Depth of Origin of the Earthquake of August 15

Abstract

THE earthquake of Sunday morning, August 15, was one of those for which no method of determining the depth of origin had been devised until recently, and offers an opportunity for applying that which was published in the Quarterly Journal of the Geological Society for February last. The extreme limits of the area over which the shock was felt appear to have been Llandrindod Wells and Blackheath, which are about 150 miles apart; the maximum violence seems to have just about reached the lower limit of VI° of the Mercalli scale, or an acceleration of about 150 mm./sec.² Applying the coefficients in the paper referred to, the resulting depth of origin is not far from thirty miles. This estimate is based on the newspaper reports and subject to correction when more precise data are available. The maximum acceleration may have been more than the figure adopted, the limiting value of acceleration, of a shock which is just sensible in England, may be less than the 20 mm./sec.² adopted, and either of these would lessen the resulting depth of origin. On the other hand, the description of the shock at the limiting stations suggests that it must have been sensible even beyond them, and this would give a greater depth. Taking all these circumstances into consideration, it seems probable that the earthquake originated at a depth of some 25 to 30 miles, say 40 to 50 kilometres, below the surface of the ground.