Crustal structure of Iceland

Abstract

A report is given of the results of a seismic field investigation in 1959 of the crustal structure of Iceland. Explosions were made at a depth of 30 meters in Graenavatn, a crater lake in southwest Iceland, and recordings were made with a 12-channel refraction apparatus at a number of stations along two profiles across Iceland, one profile across the center of the island and another in the western part. A three-layered crust was found-a top layer of lava and volcanic ash and two basaltic layers. The longitudinal wave velocities are 3.69, 6.71, and 7.38 km/sec, respectively, and the layer thicknesses 2.1, 15.7, and 10.0 km, respectively. The total crustal thickness down to the Mohorovicic discontinuity is around 27.8 km. Direct waves through the various layers, as well as reflected waves, are used in the study. Longitudinal guided waves, propagated by multiple reflections in the lava layer, were recorded out to distances of over 100 km. As a consequence of the large velocity contrast between the lava layer and the first basaltic layer, more than 83.5 per cent of the original seismic energy remains in the lava layer, leaving only a few per cent to penetrate deeper. Amplitude attenuation coefficients have been determined, which are about twice as large for the central profile as for the western profile. The main reason for the strong attenuation along the central profile is scattering of the waves in the inhomogeneous and heavfiy fractured crust. The seismic efficiency of the explosions, all carried out in the same way, varies in the mean by 20 to 25 per cent, as evidenced by the records of the Reykjavik seismograph station.