The influence of crustal scattering on translational and rotational motions in regional and teleseismic coda waves

Abstract

S U M M A R YMonte Carlo solutions to the radiative transfer equations are used to model translational and rotational motion seismogram envelopes in random elastic media with deterministic background structure assuming multiple anisotropic scattering. Observation and modelling of the three additional components of rotational motions can provide independent information about wave propagation in the Earth's structure. Rotational motions around the vertical axis observed in the P-wave coda are of particular interest as they can only be excited by horizontally polarized shear waves and therefore indicate the conversion from P to SH energy by multiple scattering at 3-D heterogeneities. To investigate crustal scattering and attenuation parameters in south-east Germany beneath the Gräfenberg array multicomponent seismogram envelopes of rotational and translational motions are synthesized and compared to seismic data from regional swarm-earthquakes and of deep teleseismic events. In the regional case a nonlinear genetic inversion is used to estimate scattering and attenuation parameters at high frequencies (4-8 Hz). Our preferred model of crustal heterogeneity consists of a medium with random velocity and density fluctuations described by an exponential autocorrelation function with a correlation length of a few hundred metres and fluctuations in the range of 3 per cent. The quality factor for elastic S-waves attenuation Q S i is around 700. In a second, step simulations of teleseismic P-wave arrivals using this estimated set of scattering and attenuation parameters are compared to observed seismogram envelopes from deep events. Simulations of teleseismic events with the parameters found from the regional inversion show good agreement with the measured seismogram envelopes. This includes ringlaser observations of vertical rotations in the teleseismic P-wave coda that naturally result from the proposed model of wave scattering. The model also predicts, that the elastic energy recorded in the teleseismic P coda is not equipartitioned, unlike the coda of regional events, but contains an excess of shear energy. The results confirm that scattering generating the teleseismic P-wave coda mainly occurs in the crustal part of the lithosphere beneath the receiver. Our observations do not require scattering of high frequency waves in the mantle, but weak scattering in the lithospheric mantle cannot be ruled out.

Constraints on the heterogeneity spectrum of Earth's upper mantle

“…In this work, we analyze the vertical component only, our justification being that the bulk of the ray path has to be in P or the arrival time would be delayed beyond the P P arrival. Although simulations of the wavefield by Gaebler et al [] suggest that multiple‐scattered shear waves dominate the teleseismic coda at high frequencies (>1 Hz) and long lapse times, in this study we focus on scattered energy at lower frequencies and shorter lapse times. We argue that most of the long‐period scattered energy characterized is not diverted far away from its original direction of propagation, as evidenced by the large transport mean free paths, and thus it is unlikely that most of the long‐period scattered energy is leaking into the horizontal components.…”

Seismic scattering and absorption parameters in the W-Bohemia/Vogtland region from elastic and acoustic radiative transfer theory

“…To ensure reflections at the interface between crust and mantle, wave speeds in the mantle are set to 6.4 km s −1 for P-waves and to 3.6 km s −1 for S-waves . The influence of scattering and intrinsic attenuation in the mantle on the propagation of seismic waves in a local frame is weak (Gaebler et al 2015), which justifies our choice of fixed mantle parameters. For each of the individual earthquakes in the different frequency bands the pre-computed forward simulations can now be used to find the best fitting set of crustal parameters to describe the observed data.…”

“…In this study, we use the ray-based LQT-coordinate system for the recording of the particles. For a detailed description of the modelling algorithm we refer the reader to Sens-Schönfelder et al (2009), for information on the modelling of the multi-component envelopes and the choice of the recording system see Gaebler et al (2015).…”

“…In the acoustic theory only one mode of energy is modelled. As S-energy dominates the seismic signal from the first arrival of the direct S-wave until the late parts of the seismic coda (Ryzhik et al 1996;Hennino et al 2001;Gaebler et al 2015), only the propagation of S-energy is simulated. The transfer of P-energy and mode conversions are ignored.…”

“…Further studies model the propagation of elastic energy (Zeng 1993;Margerin et al 2000;Przybilla et al 2006;Przybilla & Korn 2008) and therefore include the simulation of P-energy, as well as conversions between the two states of energy. The propagation of elastic energy assuming multiple anisotropic scattering in a continuous random medium with a deterministic background structure is, for example, described by Sens-Schönfelder et al (2009) and Gaebler et al (2015). This approach allows to model the entire seismogram envelope in a realistic model setup, including interfaces and the free surface condition.…”