zone. An enhanced VS gradient overlying the 660 likely results from remnants of subduction lingering at the base of the transition zone. Cool temperatures from slabs subducted since the late Cretaceous and longer-term accumulation of former ocean crust both may contribute to the high gradient above the 660. The shallow depth of the 520km discontinuity, 490–500km, implies that the regional mean temperature in the transition zone is 110–160K cooler than the global mean. A concentrated Vs gradient maximum centered near 660km depth and a low VS gradient below 675km confirms that the ringwoodite to perovskite+magnesiowüstite reaction is the dominant cause of the 660 in the region.

Relocation of the Yushu M S7.1 earthquake and its aftershocks in 2010 from HypoDD

After the Yushu MS7.1 earthquake on April 14, 2010, a large number of aftershocks were recorded by the surrounding permanent network and temporary seismic stations. Due to the distribution of stations, knowledge about velocity structure, the reliability of seismic phases, and so on, the location result from conventional method is usually of low precision, from which it is difficult to recognize the spatial and temporal distribution and the trends of aftershock activity. In this paper, by using teleseismic waveforms recorded by permanent station, the seismic velocity structure beneath the vicinity is obtained from receiver function stacking and inversion methods. And the Yushu earthquake sequences are relocated from seismic phase data by HypoDD. The results show that the Yushu MS7.1 earthquake occurred at 13 km depth; the aftershock sequences were distributed mainly in the NWW along the Garze-Yushu fault, and most aftershocks were concentrated in a 100 km length and 5–20 km depth. Combined with the velocity structure, it can be inferred that the earthquake mainly destroys the high-velocity layer of the upper crust. In the west of the seismic fault near (33.3°N, 96.2°E), the aftershock sequences were distributed like a straight column, suggesting there was a comminuted break from 25km depth to the ground.

Crustal structure of the British Isles and its epeirogenic consequences

Crustal receiver functions have been calculated for a network of 51 three-component broadband seismometers distributed across the British Isles and NW Europe. Over 3200 receiver functions were assembled for 1055 events. For each station, preliminary estimates of crustal thickness and Vp/Vs ratio were obtained from H−κ plots. Stacked receiver functions were then inverted to determine shear wave velocity as a function of depth. Each result was checked by guided forward modelling and by Monte Carlo error analysis. In this way, the robustness of our final calculated velocity profiles was carefully tested. A set of depth migrated profiles was also constructed using an average of 50 events for each station over a range of backazimuths. These profiles agree well with legacy wide-angle crustal models. Our results show that crustal thickness varies between 24 and 36 km across the British Isles. Thicker crust is found beneath north Wales and beneath central Scotland. Thinner crust occurs beneath northwest Scotland and northwest Ireland. By combining our database with the results of controlled source, wide-angle experiments and with depth-converted reflection profiles, we have produced a detailed crustal thickness map for a region encompassing the British Isles. Our synthesis of crustal thickness and structure has important implications for the tectonic and magmatic histories of this region. Complex Moho structure with lower crustal P-wave velocities of >7 km s−1 occurs beneath regions of Cenozoic magmatism, which may be consistent with magmatic underplating. Thin crust beneath northern Britain suggests that present-day long wavelength topography is maintained by regional dynamic support, originating beneath the lithospheric plate.

Point estimates of Crustal thickness Using receiver Function Stacking


 
 
 Introduction: Through receiver function analysis, this study inquires into some of the most basic properties of the crust below southern and central Quebec. Methods: This is accomplished, using receiver function technique, by stacking waveforms from 277 teleseismic events magnitude 6.0 and larger to find the delay in arrival time for several phases of the p-wave coda, relative to the initial p-wave arrival. This information is used to establish a linear relationship between thickness and p- to s-wave velocity ratio, each of which is stacked for a given station to identify a best-fit estimate for depth to the moho and Vp/Vs ratio. To determine their accuracy these results are compared with previous seismic studies, as well as synthetically generated receiver function p-wave arrivals based on simple 1d crustal models. Thickness calculations for the crust varied from 28 to 48 km; variation which was most likely the result of either complicated 3d structures or a shortage of available high-magnitude events for some stations. most of the results fell within appropriate windows outlined by studies like liTHoprobe. discussion: given that the 9 broadband stations used in this study compose an area from the superior province, grenville province and a selection of their subprovinces and intrusions, reliable evaluations of the crustal thicknesses below these seismic stations have broad relevance in understanding the crustal structure below Quebec.
 
