Mapping of crustal heterogeneity in the North Sea basin via the propagation of Lg-waves

Abstract

The propagation of &-waves within the continental area in and around the North Sea basin shows strong dependence on the path between source and receiver. Paths lying within the British Isles and Norway show very clear L , phases, but paths which cross the graben zone lying in the middle of the North Sea basin have very weak L,. Over 150 paths have now been studied across the region and the character of the &-wave has been described by comparison with the size of the S, phase. For shallow events this gives a stable measure of the efficiency of L , propagation. The regions which appear to block the transmission of L , are quite localized in the middle of the North Sea, in the region with graben structures, and extend on into The Netherlands. A weak zone of poor transmission appears to be associated with the Oslo graben.With the relatively dense areal coverage provided by paths crossing the basin with a wide range of azimuths, it is possible to attempt to invert for the pattern of crustal heterogeneity which gives rise to the observed character of the t, propagation. An iterative scheme has been devised to find the propagation properties in cells of a 1" x 1' grid, and a well-defined map of the heterogeneity is obtained. The strongest heterogeneities correlate very well with the major tectonic features of the Viking and Central Grabens running northsouth through the basin. Where the path coverage is greatest, the position of the heterogeneity can be tightly constrained and lies within a rather narrow zone around 100 km wide. Despite the number of paths employed it is not possible to obtain this level of resolution over the whole region. Seen in the

Sedimentary Basins and the Blockage of Lg Wave Propagation in the Continents

“…It has usually been interpreted as the superposition of higher-mode Rayleigh waves that primarily propagate in the crust (KNOPOFF et al, 1973) and, alternatively, as the superposition of supercritically re¯ected shear waves in the continental crust (CAMPILLO et al, 1985;KENNETT, 1986). Lg propagation has only been observed in the continents and does not appear to cross ocean basins or paths that contain 100 to 200 km of oceanic crust (EWING et al, 1957).…”

“…The region is known to have unusually thick (up to 70 km) crust due to the convergent type tectonics of the region. KENNETT et al (1985) also suggested that Lg propagation across the Norwegian Sea might be blocked by a region of crustal thinning beneath a graben structure. KENNETT (1986) has shown with ray-tracing modeling that Lg blockage can be caused by sudden crustal thinning, as might occur at a continent/ocean boundary (e.g., SHAPIRO et al, 1996).…”

Lateral variation of Lg wave propagation in southern Mexico

“…The characteristics of the propagation of Lg waves have been discussed for many regions [e.g., Ruzaikin et al, 1977;Campillo and Plantet, 1991;Campillo et al, 1993;Chazalon et al, 1993;McNamara et al, 1996;Shi et al, 1996;Benz et al, 1997;Rial and Ritzwoller, 1997;Rodgers et al, 1997;Mellors et al, 1999;Calvert et al, 2000] and have been studied using numerical simulations [e.g., Kennett, 1986;Maupin, 1989;Regan and Harkrider, 1989;Cao and Muirhead, 1993;Gibson and Campillo, 1994;Kennett, 1997, 1998;Vaccari and Gregersen, 1998]. It has been observed that Lg propagation becomes inefficient or is blocked if the crustal structure undergoes fast lateral changes [Ruzaikin et al, 1977;Baumgardt, 1990;Campillo et al, 1993;Chazalon et al, 1993] or if the travel path crosses oceanic crust [Press and Ewing, 1952;Knopoff et al, 1979;Kennett et al, 1985;Gregersen and Vaccari, 1993;Shapiro et al, 1996]. The lateral variation of coda Q has been determined on the basis of Lg coda [e.g., Singh and Herrmann, 1983;Xie and Mitchell, 1990].…”

Lg wave propagation in the area around Japan: observations and simulations

“…Lg wave propagation is also inefficient in areas with thinner crust, such as the Mediterranean, Black Sea, southern Caspian Sea (Press and Ewing 1952;Kadinsky-Cade et al 1981;Maupin 1989;Rodgers et al 1997), and the North Sea Central Graben (Gregersen 1984;Kennett et al 1985).…”