Propagation of 0.7-2.5 Hz Rayleigh waves in Scotland

Abstract

Summary. Rayleigh waves in the frequency range 0.7–2.5 Hz are recorded at stations on the Scottish LOWNET array, from local underwater explosions in the Kirkcaldy Bay region of the Firth of Forth. The fundamental mode is identified by comparing theoretical and observed particle motion plots derived from seismograms at the Edinburgh station, this being used as a guide for selection at the other stations. The selected signals are then analysed to obtain estimates of the group velocity dispersion characteristics along isolated event-station paths, and this information is interpreted in two ways: (a) The group velocity curves are inverted to obtain shear velocity profiles of each path, these penetrating to depths of between 1.2 and 3 km into the crust. The models show an increase of shear velocity with depth. Shear velocity in the upper 400 m of the crust, which includes the surface weathering, lies between 1.4 and 2.1 km s−1. The regional variation of shear velocity correlates with the surface geology, which consists of Carboniferous sediments, Old Red Sandstone and Dalradian rocks. There are wide differences in the velocities of the deepest layer, ranging from 1.9 to 3.5 km s−1. (b) The variation of group slowness with the geological expression is utilized in a linear scheme to obtain pure-provincial group velocities for several geological provinces. The dispersion curves corresponding to the Carboniferous sediments and the Firth of Forth regions decrease smoothly from 1.6 km s−1 at 0.7 Hz to 1.2 km s−1 at 1.8 Hz. The group velocities for the Old Red Sandstone formation are about 0.5 km s−1 faster. These results also demonstrate that the water layer and seabed sediments do not have a significant effect on the surface wave propagation.