234 ME. B. D. OLDHAM ON THE EARTHQUAKE OF [vol. lxxix, 



considerable areas, on either side of which the shock was recorded, 

 is real, and directly connected with the origin of the shock: this 

 point will be referred to later on, but, meanwhile, it must be 

 noted that the distribution of the reports is so much in accord 

 with the probable distribution of potential recorders, that no great 

 importance can be attached to the absence of records from certain 

 regions within the boundary drawn through the extreme localities 

 at which the shock was recorded. 



It is, consequently, apparent that none of the methods 

 ordinarily employed allows of the fixing of a definite epicentre, 

 or the drawing of successive isoseismals of decreasing intensity. 

 To a great extent this is due to the peculiar nature of the 

 shock, quite different in character from that usually noticed in 

 moderate earthquakes, for which the customary scales of intensity 

 are framed ; it was much more akin to the movement which is 

 noticed on the borders of the seismic area of a great earthquake, 

 when the movement, as it is propagated, dies out in comparatively 

 slow undulations, often more noticeable through the effect produced 

 by tilting than by actual sensation. At all the places where this 

 earthquake was recorded, an undulatory character was noticed, 

 suspended objects were set swinging, doors and windows were 

 shaken or moved. At Sestola it was especially recorded that there 

 was a sensible inclination of walls, giving to persons standing at 

 a window the impression of danger of falling out, and at some 

 of the localities the earthquake was onky recognized by a vibration 

 of doors or windows. These are all features characteristic of the 

 marginal portion of the seismic area of a great earthquake, and to 

 these the ordinary scales of intensity are not applicable, for some 

 of the results are such as would only be produced by a considerable 

 earthquake of the more common type, where the rattling and 

 swinging are set up by inertia, instead of by inclination of the 

 surface, clue to the passage of the long waves propagated outwards 

 from a great disturbance. It is not unnatural to attribute the 

 similarity of the movement, noticed in this earthquake, to a 

 similar cause, and, if this be so, the only direction in which 

 distance can be attained is downwards into the interior of the earth 

 — in other words, we are not dealing with a small disturbance of 

 shallow depth, but with the marginal area of a greater disturbance, 

 diminished in intensity by propagation before it reached the outer 

 surface. 



In attempting to estimate the depth at which this origin lay, 

 recourse was first made to the instrumental records, but no help 

 was found in that quarter. The records from within or just 

 outside the areas over which the shock was sensible, agree in fixing 

 the time at about 20h. 49 , 7rn. M.E.T., but there is no regularity or 

 evident connexion between distance from the centre of the shocks 

 and the time of record, and the variations extend from 49m. 20s. 

 to 50m. 20s. omitting those which merely give the time to the 

 nearest minute, Outside the seismic area the shock was recorded 



