A Study of the Crustal Structure beneath the Irish Sea

An explosion study was undertaken in 1965 to investigate the structure of the crust in the Irish Sea area. The experiment was designed to permit the use of the time term approach to interpret the data. Seismic waves from twenty-five depth charges were recorded at three temporary seismometer stations in Wales and one in Ireland, the Rookhope Borehole and the U.K.A.E.A. array at Eskdalemuir. Detailed geophysical surveys made across Cardigan Bay and St George's Channel have provided information about the shallow structures. P</it><inf>g</it></inf>, P</it>* and P</it><inf>n</it></inf> arrivals were identified on the records, and the velocities were calculated to be 6.1, 7.3 and 8.1 kms−1 respectively. P</it><inf>n</it></inf> was observed only at Eskdalemuir and Rookhope and the value determined represents in effect only an apparent velocity. P</it>*, however, was observed at the Irish and the three Welsh stations, and its velocity and the associated time terms are well determined. The calculated values of the P</it><inf>g</it></inf> velocity and time terms have relatively large uncertainties, due probably to the heterogeneous nature of this layer. The shallow structures are complex and as a result it is difficult to translate from time terms to interface depths. A simplified crustal model, making use of average values of the time terms, is made up of three crustal layers with interfaces at depths of 4 km and 24 km and the Moho at a depth of 30 km.