Regional spectral scaling relations of source parameters for earthquakes in the Baltic Shield

Abstract

Spectral analysis of regionally recorded Lg waves is performed to determine source parameters such as seismic moment, source radius and stress drop of earthquakes in the Baltic Shield, and to derive regional spectral scaling relations. The data consist of about 350 Lg phases on short-period, vertical-component analog seismograms from earthquakes with magnitude ( M L ) ranging from 2 to 5.2. Source radii estimated from the corner frequency show only a slight increase with increasing seismic moment in the range 3 × 10 18 to 5 × 10 20 dyn cm, while the increase is more rapid for larger events. Baltic Shield earthquakes show increasing stress drops, ranging from about 0.1 to 10 bars, for increasing seismic moment. The relatively low stress drops could, in part, be explained by bias due to band-limited analog data. The slope of the curve relating seismic moment to corner frequency is steeper than −3, which suggests a departure from a constant stress drop scaling relation proposed for large earthquakes. The derived relationship between the seismic moment ( M o ) and magnitude ( M L ) is: log M o = 16.93 + 1.01 M L (for 2 ⩽ M L ⩽ 5.2 ).