 

수신함수와 표면파 분산곡선의 복합역산 및 수신함수 H-κ 중첩법을 이용한 원주 KS31 지진관측소 하부의 S파 지각 속도구조

To estimate the S-wave velocity structure beneath the KS31 broad-band station in Wonju, Korea, we used H- κ stacking and joint inversion of receiver functions and surface-wave dispersion curves derived from 297 teleseismic events (Mw > 5.5) recorded during the period between 2002 and 2009. We thereby determined that the average depth to a nearly flat Moho is 32.4 ± 0.5 km within tens of kilometer radius of the seismic station. For the crust at this location, we estimate an average shear-wave velocity of 3.69 km/s and a ratio of P- to S-wave velocities, VP/VS, of 1.72 ± 0.04, as is typical for continental crust. A negative phase in the receiver functions at 1 s indicates the presence of a shear- wave low velocity layer in a depth interval of 10 to 18 km in the upper crust beneath the KS31 station.

Transverse Variations of Crustal Thickness and VP/VS Ratio Under the Stations in the Liaodong Anteclise‐Yanshan Belt‐Xingmeng Orogenic Belt and Their Tectonic Implications

AbstractWe obtained the crustal thickness and average crustal Vp/Vs ratio along the profiles across the Liaodong anteclise, Yanshan belt and Xingmeng orogenic belt using H‐к stacking of receiver functions. The result shows that the average crustal thicknesses of these three tectonic units are 32, 33 and 35 km, respectively, indicative of slight differences, and display distinct transverse variations in the crustal structure. The crust is thicker in the middle and thinner on each side in the Liaodong anteclise, whereas the thickness of the crust varies smoothly in the Yanshan belt. In the Xingmeng orogenic belt, the crust thickens sharply from southeast to northwest near the Solonker suture. The Vp/Vs ratio also fluctuates obviously in the study region. In particular, it varies laterally in the Yanshan belt and increases markedly nearby the boundary area between the Yanshan belt and the Xingmeng orogenic belt (near the north‐south gravity gradient zone). However, the Vp/Vs ratio is relatively low and has small change in the Xingmeng orogenic belt. The distinct crustal structures of the Yanshan belt and the Xingmeng orogenic belt are probably indicative of a more intense crustal modification in the Yanshan belt compared with the Xingmeng orogenic belt during Mesozoic‐Cenozoic time. Significant changes in the crustal structure roughly coincide with the north‐south gravity gradient zone aforementioned. This together with the concordant change in the deep lithospheric structure observed previously suggests that this gravity gradient zone may be a lithospheric‐scale intra‐continental tectonic boundary, the opposite sides of which may have evolved differently in the Phanerozoic. The thickening of the crust in the Xingmeng orogenic belt (near the Solonker suture) and the Liaodong anteclise appears to mirror the surface topography, and corresponds to relatively stable Vp/Vs ratios. This feature probably indicates relatively even Phanerozoic modifications of the crust in each region, resulting in minor lateral variations in the crustal structure and components at present.

Crustal thickness and Vp/Vs ratio of the central and western North China Craton and its tectonic implications

We obtained the crustal thickness (H) and average Vp/Vs ratio (κ) for the central (CNCC) and western North China Craton (WNCC) by H–κ stacking of receiver functions. Our results show that H and κ varies significantly but similarly in the northern and southern CNCC, reflecting possible effects of the Phanerozoic cratonotic reactivation on the region. Of the WNCC, the south is featured by κ values typical for Precambrian shields (∼1.77) and variations of H mirroring surface topography; the north presents large fluctuations of κ (1.68–1.93) and a crustal thickening (up to 50 km) uncorrelated with topography. Such N–S differences were thought to be associated with a ∼1.95-Ga suturing event occurred in the north with little effects on the south. Our observations suggest that distinct structural heterogeneities of the NCC not only reflect diverse regional tectonics in Phanerozoic, but also provide evidence for long-term survival of crustal fabrics in stable continental interior.

