11. Avoid heavy topped roofs and chimneys. If the foundations were free the roof might be heavy.
12. In brick or stone work use good cement.
13. Let archways curve into their abutments.
14. Let roofs have a low pitch, and the tiles, especially those upon the ridges, be well secured.
CHAPTER VIII.
EFFECTS OF EARTHQUAKES ON LAND.
1. Cracks and fissures—Materials discharged from fissures—Explanation of fissure phenomena. 2. Disturbances in lakes, rivers, springs, wells, fumaroles, &c.—Explanation of these latter phenomena. 3. Permanent displacement of ground—On coast lines—Level tracts—Among mountains—Explanation of these movements.
Cracks and fissures formed in the ground.—Almost all large earthquakes have produced cracks in the ground. The cracks which were found in the ground at Yokohama (February 22, 1880) were about two or three inches wide, and from twenty to forty yards in length. They could be best seen as lines along a road running near the upper edge of some cliffs which overlook the sea at that place. The reason that cracks should have occurred in such a position rather than in others was probably owing to the greater motion at such a place, due to the face of the cliff being unsupported, and there being no resistance opposed to its forward motion. It often happens that earthquake cracks are many feet in width. At the Calabrian earthquake of 1783, one or two of the crevasses which were formed were more than 100 feet in width and 200 feet in depth. Their lengths varied from half a mile to a mile.[38] Besides these large cracks, many smaller ones of one or two feet in breadth and of great length were formed. In the large fissures many houses were engulfed. Subsequent excavations showed that by the closing of the fissures these had been jammed together to form one compact mass. These cracks are usually more or less parallel, and at the same time parallel to some topographical feature, like a range of mountains. For example, the cracks which were formed by the Mississippi earthquake of 1812 ran from north-east to south-west parallel to the Alleghanies. By succeeding shocks these crevasses are sometimes closed and sometimes opened still wider. Their permanency will also depend upon the nature of the materials in which they are made.
During an earthquake large cracks may suddenly open and shut.
During the convulsions of 1692 which destroyed Port Royal, it is said that many of the fissures which were formed, opened and shut. In some of these, people were entirely swallowed up and buried. In others they were trapped by the middle, and even by the neck, where if not killed instantaneously they perished slowly. Subsequently their projecting parts formed food for dogs.[39]