Seismic-source and wave-propagation effects ofLgwaves in Scandinavia

Abstract

A set of 517 recordings of Lg waves from 151 earthquakes in and around Norway has been used for determination of seismic moment Mo, corner frequency fo and anelastic attenuation Q(f). The data used have been recorded at source-receiver distances of 20 to 1200 km, with ML magnitudes between 0.8 and 5.0, and the parameters were estimated by inverting Fourier spectra from all of the recordings simultaneously. The observed spectra were represented by a source term, a spreading term, and an attenuation term, and the inversion was made assuming the geometrical spreading to be known. Because of the non-linear behaviour of the spectral shape, the inversion was done iteratively by minimizing the differences between observed and computed spectra. A standard ω−2 source model was used in the inversion, supported by near-field observations of small-magnitude earthquakes at the regional NORESS array. A model for geometrical spreading was then established by investigating the decay of Lg waves using synthetic data modelled without anelastic attenuation, for a realistic crustal structure with a Moho depth of 40 km. The results support the standard model of spherical spreading at short distances and cylindrical spreading at longer distances, but with a transition distance closer to 200 km than to the more commonly used value of 100 km. The decay rate beyond this distance was found to be close to ,−1/2 in the frequency domain, equal to the theoretical value for cylindrical spreading, and ,−3/4 in the time domain, where the theoretical value for an Airy phase is ,−5/6. The data used in the inversion were amplitude-displacement spectra corrected for instrument response and site effects, reduced (by smoothing) to 64 spectral values and weighted to represent equidistantly spaced points in the log-frequency domain. Only spectral values with signal-to-noise ratios of at least four were accepted, with an upper limit set to 10Hz, and a lower frequency limit determined by the instrument response. The computed MO values are quite stable, and exhibit a linear dependency on ML. The corner frequencies are less well constrained, however, especially for smaller events. In using a model of the type fO∝MO−δ we found a δ around 3.4, indicating slightly increasing stress drop, with values of less than 10 MPa in all cases (using a Brune model). The resulting Q models of the type Q(f) = qfη yield q values around 440 and η values around 0.7. This is reasonably close to other anelastic attenuation models found in this area.