Lithospheric velocity structure of the Anatolian plateau-Caucasus-Caspian region

[1] The Anatolian plateau-Caucasus-Caspian region is an area of complex lithospheric structure accompanied by large variations in seismic wave velocities. Despite the complexity of the region, little is known about the detailed lithospheric structure. Using data from 31 new, permanent broadband seismic stations along with results from a previous 29 temporary seismic stations and 3 existing global seismic stations in the region, a 3-D velocity model is developed using joint inversion of teleseismic receiver functions and surface waves. Both group and phase dispersion curves (Love and Rayleigh) were derived from regional and teleseismic events. Additional Rayleigh wave group dispersion curves were determined using ambient noise correlation. Receiver functions were calculated using P arrivals from 789 teleseismic (30°–90°) earthquakes. The stacked receiver functions and surface wave dispersion curves were jointly inverted to yield the absolute shear wave velocity to a depth of 100 km at each station. The depths of major discontinuities (sediment-basement, crust-mantle, and lithosphere-asthenosphere) were inferred from the velocity-depth profiles at the location of each station. Distinct spatial variations in crustal and upper mantle shear velocities were observed. The Kura basin showed slow (∼2.7–2.9 km/s) upper crustal (0–11 km) velocities but elevated (∼3.8–3.9 km/s) velocities in the lower crust. The Anatolian plateau varied from ∼3.1–3.2 in the upper crust to ∼3.5–3.7 in the lower crust, while velocities in the Arabian plate (south of the Bitlis suture) were slightly faster (upper crust between 3.3 and 3.4 km/s and lower crust between 3.8 and 3.9 km/s). The depth of the Moho, which was estimated from the shear velocity profiles, was 35 km in the Arabian plate and increased northward to 54 km at the southern edge of the Greater Caucasus. Moho depths in the Kura and at the edge of the Caspian showed more spatial variability but ranged between 35 and 45 km. Upper mantle velocities were slow under the Anatolian plateau but increased to the south under the Arabian plate and to the east (4.3–4.4 km/s) under the Kura basin and Greater Caucasus. The areas of slow mantle coincided with the locations of Holocene volcanoes. Differences between Rayleigh and Love dispersions at long wavelengths reveal a pronounced variation in anisotropy between the Anatolian plateau and the Kura basin.

Common-conversion-point stacking of receiver functions for estimating the geometry of dipping interfaces

SUMMARY We develop a new method to estimate geometries of dipping seismic velocity discontinuities from common conversion point stacking of receiver functions. The multistage fast-marching method is applied to the ray tracing with refraction at dipping interfaces. Although geometry of interfaces dipping at 30°–70° cannot be correctly estimated with a 1-D velocity model, it can be correctly estimated when refraction at dipping interfaces is taken into account. We apply this method to synthetic and observed data and confirm that geometries of dipping interfaces are successfully estimated. This method will contribute to revealing the structures beneath subduction zones where dipping interfaces exist.

