The earthquake activity of Norway and nearby offshore areas is low to intermediate, with few events above magnitude 5. Recent significant improvements in instrumental coverage in parallel with a better utilization of older (including historical) data have shown that the seismicity in the south is predominantly confined to the coastal areas and to the Viking Graben, while from the northern North Sea to Svalbard the earthquakes in a broad sense follow the continental margin. Fifty‐one focal mechanism solutions from these areas, about half of them new, reveal stress directions that clearly indicate a connection to the plate tectonic “ridge push” force, at least for the areas at a minimum distance from the continental margin. Along the margin, stress directions also indicate a possible connection to postglacial uplift as well as to lithospheric loading effects. A dominance of normal faulting on the landward side and reverse faulting on the oceanic side agrees with this interpretation. On a regional level, the seismicity in these areas correlate quite well with geologic features such as grabens, fault zones, fault complexes, fracture zones, and the margin itself, indicating that these structures act in a general sense as weakness zones in the presence of a regionally more stable stress field. In the northern North Sea, however, an area with quite anomalous stress orientations, with strike‐slip faulting, is found in a region transitional between normal and reverse faulting. Most of the earthquake foci are confined to the presumably brittle parts of the crust, but many events are also located quite close to, and on both sides of, the Moho discontinuity.
SUMMARYStrong-motion recordings at 87 sites from 56 different intraplate earthquakes from North America, Europe, China and Australia have been used through a two-step regression analysis to develop new attenuation models for peak ground acceleration, and for pseudo-relative velocity for frequencies of 0.25, 0.5, 1.0, 2.0, 5.0 and 10.0 Hz, all for 5 per cent of critical damping. The estimates are obtained along with an analysis of residuals and scatter.A similar regression analysis has been performed also for Fourier spectra of acceleration, in which case the coefficient for the anelastic term has been interpreted in terms of a frequency dependent quality factor Q. The resulting Q-model shows a strong frequency sensitivity with values around 600-700 at 1 Hz, around 2000 at 10 Hz and around 5200 at 25 Hz.These PGA, PSV and Q results depend, however, on the underlying assumption for geometrical spreading, in particular for low frequencies.
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.
A new local magnitude ML scale has been developed for Norway, based on a regression analysis of synthesized Wood-Anderson records. The scale is applicable for distances up to more than 1000 km, and the data used comprise 741 short-period recordings at 21 seismic stations from 195 earthquakes in the magnitude range 1 to 5 occurring in and around Norway over the last 20 years. Magnitude corrections for distance have been evaluated in terms of a geometrical spreading term a and an anelastic attenuation term b, and the significant regional crustal differences in the area under investigation made it desirable to develop these for several subsets of the data base. The results for a are generally found to be around the commonly found value of 1.0 (using the Lg phase), while the values of b are found to be around 0.0008, consistent with the weak, intraplate attenuation expected for Norway. Compared to interplate California, this difference in attenuation represents more than a factor of ten in amplitude at a distance of 1000 km. New ML scales are commonly tied to Richter's original definition at the standard reference hypocentral distance of 100 km. The significantly weaker Lg wave attenuation in Norway, however, requires a smaller reference distance. We have chosen a value of 60 km, based on an overall assessment of regional coverage, focal depths, and quality of the data. The resulting ML formula for Norway reads M L = log A w a + a log ( R / 60 ) + b ( R - 60 ) + 2.68 + S , where Awa is synthesized Wood-Anderson amplitude (in mm), R is hypocentral distance (in km), and S is a station correction term that for all 21 stations is found to lie within the range ± 0.22. When using the entire data base, the spreading term a equals 1.02 (± 0.09), and the anelastic attenuation term b equals 0.00080 (± 0.00009). When only strictly continental ray paths are selected, the a value decreases to 0.91 (± 0.11) while the value of b increases to 0.00087 (± 0.00011), a difference which on the average accounts for less than 0.1 magnitude units. While all values used in the regressions have been derived for vertical amplitudes, a separate analysis has shown that these are not significantly different from the horizontal ones, and the new scale is therefore applicable to both. In order to facilitate the practical use of this new ML scale, a relation has also been established between observed seismogram amplitudes in nanometers (corrected for instrument response) and the synthesized Wood-Anderson amplitudes. This relation reads log Awa = 0.925 log Aobs − 2.32. The new ML magnitudes for the events analyzed are in good agreement with those calculated from a previously used relation developed by Båth for Sweden. The ML values have also regressively been related to a data set of Ms magnitudes, yielding the relation Ms = 0.83 ML + 1.09.
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