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The maximum intensity of the shock of the 23rd July was confined to a very limited area around the town of Paliyad. Judging
from the damage to buildings there the intenlutensity of shock of 23rd July.
sity of the shock must have just reached VII
on the Mercalli scale. I append a map showing isoseismals VII, VI and IV on this scale. The isoseismals are very approximate, as it was not possible to get accurate information in the short time (4 days) spent hy me in Kathiawar. Their general shape calls for no comment.
The times at which the various shocks are supposed to have taken place have been given in my list whenever they were avail
able. Unfortunately there are many discreTiming of shocks.
pancies. Thus the earthquake of the 23rd is reported from one source as 5:30 P.M., from another less than 25 miles distant as 5:35 P.m., and from a third about 90 miles distant as 5:45 P.M. As the slow surface waves of an earthquake travel 2 to 3 miles a second these differences are inexplicable unless some of the times given are wrong.
Without reliable information of the times at which an earthquake is felt in different areas it is impossible to calculate the rate of travel of the shocks, and I have not attempted to do so.
Opinions differed as to the direction from which the shocks in Paliyad came, but most people considered that they had come from
the south. The cracks the walls threw Direction of shocks.
no light on this point. Horizontal cracks were the commonest. Cracks parallel to one or both diagonals of a wall were frequent. Vertical cracks were rare, but there must have been a considerable vertical component to the shocks to have split the capitals of the pillars holding up the dome of the temple. On the whole the directions taken by the cracks in the walls followed the lines of greatest weakness in the buildings, and gave little indication of the direction of the shocks.
Although the intensity of most of the shocks at Paliyad was small, almost all of them were preceded by loud rumbling noises.
Many noises were also recorded unaccompanied Noises.
by any perceptible shocks. Many earthquakes are attended by similar sounds, but the general insistence on their loudness in the region all round Paliyad makes me think that they must have been an exceptional feature of the earthquakes here.
Rock is present in this part of Kathiawar everywhere within a few inches of the surface. It consists of a vast horizontally bedded sheet of basalt, Deccan trap (see p. 582). Without an extensive and careful geological survey it would be impossible to make an accurate estimate of its thickness, but it is probably between one and four hundred feet. Such a relatively thin sheet of hard rock might well vibrate like a drum under the impact of quite a small earthquake. Whether this is the real explanation of the exceptionally marked rumblings it is impossible to say without more and better evidence.
No determination of the depth of focus of an earthquake is possible unless there are a considerable number of seismographs
in the region of the earthquake which can Depth of focus of
give accurate times of arrival for the various
waves (II k, p. 104). The only seismograph, so far as I know, which picked up this earthquake, is the one at Colaba in Bombay (Statesman, 25th July 1938). One observation is of no use in fixing the depth of focus. The very local destruction caused by the earthquake suggests a shallow focus, but this in any case must be several miles below the surface. In the case of a similar swarm of earthquakes at Ito in Japan the depth of focus was about 37 miles below the surface of the ground (Davison, p. 218).
Few, if any, of the inhabitants of Paliyad and the surrounding rillages had ever felt an earthquake, so they were naturally terrified
by the loud noises and tremors. All sorts rumours
current. The shocks.
most generally believed were stories to the effect that a huge cavity had opened in the earth under Paliyad, and that the noises and tremors were due to its roof collapsing. They were very much afraid that this cavity would ultimately reach the surface and swallow up Paliyad and its surroundings. There was also great apprehension of a flood which would destroy any villages in the neighbourhood, which had not already been swallowed up. The local shopkeepers were
more terrified than the cultivators, but their fears were characteristically lined with silver. They thought the tremors a sign of coal or oil beneath Paliyad.
The panic due to the weird earthquake noises and tremors was considerably increased by these wild rumours, so that by the 14th of July all those who could afford to leave the village had gone. The cultivators, however, stayed to tend their herds and crops,
by of wild
and with them the thandar, the police and the doctor. All those who remained were living under improvised tents on high ground in case of floods.
On the 26th July, as the result of the severe shock of the 23rd, I was deputed to investigate the cause of these earthquakes, and, if possible, to reassure the local people. I left Calcutta that evening, and arrived in Paliyad on the 29th. I remained in that neighbourhood till the 31st, but unfortunately neither heard nor felt any tremors.
Paliyad lies in an undulating plain extending for many miles in all directions. The plain is varied here and there by hillocks
rising 50 feet or so above the surrounding Geology of Paliyad
country. The present land surface corre
sponds very closely to the top of a single basalt flow of Deccan trap age.
The hillocks are the remains of higher flows now almost completely worn away by erosion.
Thin red siliceous beds occur on the surface of the main flow over a wide area surrounding Paliyad. These are almost certainly the remains of a sedimentary bed deposited in the interval between successive basalt flows.
