Teleseismic P-wave Receiver Functions Research Articles

Crustal structure beneath two seismic broadband stations revealed from teleseismic P-wave receiver function analysis in the Virunga volcanic area, Western Rift Valley of Africa

The shear velocity structure beneath the Virunga volcanic area was estimated by using an average solution in the time domain inversion of stacked teleseismic receiver functions provided by two seismic broadband stations KUNENE (KNN) and KIBUMBA (KBB). These two stations are 29 km apart and located at the eastern and western escarpment of the Western Rift Valley of Africa in the Virunga area, respectively. The velocity model was presented as P-wave velocity models. From these models, the crust mantle transition zone beneath the area sampled by KNN and KBB in the Virunga area was determined at depth from about 36 to 39 km and 30 to 41 km, respectively. A low velocity zone was observed below stations KNN and KBB at depths between 20–30 km and 18–28 km, respectively, and with average velocity 5.9 km/s and 6.0 km/s. This low velocity zone may probably related to a magma chamber or a melt-rich sill. The models show also high velocity material (6.8–7.4 km/s) lying beneath stations KNN and KBB at depths 3–20 km and 3–10 km, respectively, which is indicative of magma cumulates within the volcanic edifice. The result obtained in this study was applied to the determination of epicentres during the period prior to the 27 November 2006 Nyamuragira eruption. This eruption was preceded by a swarm of hybrid volcanic earthquakes with clear P-waves onset. Using the receiver function model was found to improve the location of events. The located events correlate well with the location of the eruptive site and data provided by the INSAR observations of surface deformation associated with eruption.

Nature of the Moho beneath the Scottish Highlands from a receiver function perspective

The geology of Scotland documents a protracted geological history from Precambrian basement formation through the Caledonian Orogeny and to the Tertiary opening of the Atlantic. A temporary deployment of 22 closely spaced broadband seismic stations in Scotland traverses many of the major terrane boundaries in the region and provides a unique opportunity to place spatial and temporal constraints on variations in crustal structure. With teleseismic P-wave receiver functions, we use the H − κ method to determine variations in bulk crustal parameters: crustal thickness ( H) and V P/ V S ratio. Mean crustal thickness is ~ 28 km, varying from ~ 23 km in the NE highlands and increasing to > 30 km near the Highland Boundary Faultin the southern part of the study area. Mean V P/ V S values of ~ 1.76 show no significant variation across the study area. An abrupt increase in crustal thickness of ~ 4.5 km NW across the Moine Thrust is not easily linked to Tertiary–Recent tectonic activity. Instead it appears that Scotland's crust has retained features for hundreds of millions of years since it was first formed, despite abundant volcanic activity in Tertiary times. Our work favours the view that the Moho is a compositional boundary rather than a mineralogical one defined by a reaction in P– T space.

Deep tectonic setting of the 2008 Wenchuan M s8.0 earthquake in southwestern China

Teleseismic P-wave receiver functions at 20 broadband seismic stations in the Longmenshan fault zone (LMFZ) and its vicinity were extracted, and the crustal thickness and the P- and S-wave velocity ratio were calculated by use of the H-k stacking algorithm. With the results as constraints, the S-wave ve-locity structures beneath each station were determined by the inversion of receiver functions. The crustal structure of the Rear-range zone is similar to that of the Songpan-Garze Block, whereas the velocity structure of the Fore-range zone resembles that of Sichuan Basin, implying that the Central Principal Fault of LMFZ is the boundary between the eastern Tibetan Plateau and the Yangtze Block. Lower velocity zone exists in lower crust of the Songpan-Garze Block and the central-southern seg-ment of the Rear-range zone, which facilitates the detachment of the material in upper and middle crust. Joint analysis of the receiver functions and the Bouguer gravity anomalies supports the thesis on the detachment-thrust mode of the LMFZ. A double-detachment pattern is suggested to the tectonic setting in the Songpan-Garze Block. The upper detachment occurs at the depth of 10－15 km, and represents a high-temperature ductile shear zone. There is a lower detachment at the depth of about 30 km, below which the lower crust flow exists in the eastern Tibetan Plateau. Interpretation of the Bouguer gravity anomalies indicates that the Sichuan Basin is of higher density in upper and middle crust in compari-son with that of the Songpan-Garze Block. The LMFZ with higher density is the result from the thrusting of the Songpan-Garze Block over the Sichuan Basin. In the lower crust, higher P velocity and higher density in the Sichuan Basin are related to more rigid material, while lower S velocity and lower density in the Songpan-Garze Block are related to the softened and weakened material. The higher density block beneath the Sichuan Basin obstructs the eastward flow of lower crustal material from the Tibetan Plateau, which is driven by the compression of northward movement of Indian Plate. The eastward movement of upper and middle crustal material is also obstructed by the rigid Yangtze Block, resulting in the stress concentrated and accumulated along the LMFZ. When the stress releases sharply, the Wenchuan M s8.0 earthquake occurs.

