Short-period surface wave dispersion: noise studies and Q along seismic profiles in southern Sweden

Abstract

Short period (SP) Rayleigh wave dispersion was determined along linear seismic arrays in southern Sweden, using data from the Fennolora and the Eugeno-S seismic refraction projects on the Baltic Shield. The study area consists of crystalline and metamorphic igneous rocks of Precambrian age and has essentially no sedimentary cover. The dispersion curves were classified into dispersion regions and the regional (background) shear velocity structure was determined down to 2–3 km or more. Noise studies were undertaken in order to separate trends caused by the heterogeneous medium from the experimental uncertainties. It was shown that standard errors of the data in the 1–3 Hz band were predominantly significantly smaller than the conventional estimate based on the standard deviation. Thus, the estimates of the shear velocity resolution could be significantly improved, Q-values were determined from the SP Rayleigh waves in each dispersion region. In a broad sense similar Q-values were found in the area, except the northwestern corner that had significantly lower values. Here the lowest shear velocities were also found. However, a clear trend from higher Q in the southeast to lower Q in the northwest could be seen in the 1.7–3.3 Hz averages. The Q-values were inverted to obtain preliminary Q β -structures. The Q β -value of the upper km of the crust was 110 in the dominating part of the area and 52 in the northwestern corner. The lowest Q-values could be modelled with the same Q β -structure below 1 km as in the other parts of the area.