I surface waves from the Atlantic II The crust and upper mantle beneath Iceland

Abstract

1. Surface waves from the Atlantic It is shown that Rayleigh wave arrivals can "be used to locate small earthquakes which are not detectable using normal "body wave techniques. Using Rayleigh wave arrivals at four stations, six small earthquakes have been located on the Reykjanes Ridge, They appear to he foreshocks preceding two larger events, with mb magnitudes of 4.5 and 4.6, which were reported in the U.S.C.G.S. bulletins. It is suggested that the apparently large Rayleigh waves from the eight events are mainly due to their shallow foci. Assuming zero focal depths, the mb magnitudes of the foreshocks appear to range from 4.2 to 4.4. It is suggested that these events were not reported in the "bulletins "because of the low detection probability, using "body waves, for low-magnitude earthquakes occurring in the mid-Atlantic, If this interpretation is correct, then the appearance of surface wave trains from unlocated sources, on the records of seismic stations around the Atlantic, can "be explained in a similar manner, 2. The crust and upper mantle "beneath Iceland a description is given of the data obtained from two temporary array stations which were installed in Iceland during the summer of 1967. P-wave delay times are measured using data from stations in Iceland, Scotland, Sweden and Greenland. A delay time of 1.5 seconds, relative to Eskdalemuir in Scotland, appears to be constant over Iceland. Relative to Kiruna in Sweden, the Icelandic delay is approximately 2.3 seconds. The delay times are interpreted in terms of the crust and upper mantle beneath the recording stations and they can be explained if the 7.U km/sec layer beneath Iceland extends to a depth of 200 km.