Description: Backazimuth-dependent Ps conversion, observed in seismic receiver functions (RFs) is generated by $\acute{SV}$ and $\acute{SH}$ waves that help match the shear-polarized displacement of hybrid \it\'{P} at the interfaces of an anisotropic layer. The effect of elliptical anisotropy with a tilted or horizontal symmetry axis $\hat{\bf w}$ on body-wave propagation in horizontal layers can be expressed in terms of the first-order hybridization of upgoing and downgoing P , SV and SH plane waves with a common horizontal slowness p . The first-order perturbations to the upgoing P wave involve hybridization of its polarization eigenvector with the addition of the shear wave polarizations $\acute{SV},\grave{SV},\acute{SH},\grave{SH}$ . Although the hybridization of plane waves in an anisotropic layer is only one factor in the complicated reflection and transmission problem, these perturbations suggest that the influence of P anisotropy on RFs is much larger than the influence of S anisotropy. Perturbation terms for hybrid \it\'{P} can be grouped into functional dependencies on the tilt angle of the symmetry axis $\hat{\bf w}$ from the vertical. Terms proportional to sin 2 include four-lobed variation (cos 2, sin 2) with backazimuth , encompassing the effect of a horizontal axis of symmetry. Terms proportional to sin 2 have two-lobed variation (cos , sin ) with backazimuth , encompassing the effects of a tilted symmetry axis. In the perturbation formula (33) for the hybrid \it\'{P} polarization, the four-lobed terms have pre-factor α p , and the two-lobed terms have pre-factor α P , leading potentially to larger amplitude in the two-lobed pattern in Ps for the same amount of P anisotropy. For a dipping interface between two isotropic media, the out-of-plane deflections of the P – SV converted wave lead to a two-lobed pattern of P – SV and P – SH amplitudes that resembles the effects of anisotropy with a tilted axis of symmetry. The birefringence of the Moho-converted Ps phase influences the backazimuth harmonics of Ps amplitude, but occurs only in the case where S anisotropy is nonzero. A frequency-by-frequency regression of migrated multiple-taper-correlation RFs H R ( f ), H T ( f ) for a collection of seismic records can extract well the interleaving of their harmonic backazimuth dependence. Application of harmonic stacking to migrated RFs from 471 events recorded at station RAYN (Ar Rayn, Saudi Arabia) of the Global Seismographic Network confirms the general features reported by Levin & Park: a strong two-lobed backazimuth variation in Ps from a Moho near 40-km depth and from an interface 25–30-km deeper, both consistent with a north-striking tilted axis of symmetry. The identical polarity of these two signals indicates that they are not Ps conversions from the top and bottom of a single subcrustal anisotropic layer.

Description: We investigated the infrasound signals from seismic ground motions induced by North Korea's underground nuclear explosions, including the recent third explosion on 2013 February 12. For the third explosion, the epicentral infrasound signals were detected not only by three infrasound network stations (KSGAR, ULDAR and YAGAR) in South Korea but also by two nearby International Monitoring System infrasound stations, IS45 and IS30. The detectability of the signals was limited at stations located on the relatively east side of the epicentre, with large azimuth deviations due to very favourable atmospheric conditions for eastward propagation at stratospheric height in 2013. The stratospheric wind direction was the reverse of that when the second explosion was conducted in 2009 May. The source location of the epicentral infrasound with wave parameters determined at the multiple stations has an offset by about 16.6 km from the reference seismic location. It was possible to determine the infrasonic location with moderate accuracy by the correction of the azimuth deviation due to the eastward winds in the stratosphere. In addition to the epicentral infrasonic signals, diffracted infrasound signals were observed from the second underground nuclear explosion in 2009. The exceptional detectability of the diffracted infrasound was a consequence of the temporal formation of a thin atmospheric inversion layer over the ocean surface when the event occurred.

Description: The multiple-taper correlation (MTC) algorithm for the estimation of teleseismic receiver functions (RFs) has desirable statistical properties. This paper presents several adaptations to the MTC algorithm that exploit its frequency-domain uncertainty estimates to generate stable RFs that include moveout corrections for deeper interfaces. Narrow-band frequency averaging implicit in spectral cross-correlation restricts the MTC-based RF estimates to resolve Ps converted phases only at short delay times, appropriate to the upper 100 km of Earth's lithosphere. The Ps conversions from deeper interfaces can be reconstructed by the MTC algorithm in two ways. Event cross-correlation computes a cross-correlation of single-taper spectrum estimates for a cluster of events rather than for a set of eigenspectrum estimates of a single P coda. To extend the reach of the algorithm, pre-stack moveout corrections in the frequency domain preserves the formal uncertainties of the RF estimates, which are used to weight RF stacks. Moving-window migration retains the multiple-taper approach, but cross-correlates the P -polarized motion with time-delayed SH and SV motion to focus on a Ps phase of interest. The frequency-domain uncertainties of bin-averaged RFs do not translate directly into the time domain. A jackknife over data records in each bin stack offers uncertainty estimates in the time domain while preserving uncertainty weighting in the frequency-domain RF stack.