Upper mantle anisotropy and transition zone thickness beneath southeastern North America and implications for mantle dynamics

A variety of models for mantle flow beneath southeastern North America have been proposed, including those that invoke westward driven return flow from the sinking Farallon slab, small‐scale convective downwelling at the edge of the continental root, or the upward advective transport of volatiles from the deep slab through the upper mantle. We use shear wave splitting observations and receiver function analysis at broadband seismic stations in the southeastern United States to test several of these proposed mantle flow geometries. Near the coast, stations exhibit well‐resolved null (no splitting) behavior for SKS phases over a range of back azimuths, consistent with either isotropic upper mantle or with a vertical axis of anisotropic symmetry. Farther inland we identify splitting with mainly NE–SW fast directions, consistent with asthenospheric shear due to absolute plate motion (APM), lithospheric anisotropy aligned with Appalachian tectonic structure, or a combination of these. Phase‐weighted stacking of individual receiver functions allows us to place constraints on the timing of arrivals from the 410 and 660 km discontinuities and on average transition zone thickness beneath individual stations. At most stations we find transition zone thicknesses that are consistent with the global average (∼240 km), with two stations showing evidence for a slightly thickened transition zone (∼250 km). Our results are relevant for testing different models for mantle dynamics beneath the southeastern United States, but due to the sparse station coverage, we are unable to uniquely constrain the pattern of mantle flow beneath the region. Our SKS splitting observations support a model in which mantle flow is primarily vertical (either upwelling or downwelling) beneath the southeastern edge of the North American continent, in contrast to the likely horizontal, APM‐driven flow beneath the continental interior. However, our receiver function analysis does not provide unequivocal support either for widespread hydration of the transition zone or for widespread thickening due to the downwelling of relatively cold mantle material. We expect that the necessary data to constrain such models more tightly can be obtained from the operation of denser seismic networks, including the Transportable Array and Flexible Array components of USArray.

Slab morphology in the Cascadia fore arc and its relation to episodic tremor and slip

Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of nonvolcanic tremor to the principal structural elements within the subduction complex. In Cascadia, active and passive source seismic data image a highly reflective, dipping, low‐velocity zone (LVZ) beneath the fore‐arc crust; however, its continuity along the margin is not established with certainty, and its interpretation is debated. In this work we have assembled a large teleseismic body wave data set comprising stations from northern California to northern Vancouver Island. Using stacked receiver functions we demonstrate that the LVZ is well developed along the entire margin from the coast eastward to the fore‐arc basins (Georgia Strait, Puget Sound, and Willamette Valley). Combined with observations and predictions of intraslab seismicity, seismic velocity structure, and tremor hypocenters, our results support the thesis that the LVZ represents the signature of subducted oceanic crust, consistent with thermal‐petrological modeling of subduction zone metamorphism. The location of tremor epicenters along the revised slab contours indicates their occurrence close to but seaward of the wedge corner. Based on evidence for high pore fluid pressure within the oceanic crust and a downdip transition in permeability of the plate interface, we propose a conceptual model for the generation of ETS where the occurrence and recurrence of propagating slow slip and low‐frequency tremor are explained by episodic pore fluid pressure buildup and fluid release into or across the plate boundary.

Crustal structure in the Pakistan Himalaya from teleseismic receiver functions

Subsurface structure in the Pakistan Himalaya is poorly known despite its geological importance in understanding the trend and growth of the Himalaya. In this paper, we compute receiver functions from a broadband seismic network in northern Pakistan and construct crustal structure beneath the stations. P to S converted phases at the Moho are clearly observed on stacked receiver functions. We model the conversions from the Moho and shallower interfaces using the trial and error method. Assuming a Vp/Vs ratio of 1.75, the modeled crustal thickness is 52–53 km in the Peshawar Basin, 55–62 km in the Lesser Himalaya, and 65–76 km in the Higher Himalaya. The variation of the Moho depth correlates well with surface topography, indicating an Airy type of isostasy in the Pakistan Himalaya. A large negative phase before the Moho conversion is consistently observed at all three stations to the north of the India‐Asia suture, which is best modeled by a low velocity layer in the lower crust. The slow lower crust is likely caused by partial melt resulted from crust thickening. Volatile in the crust of the Kohistan island arc and the subduction and melting of the Indian upper crust may also contribute to the low velocity layer. Like the weak middle‐lower crust in Tibet, this slow lower crust probably has an important role in the formation and growth of the western Himalaya.

