The North Sea Lg-blockage puzzle

Abstract

S U M M A R YThe North Sea Lg blockage for wave paths across crustal graben structures is a wellestablished observational fact. Analysis of such observations implies that Lg blockage takes place in graben areas associated with sedimentary basin formation and crustal thinning. These intriguing observations have triggered many theoretical studies aimed at highlighting specific Lg loss mechanisms, albeit so far with only moderate success. Our approach to this problem is to simulate seismic wavefield propagation through the crustal waveguide using 2-D finite-difference techniques. The graben structures are known in detail from oil exploration works in the North Sea, which has enabled us to use realistic crustal models in our Lg synthetics. In the most extreme model tested, the crystalline crust thickness beneath the graben amounted to only 5 km, while the overlying sedimentary pile is nearly 10 km thick. At the base of the crust in the graben area the Moho is elevated nearly 10 km. This model has similarities to the oceanic crustal waveguide, where total Lg blockage is claimed for path lengths exceeding 100 km. The synthetic wavefields are displayed in terms of snapshots, semblance velocity analysis and time-space rms amplitudes. The dominant structural Lg loss mechanisms are the delay of the Lg waves in the thick sediments, Lg-to-Rg conversions (scattering) by lateral heterogeneities in the sediments, and S-wave leakage out of the crustal waveguide and into the upper mantle. A fraction of these upper-mantle S waves return to the crust and appear as Sn coda. Observationally, strong Sn phases of long duration are often associated with weak Lg phases and vice versa. Our synthetics produced Lg amplitude decay amounting at most to 6-10 dB, while observational data imply blockage amounting to 15-20 dB. The latter is equivalent to a Pn-Lg magnitude difference of nearly one magnitude unit. The main outcome of this study is therefore that Lg-wave propagation is very robust and that a dominant blockage effect associated with intrinsic attenuation, that is Q values of the order of 100 at 2 Hz for a path length of minimum 100 km, is necessary to conform to observations.

A finite-difference approach for simulating ground responses in sedimentary basins: quantitative modelling of the Nile Valley, Egypt

“…7). The latter phenomenon is consistently predicted in theory and such results are also reported in other studies (Mendi et al 1997; Ruud & Hestholm 2001).…”

“…Rift and graben structures are often associated with an elevated Moho and as such may influence wave propagation efficiency, in particular of Lg waves in the North Sea (Mendi et al 1997). We have therefore inserted a Moho bump in one of our models, although there is no observational evidence for its existence beneath the Nile Valley.…”

“…Also we have inserted Q p = 70, 50 and 50, respectively, for the three topmost sedimentary layers within the basin to investigate the influence of near-surface attenuation (≈500 m) within the basin on the synthetic wavefield (Tables 1 and 2b). Rift and graben structures are often associated with an elevated Moho and as such may influence wave propagation efficiency, in particular of Lg waves in the North Sea (Mendi et al 1997). We have therefore inserted a Moho bump in one of our models, although there is no observational evidence for its existence beneath the Nile Valley.…”

The Arctic plate boundary

“…M S yields the largest proportion of unconverted magnitudes in the catalog (53%) while being an appropriate magnitude scale for the shallow and moderately large Arctic earthquakes. However, surface waves traveling across the continentocean transition (COT) may be considerably affected, both by Lg blockage [Kennett et al, 1985;Mendi et al, 1997;Baumgardt, 2001] and surface wave reflections and refrac- 1959-1999NEIC 9 1988-1999Harvard 91 1977-1999Jemsek et al [1986] 5 1964-1976Chung and Gao [1997 1 1987 Franke et al [2000] 7 1984Avetisov [1993 1 1987 Stein et al [1979] 1 1976 Bungum [1977] 1 1976 Savostin and Karasik [1981] 12 1964Cook [1988] 4 1960-1973Sykes and Sbar [1974 1 1971 Horsfield and Maton [1970] 2 1967 Lazareva and Misharina [1965] 2 1959 a Agencies are ranked by number. Focal mechanism solutions include published studies and routinely computed solutions.…”

Average QLg, QSn, and observation of Lg blockage in the Continental Margin of Nova Scotia

“…This also has been reported by Mendi et al . [] for the North Sea. In addition to the seismograms in Figure , this can be seen from comparing the paths with strong Lg and paths of strong Sn in Figure .…”