Most probabilistic seismic-hazard analysis procedures require that at least three seismic source parameters be known, namely the mean seismic activity rate λ, the Gutenberg-Richter b-value, and the area-characteristic (seismogenic source) maximum possible earthquake magnitude m max . In almost all currently used seismic-hazard assessment procedures that utilize these three parameters, it is explicitly assumed that all three remain constant over time and space. However, closer examination of most earthquake catalogs has indicated that significant spatial and temporal variations existed in the seismic activity rate λ, as well as in the Gutenberg-Richter b-value. In this study, the maximum likelihood estimation of these earthquake hazard parameters considers the incompleteness of the catalogs, the uncertainty in the earthquake magnitude determination, as well as the uncertainty associated with the applied earthquake-occurrence models. The uncertainty in the earthquake-occurrence models is introduced by assuming that both the mean seismic activity rate λ and the Gutenberg-Richter b-value are random variables, each described by the gamma distribution. This approach results in the extension of the classic frequency-magnitude Gutenberg-Richter relation and the

S U M M A R YA high-quality multichannel seismic reflection line was acquired in 1987 along a 175 km long profile across the continental shelf off Lofoten, northern Norway. A seismic wide-angle experiment was performed in 1988 along the same profile, using seven three-component Ocean Bottom Seismographs (OBS) with 20-25 km spacing and shotpoint intervals of 240 m.The study of the data has shown that the combination of the multichannel reflection and the wide-angle (OBS) technique provides information about the crustal structure beneath the Lofoten shelf that could not have been achieved using only one of the techniques. The multichannel reflection data provide a detailed image of the shallow (Cretaceous) structures, which represents an important basis for inversion of the OBS data. The lower crust and the Moho are also well mapped in some parts of the area with the multichannel reflection technique.The OBS data reveal that significant amounts of pre-Cretaceous sediments exist along almost the entire profile, with a maximum thickness of about 5 km in the Vestfjorden Basin. From the OBS data the thickness of the lower crust is inferred to decrease from about 11.5 km under the R@st High to about 2 km below the Lofoten Ridge. The OBS data indicate further that the Moho position under the Vestfjorden Basin is considerably deeper than can be inferred from the reflection data.About 10 km below Moho a strong dipping event is observed in the OBS data. This upper mantle reflection might be related to a possible seaward dipping master fault, and/or presence of layers of partially hydrated peridotite.

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S U M M A R YThis paper presents a crustal structure model derived from an extensive oceanbottom seismographic refraction experiment conducted on a 150 km profile line along the Sognefjord, south-west Norway. The main part of the profile was located in the Western Gneiss Region (WGR) which is characterized by deformation and metamorphic overprinting during the Caledonian Orogeny. The western and eastern ends of the profile crossed the Solund Devonian Basin and the allochthonous unit of the Jotun Nappe, respectively. Within the WGR, the fjord bottom is covered with 200-250m thick glacial sediments with a low P-wave velocity (1600m) and a high Poisson's ratio (0.48-0.49). The P-wave velocity and velocity gradient of the uppermost crystalline basement is 6.05 km s-' and 0.03 s-', respectively. The Poisson's ratio within the upper 12 km crust has an almost constant value of 0.26 in spite of the westward increase in the Caledonian metamorphism. This may indicate that the uniform bulk composition of the WGR is the predominant factor on the seismic wave velocities rather than the 'fossil' Caledonization. The P-wave velocity and the Poisson's ratio in the lower crust (deeper than 19-20 km) are 6.6-7.0 km s-' and 0.27, respectively, which are comparable to those for an amphibolitic-granulitic rock type. The Moho gently deepens eastward from 31 to 36km. The velocity contrast at the Moho is large (1.0-1.2kmsp1), as seen in the other areas of the WGR. At the eastern end of the profile (the Jotun Nappe), the P-wave velocity at the uppermost basement shows a higher value (6.20 km s-') than that within the WGR (6.05 km s-'), representing relatively mafic rock components in the Jotun Nappe. Such a lateral velocity change becomes obscure at depths deeper than 6-7km, which indicates that the detachment between the WGR and the Jotun Nappe is situated at a very shallow depth. This strengthens the hypothesis that the WGR is a westward extension of the Precambrian Baltic Shield buried beneath the Caledonian Nappes. A mid-crustal interface with a velocity jump of 0.2-0.3 km s-'