P-SHconversions in a flat-layered medium with anisotropy of arbitrary orientation

Abstract

S U M M A R YP-SH conversion is commonly observed in teleseismic P waves, and is often attributed to dipping interfaces beneath the receiver. Our modelling suggests an alternative explanation in terms of flat-layered anisotropy. We use reflectivity techniques to compute three-component synthetic seismograms in a 1 -D anisotropic layered medium. For each layer of the medium, we prescribe values of seismic velocities and hexagonally symmetric anisotropy about a common symmetry axis of arbitrary orientation. A compressional wave in an anisotropic velocity structure suffers conversion to both SVand SH-polarized shear waves, unless the axis of symmetry is everywhere vertical or the wave travels parallel to all symmetry axes. The P-SV conversion forms the basis of the widely used 'receiver function' technique. The P-SH conversion occurs at interfaces where one or both layers are anisotropic. A tilted axis of symmetry and a dipping interface in isotropic media produce similar amplitudes of both direct ( P ) and converted (Ps) phases, leaving the backazimuth variation of the P-Ps delay as the main discriminant. Seismic anisotropy with a tilted symmetry axis leads to complex synthetic seismograms in velocity models composed of just a few flat homogeneous layers. It is possible therefore to model observations of P coda with prominent transverse components with relatively simple 1 -D velocity structures. Successful retrieval of salient model characteristics appears possible using multiple realizations of a geneticalgorithm (GA) inversion of P coda from several backazimuths. Using GA inversion, we determine that six P coda recorded at station ARU in central Russia are consistent with models that possess strong (> 10 per cent) anisotropy in the top 5 km and between 30 and 43 km depth. The symmetry axes are tilted, and appear aligned with the seismic anisotropy orientation in the mantle under ARU suggested by SKS splitting.

Mantle and crust anisotropy in the eastern China region inferred from waveform splitting of SKS and PpSms

“…When evaluating the magnitude of anisotropy at each layer by using different converted waves, one must allow for the frequency dependent effects on the observed anisotropic parameters, as well as possible effects caused by multiple anisotropic layers and heterogeneity (e.g., Marson-Pidgeon and Savage, 1997). The converted phases could also be modified by dipping boundaries in flat-layered anisotropic medium (Levin and Park, 1997;Savage, 1999), which might affect the determination of splitting parameters. However, our observation of PpSms is very robust, and we do not expect that our measurement could be significantly affected by the above elements.…”

Measuring and crust-correcting finite-frequency travel time residuals – application to southwestern Scandinavia

“…It generates the three-component impulse response of the layered structure, assuming a plane wave incident from below with a prescribed incidence angle. We computed the response of the crustal layer for P and SH waves using the reflectivity software of Levin and Park (1997) and defined the frequency-dependent crustal travel times as the time of the maximum of the impulse response after filtering with the same zero-phase bandpass filter as applied to the data. In order to ensure a common reference time and include the effect of the topography, the crustal corrections are computed at each station by calculating the travel times from 50 km below sea level and up to the free surface, for waves with the prescribed incidence.…”

Crust and uppermost mantle structure beneath central Japan inferred from receiver function analysis

“…To correct for the latter effect, we estimated simple velocity structures beneath each station presented by Igarashi (2009). First, we calculated many synthetic receiver function traces using the reflectivity algorithm (Levin and Park, 1997) in advance. Multiple reflected phases are considered for this calculation.…”