
NATURE 
[MarcH 31, 1923 
The Character and Cause of Earthquakes.* 
By R. D. Orpuam, F.R.S. 
HE character of earthquakes, that is, of the | 
disturbance which can be felt and causes | 
damage, has long been established as a form of elastic | 
wave motion, originated by some sudden disturbance 
in the substance of the earth. In some cases, such as 
the Japanese earthquake of 1891 or the Californian 
of 1906, the earthquake was accompanied by visible | 
fractures and displacements of the solid rock, and where 
these have been observed it has also been noticed that 
the violence of the disturbance reached its maximum 
close by, and became less as the distance from the | 
fracture increased. From this it is evident that, in 
such cases at least, the earthquake originated from 

the jar caused by sudden rupture of the rocks, and 
the fault, or fracture, may be regarded as the cause 
of the disturbance to which the earthquake was due. 
In many other cases, where no actual faulting or 
fracture is visible at the surface, and especially in 
earthquakes of moderate intensity and extent, a study 
of the observations makes it very probable that the 
immediate cause of the disturbance was a fresh move- 
ment along an old fault, or the formation of a new one, 
Su pece Level of River Berk. (8544. 


Bed of Puran. /844 
Seale { bert ae Se a f. = 

\ Alood Leve; 
Eastern Sind, across the Runn ta the Kori creek, and 
on the banks of this river were fertile and populated 
tracts ; also on this river was situated a frontier fort 
of the Government of Cutch, where customs duties 
were collected. Then, in the eighteenth century, | 
through changes in the river courses far inland, the — 
supply of water in this river began to fail, and a series ~ 
of bunds, or what we would now call barrages, was 
built to hold up the water and divert it for irrigation. 
Towards the end of the century, the whole of the 
water supply was intercepted, and the region below - 
relapsed into a state of desolation; but the fort of 
Sindri was still maintained, with a small garrison and 
a few officials to collect the dues, and so things con- 
tinued until June 16, 1819, when the classic earth 
quake of Cutch occurred. 
The fort of Sindri was not only ruined, but the 
ground on which it lay was also lowered in level, 
water flowed in from the sea, and the small garrison 
of Sindri saved themselves from drowning by taking 
refuge in the main tower, whence they were rescued 
by boat the next day. Nor was this subsidence the 


SNadrié 
position Of surface in 78/9. 




probable 
feet 
miles. 
Fic. 1.—Section across the Allah Bund and Sindri depression of June 16, 1819. 
and so may be found in many text-books the statement, 
put forward and elaborated, that faults are the cause 
of those earthquakes classed as tectonic. Thus it 
might seem that the cause of earthquakes had been 
explained, but this is only the beginning of the story, 
for we need to know what causes the fracture which 
gives rise to the earthquake. 
In pursuing this object, reference may be made to 
an earthquake which occurred more than a hundred 
years ago, at a time when the observation of earth- 
quakes was in its infancy, and when little information 
of present value could be expected, had it not been 
for certain peculiarities in the country affected, and 
in the effect of the earthquake. Just beyond the 
north-western angle of the Indian Peninsula lies one of 
the most extraordinary regions of the world, known as 
the Runn of Cutch ; more than 200 miles in length, and 
some 30 in width, it is a level barren plain, so flat and 
so near the sea-level that when the waters of the sea 
are heaped up, by the south-west monsoon, and the 
streams from the surrounding country come down in 
floods, the greater part of the surface is covered with 
a sheet of water, varying from a few inches to five or 
six feet in depth. 
The whole of this region, however, has not always 
been so barren as at present, for, up to the seventeen 
hundreds, a large river, fed by the waters of the Punjab, 
and by overflow from the Indus, flowed down through 
+ Condensation of a course of two lectures delivered at the Royal Institu- 

tion on January 30, February 6. 
NO. 2787, VOL. 111] 
only change noticed, for, about four miles to the north, 
where before had been only a dead level plain, the 
survivors observed a long low mound, stretching east 
and west with a height of about 20 feet, along the 
northern edge of the flooded area. This mound, so 
like an artificial embankment, was immediately named 
the ‘‘ Allah Bund ” or “‘ God’s Barrage,” on the same 
principle which named the bunds or barrages higher 
up the stream after the names of their makers. 
Ten years later, the ruins of the fort were seen still 
standing out of a waste of waters, and twenty-five 
years later, in 1844, a careful survey was made and 
levels taken, and this survey revealed a remarkable con- 
dition of things (Fig. 1). From the north the surface 
of the delta sloped southwards, at about eight to ten 
inches in the mile, to within six miles from the crest 
of the Allah Bund, when a reverse slope was met, and 
the surface gradually rose to nearly 20 feet above the 
level of the continuation of the southerly slope, or the 
level at which it presumably stood before the earth- 
quake. Thence there was a steep slope downwards, 
to the water of the Sindri lake. On the south the 
original reports mention a depth of twelve feet of water 
close to the shore, immediately after the earthquake, 
and, as the original surface level must have been a few 
feet above that of the sea, we have a depression of some 
fifteen feet, which gradually died out in a distance of 
some six miles to the southward. From these facts — 
it is clear that there was no appreciable change of — 
level at a distance of about six miles on either side 

