Abstract

W. E. Tremlett writes: In their recent paper Blenkinsop et al. state that the epicentre of the North Wales earthquake of 19 July 1984 must 'be attributed to a deep fault which has no surface expression' (1986, p. 332). This statement results essentially from wrong positioning of the epicentre in their fig. 3. The epicentre is stated as being at SH 237343 but is plotted at about SH 350390 since the authors overlooked the fact that the origin for grid square SH corresponds to 200300 on their diagram. In fact the epicentre was fixed intially at SH 371451 ±5 km (Turbitt et al. 1985, p. 567). Subsequently, when a network of seismographic stations had been set up, the epicentres of some 40 aftershocks fixed the activity more accurately at about 3 km ESE of the initial assessment (Turbitt et al. 1985, fig. 3a). This puts it into the vicinity of more than one fault revealed by my geological mapping of the 1960s. The apparent positions of these fault traces in relation to the aftershock epicentres given by Turbitt et al.are shown in fig. 1.

The Caledonoid fault in the eastern part of this area was given a different name (Tremlett 1964) from that in the western part (Tremlett 1962) before it was realized that they appear to be the same fracture displaced sinistrally by about 0.5 km by the Llanaelhaiarn Fault (Tremlett 1970). This NNW fault, as frequently happens, has been eroded into a drift-filled valley and is not exposed.

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