Destruction due to the nature of the underlying rocks.—That the nature of the ground on which a building stands is intimately related with the severity of the blow it receives is a fact which has often been demonstrated.
One cause of destruction is due to placing a building on foundations which are capable of receiving the full effects of a shock, and transmitting it to the buildings standing on them.
For instance, the reason why a soft bed might possibly make a good foundation, is, as has been pointed out by Messrs. Perry and Ayrton, because the time of transmission of momentum is increased; in fact, the soft bed is very like a piece of wood interposed between a nail and the blows of a hammer—it lengthens the duration of impact. For this reason we are told that a quaking bog will make a good foundation. When a shock enters loose materials its waves will be more crowded, and it is possible that a line of buildings may rest on more than one wave during a shock. There are many examples on record of the stability of buildings which rested on beds of particular material at the time of destructive earthquakes. As the observations which have been made by various writers on this subject appear to point in a contrary direction, I give the following examples:—
In the great Jamaica earthquake of 1692, the portions of Port Royal which remained standing were situated on a compact limestone foundation; whilst those on sand and gravel were destroyed (‘Geological Observer,’ p. 426). Again, on p. 148 of the same work, we read, ‘According to the observations made at Lisbon, in 1737, by Mr. Sharpe, the destroying effects of this earthquake were confined to the tertiary strata, and were most violent on the blue clay, on which the lower part of the city is constructed. Not a building on the secondary limestone or on the basalt was injured.’
In the great earthquakes of Messina, those portions of the town situated on alluvium, near the sea, were destroyed, whilst the high parts of the town, on granite, did not suffer so much. Similar observations were made in Calabria, when districts consisting of gravel, sand, and clay became, by the shaking, almost unrecognisable, whilst the surrounding hills of slate and granite were but little altered. At San Francisco, in 1868, the chief destruction was in the alluvium and made ground.
At Talcahuano, in 1835, the only houses which escaped were the buildings standing on rocky ground; all those resting on sandy soil were destroyed.
From the results of observations like these, it would seem the harder rocks form better foundations than the softer ones. The explanation of this, in many cases, appears to lie in the fact that the soft strata were in a state of unstable equilibrium, and by shaking, they were caused to settle. Observations like the following, however, point out another reason why soft strata may sometimes afford a bad foundation.
‘Humboldt observed that the Cordilleras, composed of gneiss and mica-slate, and the country immediately at their foot, were more shaken than the plains.’[30]
‘Some writers have asserted that the wave-like movements (of the Calabrian earthquake in 1783) which were propagated through recent strata from west to east, became very violent when they reached the point of junction with the granite, as if a reaction was produced when the undulatory movement of the soft strata was suddenly arrested by the more solid rocks.’
Dolomieu when speaking of this earthquake says, the usual effect ‘was to disconnect from the sides of the Apennines all those masses (of sand and clay) which either had not sufficient bases for their bulk, or which were supported only by lateral adherence.’