Seismic-source and wave-propagation effects ofL_gwaves in Scandinavia

Abstract

S U M M A R YA set of 517 recordings of L, waves from 151 earthquakes in and around Norway has been used for determination of seismic moment M,, corner frequency f;, and anelastic attenuation Q(f). The data used have been recorded at source-receiver distances of 20 to 1200km, 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 w -* 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 L, waves using synthetic data modelled without anelastic attenuation, for a realistic crustal structure with a Moho depth of 40km. 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 -112 in the frequency domain, equal to the theoretical value for cylindrical spreading, and -314 in the time domain, where the theoretical value for an Airy phase is -516.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 lOHz, and a lower frequency limit determined by the instrument response. The computed Mo values are quite stable, and exhibit a linear dependency on M L . The corner frequencies are less well constrained, however, especially for smaller events. In using a model of the type fo"M;' we found a 6 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) = q f v yield q values around 440 and 7 values around 0.7. This is reasonably close to other anelastic attenuation models found in this area.

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“…These values are used for the stochastic model for this region. Kvamme et al (1995) find that near-field earthquake spectra support the use of a source spectrum that falls off not steeper than £ -squared and possibly less steeply (a power of 1.5).…”

“…Dahle et al (1991) use r -1 for r<100km and r -5/6 for greater distances. Kvamme et al (1995) find that a decay of the form r -1 for distances r<200km and r -1/2 for greater distances is appropriate using simulations and a crustal structure for Scandinavia. The two previous studies using the stochastic method for this region (Singh et al, 1990;Bungum et al, 1992) have both adopted a decay of r -1 for distances r<100km and r -1/2 for greater distances, as do Sereno et al (1988).…”

“…Hansen et al (1989) give Q(f)=90+110f 1.20 . Singh et al (1990) have used Q(f)=290f from fitting the observed spectra of three larger offshore earthquakes, while Kvamme et al (1995) give Q(f)=438f 0.71 for a Brune The rock in the target region is granite or metamorphic rock, which has V p values of between 4.5 and 5.5km/s (Grant & West, 1965, fig. 1-1, p. 8), which corresponds to V s between 2.6 and 3.2km/s (by dividing by 3 , corresponding to a Poisson's ratio of 0.25).…”

Empirical Relations of Seismic Moment and Earthquake Moment Magnitude to Earthquake Local Magnitude for the Vardar and West Macedonia Seismic Zones

“…13,16,20,21] In this study, seismic moment determinations were done by inverting the amplitude spectra of the vertical components of the observed g S and g L waves, on the Brune dislocation model. One of the main selection criteria for the data was the preferring of source and receiver locations within the same seismic zone.…”

Near-surface wave attenuation (kappa) of Far West Rand micro-events

“…Q has been observed to have strong regional variations in the crust and a frequency dependence of the form [2] (e.g. Kvamme and Havskov, 1989;Kvamme, Hansen, and Bungum, 1995;Malagnini, Herrmann, and Koch, 2000) when f > 1 Hz and Q( f ) is nearly constant for 0.1 Hz < f < 1.0 Hz (e.g. Stein and Wysession, 2003).…”