Fig. 28.—Epicentral isoseismal lines of Charleston earthquake according to Mr. Sloan. (Dutton.)[ToList]
Fig. 29.—Epicentral isoseismal lines of Charleston earthquake according to Major Dutton. (Dutton.)[ToList]
More important, however, than this divergence of opinion is the agreement in one respect between the two sets of curves. Both show a marked expansion around the points known as the Woodstock and Rantowles epicentres, especially about the former, and a contraction in the intermediate region. The evidence of these isoseismals therefore confirms that of the damaged railway lines, and establishes Major Dutton's inference that there were two distinct foci, the epicentres of which were about thirteen miles apart.
ORIGIN OF THE DOUBLE SHOCK.
In the last chapter, it was shown that the double shock of the Andalusian earthquake could be due only to two distinct impulses taking place either within the same focus or, more probably, in two detached foci. Similar reasoning applies to the Charleston earthquake. The double maximum or double shock was observed in no less than fourteen States. Moreover, the interval between the two maxima at Charleston appears from Fig. 26 to have been about 34 seconds in length. Thus, the duplication of the shock cannot have been a merely local phenomenon, nor can it have resulted from the separation into two parts of the earth-waves proceeding from a single disturbance. Each maximum must therefore be connected with a distinct impulse.
Combining this inference with Major Dutton's discovery of the double focus, no doubt can remain as to the origin of the repeated shock. It is clear, also, that the impulse at the Woodstock focus was the stronger of the two; for the isoseismals spread out more widely round the corresponding epicentre, and there was no rapid decline of intensity from that point, such as might be associated with a weaker disturbance within a shallow focus.