The main basalt flow is of rather an unusual type, as it contains numerous large olivine crystals easily visible to the naked eye. Such flows are unknown in typical Deccan trap country. The occurrence of such an unusual type of flow over wide area, with its surface everywhere about the same height above sea-level, indicates that there is no important faulting. This indication is strongly supported by the presence in so many places of the abovementioned red bed at a slightly higher level. The only sign of faulting that I could see was the presence of a few dykes just west of Nana Paliyad. Dykes such as these occasionally occur along faults, but in this case there was no indication that this was so.
If they do represent faults they must be small ones, because the same unusual olivine-basalt flow is seen on either side of them.
I conclude that there are no strong faults in the vicinity of Paliyad. The earthquakes are, therefore, not likely to be due to the slipping of any faults exposed at the surface there.
The suggestion that the Kathiavar earthquakes were due to the presence of an oil-field is unfortunately an unlikely one. The basalt flows at Paliyad are older than any of the oil-bearing rocks
in India. It is most unlikely, therefore, that they conceal beds containing oil.
The seam of coal said to occur near Paliyad turned out to be a flow of obsidian (volcanic glass). This is an interesting occurrence, but can have no possible connection with the present series of earthquakes.
Thus there is, so far as I could ascertain from surface indicacations, no local reason why the earthquakes should have been more
at Paliyad than elsewhere. It is a No obvious local
for Paliyad remarkable fact, however, that in the only earthquakes.
available record (Oldham, 1926) of an earthquake of destructive violence in Kathiawar “Rampoor near Pullgarde' was badly damaged. This is believed to refer to Rampur, 12 miles from Paliyad. The fact that the only two earthquakes recorded from Kathiawar have both been felt in the vicinity of Paliyad suggests that there is some subterranean feature favourable to the development of earthquakes in this area.
The recent geological history of Kathiawar (Fedden, pp. 53-59) indicates that there has been a general uplift of the land amounting
to 1,200 feet in places. The evidence on which Recent geological his.
this statement is based is the presence of tory of Kathiawar.
miliolite (Porbandar stone), a sub-recent marine deposit, on the top of Chotila hill 1,173 feet above sea-level, and the common occurrence of raised beaches, oyster beds, and coral reefs far above the present high-water mark.
Where surface changes of this magnitude are taking place, the earth's crust must be in a severe state of strain. If the stress
increases till it exceeds the rigidity of the Stress of earth's crust in Kathiawar.
earth's crust distortion takes place. This is
usually accompanied by earthquakes. These may be violent or mild according as the crust is distorted suddenly or gives gradually. Stress in the earth's crust due to the relatively rapid uplift of the Kathiawar peninsula is almost certainly the ultimate cause of the present series of earthquakes.
The past earthquake history of Kathiawar, so far as it is known, suggests that the uplift of the land there has not been associated
with any great earthquake activity. In fact, Earthquake history if de Montessus de Ballore's map of Indian of Kathiawar. earthquakes (1904, Pl. 1) can be relied on, this series of earthquakes is the first whose epicentral area is known to lie inside Kathiawar.
As great earthquakes are almost entirely confined to well-known earthquake zones, and as Kathiawar is certainly not one of these, there seems little likelihood that the present state of instability will culminate in a major earthquake. It is far more likely that the shocks will fade away gradually as the stress in the earth's crust is relieved. This is the normal course of events where swarms of earthquakes like the present ones are felt (Heck, p. 46).
The chief danger from earthquakes in Kathiawar is due to its propinquity to the Indus delta. This is a well-known earthquake area from which two disastrous shocks have been recorded in historical times (Oldham, T., pp. 6, 13 and 14). The more recent of these, in 1819, had its epicentre north of Cutch. Though this is 100 miles from Paliyad it caused the destruction of many buildings between that town and Ahmedabad, and also wrecked the alluvial tracts along the north and west coasts of Kathiawar.
The danger to Kathiawar from major earthquakes in the Indus delta should not be exaggerated. In the first place such great earthquakes have only occurred at very long intervals in the past. Thus there has only been one in the Indus delta since 1800 compared with 15 in the last 90 years in Baluchistan (West, Pl. 24). Then the intensity of an earthquake in Kathiawar with
Kathiawar with its epicentre in the Indus delta would be much less than at its source, though it could still be extremely destructive, as was proved by the earthquake of 1819. Finally, very severe damage in Kathiawar would probably be confined to the margins of that country where alluvial plains and unconsolidated rocks are common. Central Kathiawar is a great basalt shield with solid rock exposed everywhere a few inches below the surface. If the results of many great earthquakes are analysed it is found that destruction on massive tough rocks like basalts is much less than that on alluvium or badly consolidated rocks. This almost universal experience was borne out in the case of the great earthquake of 1819 when central Kathiawar escaped the wholesale destruction experienced in the alluvial regions of the north and west coast. (Oldham, 1926, pp. 40-42).
The history of Kathiawar shows that a great earthquake with epicentre there is most unlikely. It is, however, possible. If it came, it would destroy buildings over a very wide area ;
nor is there any way in which one can predict what that area would be.