_Position and Dimensions of the Two Foci._--There can be little doubt
that the impulse at the north-west focus was responsible for the
greater damage to buildings at Hereford, Dinedor, Fownhope, etc. The
centre of its epicentral area must therefore lie about three miles
south-east of Hereford. It is probable, also, that the corresponding
centre of the other focus is similarly placed with respect to the
south-east portion of the isoseismal 8--that is, about two or three
miles north-east of Ross. These two points are eight or nine miles
apart. Now, since, as we shall see, the mean surface-velocity of the
earth-waves was about 3000 feet per second, and the mean duration of
the quiet interval between the two series was 3-1/2 seconds, the
nearest ends of the two foci must have been separated by a distance of
not less than two miles. Moreover, since the series of vibrations from
the north-west or Hereford focus lasted a few seconds longer than that
from the south-east or Ross focus, the former must have been about two
miles longer than the latter, and we may therefore estimate their
lengths at about eight and six miles respectively. Including the
undisturbed intermediate portion, this would give a total length of
focus of about 16 miles, a result we have already inferred from the
dimensions of the isoseismal 8.
DIRECTION OF THE SHOCK.
Although no question was asked with regard to the direction of the
shock, no fewer than 469 observers made notes on this point. As a
general rule, their determinations are extremely rough, few referring
to more than the eight principal points of the compass. Moreover, in
any one place, the directions assigned to the shock are very varied.
For instance, in the city and suburbs of Birmingham, eight observers
give the direction along a north and south line, eight east and west,
eleven north-west and south-east, and five north-east and south-west,
while there are five other intermediate estimates. But, when these
directions are plotted on a map of the district, it is seen at once
that they are either nearly parallel or perpendicular to the roads in
which the observers were living; that is, the apparent direction of
the shock was at right angles to one of the principal walls of the
house. This, of course, is a result to be anticipated, for, whatever
be the direction of the earthquake-motion, a house tends to oscillate
in a plane perpendicular to one or other of its walls.
It is extraordinary to how great a distance the direction of the shock
is perceptible. Records come from Brighton (137 miles from the
epicentre), Maldon in Essex (144 miles), Harrogate (147 miles),
Douglas in the Isle of Man (167 miles), Dublin (176 miles), and
Baltinglass in Co. Wicklow (180 miles).
Public-domain text, read in full here on John Shaqi.
Reviews
Reviews
No reviews yet
Be the first to share your thoughts on this work.
Elsewhere in the archive
Join the Discussion
Join the discussion
Sign in to leave a comment or review.
Sign InorCreate an account