No abstract

S U M M A R YWe jointly invert teleseismic radial-component receiver functions and regional Rayleigh and Love surface-wave group velocities for 1-D shear-wave velocity structure beneath station TBZ located on the northern side of the eastern Pontides. An influence factor is employed to control the relative influence of receiver function and surface-wave dispersion on the resultant velocitydepth profile. Radial-and transverse-component receiver functions at station TBZ exhibit an azimuthal amplitude and polarity pattern consistent with 2-D receiver structure that has a general dip direction towards approximately south. The radial-component receiver functions are least affected by the dipping structures along the strike direction and thereby we prefer teleseismic events sampling along-strike structures to alleviate the deflecting effect of dipping interfaces on the 1-D solution. The 1-D inversion effectively reveals the two-layer nature of the crust which is perturbed by high-and low-velocity layers, and serves as a provisional model for the 2-D forward modelling. Minor-to-moderate changes to the 1-D model, such as changing depth to and velocity contrast across an interface, are needed to achieve the results with the 2-D modelling. Dipping interfaces and seismic anisotropy are included in the 2-D modelling to fit both radial-and transverse-component receiver functions.The upper crust is characterized by a shear velocity of ∼3.5 km s −1 and cut through by a 4 km thick high-velocity (i.e. ∼3.8 km s −1 ) layer. Overlying the upper crust, the sedimentary cover (i.e. the top 5 km) has velocities within the range ∼2.0-3.5 km s −1 . A mid-crustal velocity discontinuity between the upper granitic crust and the lower basaltic crust is identified at ∼16-km depth. This boundary is analogous to the mid-crustal discontinuity found under the Black Sea basin across which the shear velocity jumps from 3.5 to 4.1 km s −1 . A relatively thick (i.e. ∼12 km) low-velocity layer in the lower crust with a velocity reversal from 4.1 to 3.7 km s −1 is needed to better explain reverberations off this depth range. We infer a 2-D Moho discontinuity placed at ∼35-km depth beneath the station. The proposed dip angle for the Moho is rather steep (i.e. ∼25 • ), although coincident with regional gravity studies. The associated Sn velocity (i.e. ∼4.4 km s −1 ) is rather low, indicating disturbed upper-mantle structure beneath the region. Initial amplitudes of transverse-component receiver functions are rather energetic, for which we propose substantial P and S velocity anisotropy (∼12 per cent) for the topmost depths (<5 km).

No abstract

S U M M A R YWe used the Green's function method to describe the multiple scattering of surface waves generated by interactions with the complex 3-D earth structures. The basic process involves an integral equation of convolutional type. An efficient multilevel fast multipole method is utilized to accelerate the matrix-vector products that correspond to the integrals on the horizontal plane. This new algorithm is shown to be quite successful when compared to its more traditional complement (e.g. direct integration method, DIM) with particularly the large number of data points. The fast execution is achieved through well-organized truncated multipole expansions, functions grouping and translation operators. In general, the new algorithm has a fruitful logarithmic time complexity as opposed to an uncomfortable exponential time complexity attainable with the traditional algorithms. This algorithm requires the user provide some important parameters to operate. One of them is the truncation number of the infinite series associated with the multipole expansions for which two linear relationships are derived for an automatic determination. The other central parameter is the clustering number that is used to group data points in the data structure. We showed that the clustering number around 5 is mostly an optimum value providing a minimum run time. However, greater clustering numbers (i.e. ∼10) become necessary for an optimum operation when the number of data points gets real large.In order to test the convergence of the current algorithm we compared our numerical results with analytical solutions provided for cylindrical obstacles by other researchers. A halfspace model in which the total wavefield is represented by a finite number of propagating modes describes the reference structure. Not all waves are trapped in this representation since leaking modes and body waves downward radiating into the half-space can exist. The comparisons with the exact solutions revealed that the quality of vertical component amplitudes is reasonably well while some amplitude discrepancies on the horizontal components particularly near and inside the heterogeneities exist. Extra fast half-space velocities complementing the deficiencies on the modal structure greatly help reduce the discrepancies on the horizontal components.

No abstract

The teleseismic P receiver functions are customarily inverted to attain the seismic velocities beneath a seismic station. Surface wave dispersion data are often added to reduce the effect of the non-uniqueness. The combination of P receiver function and surface wave works well in resolving the structures in the crust and uppermost mantle, but is less effective in characterizing greater (lithosphere and asthenosphere) depths due to the interference from crustal multiples. A solution to this problem is jointly to model teleseismic S receiver functions with surface wave and P receiver functions. This study adopts a fast, one-dimensional (1-D) inversion scheme. To avoid the effect of multidimensional structures away from the seismic station, we eliminate multiples that reverberate between the surface and interfaces below a restriction depth (RD), as well as S-to-P conversions below an inversion depth (ID). P-to-S conversions off the interfaces above the half-space and S-to-P conversions above the ID and multiples above the RD are properly modelled. This approach favours ray paths travelling close to stations and is, therefore, more suitable for 1-D inversions. We perform numerical experiments with and without noise and highlight the advantages of a joint receiver function and surface wave analysis.

No abstract

We numerically simulate the field measurements of Rayleigh surface waves and electrical resistivity in which the target depth is set to be less than 50-m. The Rayleigh surface waves are simulated in terms of fundamental mode group and phase velocities. The seismic field data is assumed to be collected through a conventional shot-gather. The group velocities are found from the application of the multiple filter technique in a single-station fashion while for the phase velocities the slant stacking, or linear radon transform are applied in fashion of multichannel analysis of surface waves (MASW). The average seismic structure from the source to the receiver (or geophone) is represented by the group velocity curve while the average seismic structure underneath the geophone array is represented by the phase velocity curve. The single-station group velocity curves are transformed into local group velocity curves by setting a linear system through grid points. The shear-wave velocity cross section underneath the examined area is constructed by inverting these local group velocity curves. The electrical resistivity structure of the underground is similarly studied. The field compilation of the resistivity data is assumed to be completed by the application of the multiple electrode Pole-Pole array. The actual resistivity assemble underneath the analyzed area is inverted by considering the apparent (measured) resistivity values. Unique forms such as ore body, cavity, sinkhole, melt, salt, and fluid within the Earth may be examined by joint interpretation of electrical resistivities and seismic velocities. These formations may be better outlined by following their distinct signs such as high/low resistivities and high/low seismic velocities. Doi: 10.28991/HEF-2021-02-03-01 Full Text: PDF