244 ME. E. D. OLDHAM ON THE [vol. lxxix, 



reached a destructive degree of violence, there are some general 

 considerations pointing to the same conclusion. It is known that 

 in some cases earthquakes, of great violence in the central area, 

 were only sensible for a comparatively short distance from it. 

 The classic instance of this last-named type is the Ischian earth- 

 quake of 1883, which levelled Casamicciola with the ground, and 

 caused 1800 deaths in that town alone, but was only felt by a few 

 persons at Naples, not more than 20 miles away. This restriction 

 of the seismic area is commonly attributed, and seemingly with 

 justice, to a small depth of origin, less than half-a-mile in the 

 Ischian earthquake, while the more extended shocks originate at a 

 greater depth. 



If we compare the extent of the Pamir earthquake of 1911 with 

 the Ischian of 1883, both being of about the same degree of 

 maximum violence, we are faced with very different conditions ; 

 although in both cases the maximum degree of violence was not 

 very different, the area over which the one reached a destructive 

 degree of violence was as great as the whole area over which the 

 other could be felt at all. The great development of landslips in 

 the Pamirs was due to the accidental coincidence of the epicentral 

 area with a region where the carving of deep, narrow, and steep- 

 sided river-valleys, through lofty mountains, had given rise to 

 unstable conditions of the hillsides ; but, apart from this, the 

 earthquake differed in no material respect from the general run of 

 great earthquakes, which give good records at long distances from 

 the origin. 



Of disturbances known to have originated on the surface, we 

 have had, in the last few years, explosions of great magnitude and 

 violence, which gave rise to surface-waves capable of record by 

 seismographs at a distance ; but, in all cases, these have been 

 marked by the very restricted area over which the disturbance 

 was sufficiently great to cause material damage : within a distance 

 measurable in yards, damage, directly due to vibration of the 

 ground, had ceased, and within a very few miles at most no 

 vibration could be felt, even by those who were specially favourably 

 situated. From these analogies we may conclude that, even if the 

 fall of the Usoi landslip could have produced a shock sufficiently 

 great to cause damage, this would have been limited to the imme- 

 diate neighbourhood, and the earthquake would have ceased to 

 become sensible before the limits of the region in which damage 

 was actually done were reached. 



The facts known of this earthquake show that it cannot have 

 been of surface origin, but must, like other similar shocks, have 

 had a deep-seated origin, not necessarily the 1200 or even the 

 200 kilometres, which have been claimed for some of the world- 

 shaking earthquakes, but at any rate of the order of 50 kilometres 

 or 30 miles. That it could not have been due to the fall of the 

 landslip is evident; the landslip was determined by the earthquake 

 and, so far as the time of occurrence is concerned, was a con- 

 sequence, not a cause. 



