EARTHQUAKE

a shaking of the earth’s surface, propagated from place to place by a wave motion. It may vary in intensity from the slightest perceptible tremor to a violent shock which bursts open chasins and changes the appearance of the ground. Karthquakes originate in the crust of the carth, generally at only a very few miles’ depth, and probably never lower than about 30 miles. The point of origin is called the centre or seismic focus, and the place on the surface vertically over it the epicentre. It is rather alitienlt to tell the depth of the focus. Mallet estimated this by projecting backward the direction of travel of the wave at different points, ag judged from the inclinations of the rents in buildings, ., a umed to be at right angles to the line of propagation. The accuracy of this method has 37 EARTHQUAKE been improved by substituting evidence of direction as given by seismographs, The focus of an earthquake is often submarine, and subsequent to the shock transmitted through the solid earth a great sea-wave may invade the land and produce far more disastrous effects. Causes.—In some cases an earthquake may be caused by a fall of rock in some subterranean cavity. This gives only a minor and local shock. The vast majority of earthquakes are certainly tectonic, originating from the snapping of strata under great strain, or the further slipping of portions of the earth’s crust along previously existing fault planes. Such dislocations probably arise sometimes from the variations of weight supported by the earth’s crust in neighbouring regions, due to the transport by rivers of material, which they erode at one place and deposit at another. <A further cause is the contraction undergone by the earth inits secular cooling. There are also earthquakes of volcanic origin, accompanying eruptions, but these are not usually of any great violence, nor do they involve any large area. The coasts of the Pacific Ocean— American, Asiatic, and East Indian —are much visited by earthquakes, in especial the Japanese Islands. The other band of greatest frequency has a direction outlined by the Azores, Alps, Mediterranean, and the Caucasus and Himalaya Mountains. It may be noted that all the regions specially affected are distinguished by steep gradients of the earth’s surface. Seismograph.—In recent years much information has been obtained by the investigation of earthquakes by various kinds of seismograph. One single instrument at a particular station, e.g. a Milne seismograph, will enable the distance of the epicentre to be calculated. From the results of three stations, the precise locality can eens | always be told. With additional or particular forms of instruments, this may even be po ible by means of the records at one station. Earthquake waves are found to consist of distinctly defined types. The first to arrive are the preliminary tremors or first-phase waves, then the second-phase waves, next the third-phase or large waves, and Jastly the concluding waves, consisting largely of ‘echoes’ or reflected vibrations. The speed of the preliminary tremors is found to be only about 2 miles per second for very short distances, but for a quadrant of the earth’s surface they travel at an average of about 7 miles per second, a speed which is only slightly exceeded for still greater distances. The second phase waves travel with a little under two-thirds of these velocities. These two cla es of wave have been proved to travel through the earth, approximately along chords, but with the path ey. bent, convex towards the earth’s centre. he first-phase waves are longitudinal, or waves of compre ion; the second-phase are transverse, or waves of distortion. The greater speed for greater distances is due to the track being more through the earth’s interior and le through its outer portions, as the interior transmits wave-motion much more rapidly than the crust. The rigidity at some depth from the surface has been shown to be of the same order of magnitude as the rigidity of steel. The third-phase waves are of much longer vibration period and wide amplitude, and have been compared to a ground-swell on the sea. Their time of pa age from the epicentre to any place is proportional to the distance measured round the earth’s surface, and it is clear that they travel on the surface, and not through the interior. Their speed is nearly 2 miles per second. The difference in time between the arrival of the preliminary tremors at any station and the arrival of the second-phase waves, or between the second-phase and third-phase waves, enables the distance of the epicentre to be easily found, as these differences, of course, become greater with increasing distance. The number of earthquakes has been found to be enormously greater than was at one time supposed; in fact, small tremors are occurring daily in one part or another of the earth. Among the most remarkable earthquakes of modern times were those which destroyed Lima in 1746 and Lisbon in 1755; more recently destructive earthquakes visited Calabria in 1857, Peru and Ecuador in 1868, the Island of Ischia in 1884, Japan in 1896, North India and Calabria in 1905, San Francisco in 1906, Me ina. and Reggio in 1908, and the provinces of Kansu and Shensi in North-West. China in 1920. Yokohama was completely and Tokio partially destroyed and 98,000 people killed by the Japanese earthquake in Sept., 1923. In Feb., 1931, an earthquake in the Hawke’s Bay area of New Zealand devastated the towns of Hastings and Napier and did damage amounting to £10,000,000. See SEISMOGRAPH.— BIBLIOGRAPHY: J. Milne, Karthquakes and other Harth-Movements; C. Day is on, A Study of Recent Karth(Cason C. G. Knott, The Physics of arthquake Phenomena.