A Study of Primary and Secondary Microseisms recorded in Anglesey

Abstract

Summary The paper describes a theoretical investigation of the generation of primary microseisms which have the same period as the generating sea waves and secondary microseisms which have half the period, following the work of Longuet-Higgins and Hasselmann. In particular, the mechanism in both cases by which a high phase velocity pressure wave can be formed on the sea bottom to resonate with the ground wave, as suggested by Hasselmann, is investigated. By introducing a damping term in the elastic equations for the ground movement, quantitative estimates can be made of the ratio of the wave to microseisms intensity. In the case of secondary microseisms, the theory is confined to generation in shallow water and a value has to be assumed for the reflection coefficient of waves off the coast. In the case of primary microseisms, the ratio increases with the fifth power of the period, the steepness of the coast and inversely with the width of the breaker zone. These theoretical relations are checked with simultaneous observations of waves on the coast of Anglesey and on weather ships in the North Atlantic Ocean and microseisms recorded at Menai Bridge. Three types of microseisms could be identified: (1) primary microseisms generated in the Atlantic, probably off the coasts of Ireland and Iceland; (2) secondary microseisms generated in the Atlantic; (3) primary microseisms generated in the Irish Sea. In the case of (1) and (2), there was agreement with theory to about one order of magnitude. In the case of the Irish Sea off the coast of Anglesey, the coast is very steep down to a depth of 5 to 10 fathoms so that a steep coast is presented to short waves of periods less than 9 s and these tend to be more effective in generating primary microseisms than longer waves. This is borne out by the observations but the numerical factor is much greater than the one derived theoretically and as yet no satisfactory explanation can be found for this.