Improved Statistical Processing for Common-Conversion-Point Stacked Receiver Functions

The interpretation of teleseismic receiver functions (RFs) is typically limited by poor constraints on the uncertainty of amplitudes of converted phases. In continental regions these problems are overcome by stacking large amounts of data. In oceanic regions, however, data quality is notoriously noisy and the number of events are often limited by significantly shorter station deployment times. Estimates of RF pulse amplitude uncertainty are, therefore, necessary to allow an analyst to differentiate between real structure in the Earth and random correlations in ambient noise. Here we combine a common-conversion-point stacking technique with multiple-taper correlation RF estimates, which allow frequency-domain weighting. We then compute jackknife statistics to estimate local uncertainties in RF amplitude. We apply this technique to a continental station in Arabia (RAYN) and to the ocean island station at Raratonga, Cook Islands (RAR) in order to compare our method to results obtained via other techniques. The structure we recover matches previous crustal studies at both stations and provides new interpretations of conversions in the upper mantle. At single stations this technique works well to resolve crust and mantle structure up to a depth of 100 km. Geographical dispersion of ray paths at greater depths decreases the number of events per bin and, therefore, increases the uncertainty in converted amplitude. The ability to stack crossing ray paths implies that this method will be well suited to the analysis of data from seismic arrays.

The crust and upper mantle structure beneath southeastern China

We analyzed teleseismic waveforms recorded by 44 stations in the Fujian and Taiwan provinces of China and obtained 5344 high quality receiver functions. The crustal thickness ( H) and average crustal V P / V S ratio ( k) beneath every station were estimated using the H– k stacking method. Crustal thicknesses near the Fujian Province range from 28.3 to 32.8 km with an average of 31.1 km, and the corresponding V P / V S ratios vary from 1.70 to 1.84 with a mean of 1.76. From inland to offshore of the Fujian Province, the crustal thicknesses decrease and Poisson's ratios increase. These may indicate decreasing SiO 2 and increasing calc-alkaline contents in the crust. The discontinuity structures such as the Moho, subducting slab, the 410- and 660-km discontinuities (hereafter we call them the 410 and the 660) are also studied using common converted point (CCP) stacking of receiver functions. Along two NW–SE lines of central and northern Taiwan, the CCP stacking results show a western dipping structure at depths above 50 km, suggesting that the Philippine Sea plate is probably subducting beneath the Eurasian continent plate near the central and northern Taiwan. The CCP stacking results show sharp and flat 410- and 660-km discontinuities, and the transition zone thickness (TZT) is the same as that of ambient mantle beneath Fujian and Taiwan Strait, but thickens in the east of Taiwan. These results suggest that (1) the subducting Eurasian continent plate is confined to the depths above 410 km beneath Fujian and Taiwan Strait; and (2) the South China Sea slab may reach the transition zone beneath the east of Taiwan.

Illuminating the plate interface structure beneath Cook Strait, New Zealand, with receiver functions

Using teleseismic receiver functions derived from broadband seismic arrays on the north end of the South Island and south end of the North Island of New Zealand, we image seismic impedance discontinuities in the upper mantle beneath Cook Strait using common conversion point (CCP) and Kirchhoff migration methods. Our primary findings are observations of discontinuities associated with the top of the subducting Pacific Plate. Our results suggest that contrary to recent suggestions, the plate is continuous under the northern South Island through the region of seismicity deeper than 50 km. West of Cook Strait, the slab dips steeply to the northwest. We find evidence for a low‐velocity layer at the top of the slab, near which much of the seismicity is concentrated. We see hints of two crustal discontinuities, consistent with observations from previous studies. We also see substantial and continuous energy on the transverse receiver function stacks above the mantle wedge extending to the northwestern edge of our stacks, which may indicate seismic anisotropy above 50 km depth.