Joint Analysis of Teleseismic Receiver Functions and Surface Wave Dispersion using the Genetic Algorithm

Teleseismic P-wave receiver function data and Rayleigh-wave phase velocity measurements are combined using the genetic algorithm, a global optimi- zation technique, to model crustal structure in southern Korea. The two datasets complement each other because receiver functions are sensitive to shear-wave ve- locity contrasts in layered structures, while surface wave dispersion is sensitive to averages of shear-wave velocities. The genetic algorithm is more useful than line- arized inversion in regions where there is little a priori information about local velocity structure because it is not sensitive to the initial model. The stability and variability of resulting crustal model parameters are quantified by using a Monte Carlo technique in specifying a suite of initial models. Depths to the Moho discon- tinuity in southern Korea were estimated to be 29-30 km for stations near the western coast and 33-36 km for inland stations. A well-resolved crustal low-velocity zone was inferred for some stations.

S-wave velocity structure beneath the Mátra Mountains (Hungary) inferred from teleseismic receiver functions

The crustal and uppermost mantle structure beneath the Matra Mountains has been investigated by means of the inversion of teleseismic P-wave receiver functions. Earthquakes recorded at the PSZ station in the period 1995-2000 with epicentral distances of between 20° and 100° and with magnitudes greater than 5.5 have been selected for analysis if their signal-to-noise ratio was sufficiently high. Source equalisation has been carried out to gain the radial and tangential receiver functions. The traces from similar backazimuths and distances were stacked to improve the data quality. The inversion of teleseismic waves coming from the north-east and the west shows that in the upper part of the crust the S-wave velocity has strong positive gradient, the middle crust exhibit almost uniform velocities, while in the lower crust and uppermost mantle the velocity increases monotonically. Velocity profiles for the eastern backazimuths are similar to the previous ones in the case of the upper and middle crust and upp...

Joint inversion of receiver function and surface wave dispersion observations

We implement a method to invert jointly teleseismic P-wave receiver functions and surface wave group and phase velocities for a mutually consistent estimate of earth structure. Receiver functions are primarily sensitive to shear wave velocity contrasts and vertical traveltimes, and surface wave dispersion measurements are sensitive to vertical shear wave velocity averages. Their combination may bridge resolution gaps associated with each individual data set. We formulate a linearized shear velocity inversion that is solved using a damped least-squares scheme that incorporates a priori smoothness constraints for velocities in adjacent layers. The data sets are equalized for the number of data points and physical units in the inversion process. The combination of information produces a relatively simple model with a minimal number of sharp velocity contrasts. We illustrate the approach using noise-free and realistic noise simulations and conclude with an inversion of observations from the Saudi Arabian Shield. Inversion results for station SODA, located in the Arabian Shield, include a crust with a sharp gradient near the surface (shear velocity changing from 1.8 to 3.5 km s−1 in 3 km) underlain by a 5-km-thick layer with a shear velocity of 3.5 km s−1 and a 27-km-thick layer with a shear velocity of 3.8 km s−1, and an upper mantle with an average shear velocity of 4.7 km s−1. The crust–mantle transition has a significant gradient, with velocity values varying from 3.8 to 4.7 km s−1 between 35 and 40 km depth. Our results are compatible with independent inversions for crustal structure using refraction data.

The receiver structure beneath the China Digital Seismograph Network stations

Abstract Six new crustal models for the region beneath the China Digital Seismograph Network stations are obtained by inversion and forward modeling broadband teleseismic P-wave receiver functions. The seismic phases modeled are primarily P-to-S conversions and reverberations generated at the two highest impedance interfaces: the Moho and the sediment-basement contact; these are the best-constrained features of the crustal models. Receiver functions from all of the stations show some indication of laterally varying structure, but relatively simple 1D models can represent the crustal structure beneath the three stations in northeast China and the two stations on the margin of the Tibetan Plateau. The station on the northern flank of the Tien Shan mountains is on a complex upper crustal structure that we represent with a 2D crustal model. Crustal velocity models and Moho depths from the receiver function analysis are compared with other compilations of Chinese crustal structure. These closely agree for the more uniform crustal region in northeastern China, but the differences are greater in southern and western China, where the crustal structure is more complex.

Effect of sinusoidal interfaces on teleseismic P-wave receiver functions

Using the T-matrix, or extended boundary condition, method, we model seismic wave scattering from earth models with irregular interfaces. This is motivated by numerous observations that surface and subsurface topography is responsible for the amplification and de-amplification of seismic waves and the generation of coda waves. We demonstrate that irregular Moho and free-surface scattering significantly affects teleseismic P-wave receiver function waveforms. Receiver functions are routinely used to obtain detailed crustal and upper mantle structure. For a layer-over-halfspace model, we perform a sensitivity study on the effects of ray parameter, interface roughness and wavelength, receiver position, and incident wave frequency content. The free-surface responses vary significantly with receiver position, interface wavelength and with incident wave frequency content. Using frequency-wavenumber analysis, the coda is shown to contain scattered Rg waves and S waves. Receiver functions are formed from the free-surface responses by deconvolution, and these are then inverted, as synthetic ‘data’, for plane-layered velocity structure, to show how scattered energy maps onto the final velocity models. Spurious structure models result, with the amplitude of the spurious features (for example low-velocity zones) decreasing with interface roughness. The resulting velocity models also vary appreciably with receiver position.