EXPLOSION-GENERATED SHORT-PERIOD SURFACE-WAVE DISPERSION AND NOISE STUDIES ALONG LINEAR SEISMIC ARRAYS IN SOUTHERN SWEDEN

The dispersion of short-period fundamental-mode Rayleigh waves (Rg) wa s measured along linear arrays on the Baltic Shield. The data used is a selected set of 176 records along three refraction profiles in south ern Sweden. The phase velocity, determined in the 0.2-3.5 Hz band, cou ld be grouped into a number of dispersion regions. Each regional dispe rsion showed a high internal consistency and the slowness as function of frequency was almost linear in the 1-3 Hz band. Linear regression w as used to separate the effects of the medium from the experimental un certainties. Systematic studies of the noise-to-signal ratio showed th at the standard deviation of the dispersion, within each region and th e above frequency band, essentially reflected the lateral heterogeneit ies. From the dispersion shear velocity models were inverted down to a bout 2-3 km, in one case down to 6 km. The S-wave velocity was weakly constrained to a non-decreasing function of depth with decreasing grad ient. The P-wave velocity was constrained using recent results on Pois son's constant ranging from 0.28 to 0.25 in the 0-2 km interval. The d ensity was constrained using geological knowledge of the area. Using t hese constraints, the S-wave velocity increased rapidly with depth in the first few hundred metres of the crust. The models could be natural ly grouped into three sets with similar character, in close agreement with the large-scale surface geology. The highest velocities were foun d in the east, in the Smaland-Varmland Granitic Belt and the smallest in the northwest, in the Sveconorwegian crust.