Seismic wave propagation in complex crust-upper mantle media using 2-D finite-difference synthetics

Abstract

SUMMARY A description is given of the numerical FD scheme used to solve the elasti wave equation, including a few remarks on the source functions used. Our FD method has been used for computing synthetic seismograms for 2‐D crust/upper mantle models of size 150 × 400 km2, with options for free‐surface topography. The strategy was to introduce successively more complex lithosphere models for generating the synthetics; the reference model was laterally homogeneous lithosphere. The interface scattering was visualized through displays of free‐surface synthetic waveforms and snapshots for models with a corrugated Moho only and free surface topography only. Near the free surface the latter seems to dominate, in the form of P‐to‐Rg and S‐to‐Rg conversions. Lithosphere randomizations were introduced through von Kármán functions of order n= 0.3, with rms velocity fluctuations of 3–4 per cent and correlation distances (horizontal and vertical) at 2.5 or 10 km. In case of a medium with only sub‐Moho heterogeneities, those with horizontal anisotropy (ax= 10 km; az= 2.5 km) produced relatively strong Pn and Sn phases. The respective codas were dominated as in most of our experiments by P‐to‐S and S‐to‐S scattering wavelets excluding Rg scattering at a free surface with topography. For a medium with crustal heterogeneities, the distortions of the P and S wave trains with distance were clearly demonstrated. For full‐scale heterogeneous lithosphere models, characteristic features of the synthetics were quantitatively similar to observational records of local events. Dominant attributes were a pronounced P coda consisting mainly of P‐ and Rg‐scattered wavelets, and a relatively strong S coda consisting mainly of P‐to‐S and S‐to‐S scattered wavelets. The P and S waveforms are severely distorted pointing at the futility of reliably picking many secondary arrivals in local event recordings. Most of the scattering wavelets are confined to the crustal waveguide and to surface waves, since coda excitations for sensors at a depth of 100 km were weak and, moreover, consisted mainly of S wavelets. This implies that a strong teleseismic P coda does not reflect scattering within the crust in the source region but, rather, a complex source. Observational results from analysis of NORESS and ARCESS local event recordings are also presented. Clearly the lithosphere is not isotropically inhomogeneous. The essence of our 2‐D FD synthetic seismogram experiments is that a simple lithosphere model, being moderately heterogeneous, gives rise to complex seismograms which are grossly similar to the observational recordings. In contrast, complex models derived from profiling surveys (but lacking the fine‐scale random variations) give simple,‘ray tracing’like synthetics, not necessarily similar to the observed records.

The North Sea Lg-blockage puzzle

“…a considerable fraction of Lg energy leaking into the mantle reappears in the Sn coda. These features are not clear in our synthetics, as they would require a strong positive velocity gradient beneath the Moho and/or a more laminated or heterogeneous sub-Moho structure for proper modelling (Hestholm et al 1994). …”

“…The idea here is that by subdividing the crust near the free surface and close to the Moho we can easily change the model types from the extreme of one layer over a half-space to others with thick unevenly distributed sediment strata, Moho ramps and associated crustal thinning. This approach allows great flexibility in model building; details are given in Table 1 Hestholm et al 1994) in the central part of the model (see Fig. 11).…”

Seismic Wave Propagation and Scattering in the Heterogeneous Earth : Second Edition

“…We can see how scalar waves are deformed by using finite difference (FD) simulations for 2-D random media. Hestholm et al (1994) numerically simulated wave propagation through a complex heterogeneous 2-D medium consisting of a random velocity structure characterized by a nonisotropic von Kármán type ACF superimposed on a layered velocity structure. 3.10a, where the wavenumber corresponding to 2 Hz is marked by a vertical gray bar.…”

First breaks—automatic phase pickings of P‐ and S‐onsets in seismic records

“…Also, ambient noise tends to mask the first cycle of the P-onset so picking errors are non-Gaussian. Such problems are more adverse for S-signals which arrive in the tail of the P-coda and besides have a more complex propagation path including phase conversions and scattering contributions [Hestholm et al, 1994; $ato and Fehler, 1998; Takanami and Kitagawa, 1993]. The aim of this study is to develop a robust phase picking scheme with focus on signal envelope processing since both P-and S-envelopes are far more stable than their respective original waveforms [Husebye et al, 1998].…”