Mantle transition zone structure and upper mantle S velocity variations beneath Ethiopia: Evidence for a broad, deep‐seated thermal anomaly

Ethiopia has been subjected to widespread Cenozoic volcanism, rifting, and uplift associated with the Afar hot spot. The hot spot tectonism has been attributed to one or more thermal upwellings in the mantle, for example, starting thermal plumes and superplumes. We investigate the origin of the hot spot by imaging the S wave velocity structure of the upper mantle beneath Ethiopia using travel time tomography and by examining relief on transition zone discontinuities using receiver function stacks. The tomographic images reveal an elongated low‐velocity region that is wide (>500 km) and extends deep into the upper mantle (>400 km). The anomaly is aligned with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but its center shifts westward with depth. The 410 km discontinuity is not well imaged, but the 660 km discontinuity is shallower than normal by ∼20–30 km beneath most of Ethiopia, but it is at a normal depth beneath Djibouti and the northwestern edge of the Ethiopian Plateau. The tomographic results combined with a shallow 660 km discontinuity indicate that upper mantle temperatures are elevated by ∼300 K and that the thermal anomaly is broad (>500 km wide) and extends to depths ≥660 km. The dimensions of the thermal anomaly are not consistent with a starting thermal plume but are consistent with a flux of excess heat coming from the lower mantle. Such a broad thermal upwelling could be part of the African Superplume found in the lower mantle beneath southern Africa.

A N-S receiver function profile across the Variscides and Caledonides in SW Ireland

SUMMARY Teleseismic receiver functions have been calculated from data of a temporary seismological network of broad-band three-component stations to investigate the lithospheric and asthenospheric structure across the Late Caledonian Iapetus Suture Zone (ISZ) in southern Ireland. The stations were deployed during the Irish Seismological Lithospheric Experiment (ISLE 2002/3) and straddle the Killarney-Mallow Fault Zone, a remnant of the Variscan orogeny, and the ISZ, the inferred boundary between the Laurentia and Eastern Avalonia plates fused together during the Caledonian orogeny. Receiver functions from the western part of the network were projected onto the N‐S VARNET 1996 seismic refraction profile, extending from the Old Head of Kinsale to Galway Bay in SW Ireland. Laterally continuous P to S conversions from the Moho at delay times of about 3.8‐4.1 s are clearly observed, and correspond to Moho depths of about 29‐32 km. The Moho has a transitional character to the south of the ISZ. Synthetic receiver functions, calculated from a 2-D velocity model of the previous VARNET experiment, show Moho conversions and multiple crustal phases compatible to those observed in the ISLE data. Furthermore, P to S conversions from the 660 km discontinuity (66‐68 s delay time) are well determined at the stations. In comparison, the conversion from the 410 km discontinuity at about 43‐45 s delay time is considerably weaker. Delay times of stacked receiver functions from the mantle transition zone are in agreement with the standard iasp91 earth model and thus no structural changes are observed across the ISZ at this depth interval.

New constraints on crustal structure in eastern Afar from the analysis of receiver functions and surface wave dispersion in Djibouti

Abstract Crustal structure beneath the GEOSCOPE station ATD in Djibouti has been investigated using H -κ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities. We obtain consistent results from the two methods. The crust is characterized by a Moho depth of 23 ± 1.5 km, a Poisson’s ratio of 0.31 ± 0.02, and a mean V p of c. 6.2 km s −1 but c. 6.9–7.0 km s −1 below a 2–5 km-thick low-velocity layer at the surface. Some previous studies of crustal structure for Djibouti placed the Moho at 8 to 10 km depth, and we attribute this difference to how the Moho is defined (an increase of V p to 7.4 km s −1 in this study vs. 6.9 km s −1 in previous studies). The crustal structure we obtained for ATD is similar to crustal structure in many other parts of central and eastern Afar. The high Poisson’s ratio and V p throughout most of the crust indicate a mafic composition and are not consistent with models invoking crustal formation by stretching of pre-existing Precambrian crust. Instead, we suggest that the crust in Afar consists predominantly of new igneous rock emplaced during the late syn-rift stage where extension is accommodated within magmatic segments by dyking. Sill formation and underplating probably accompany the dyking to produce the new and largely mafic crust.