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Earthquake, a sensible tremor of the earth's surface as if from a concussion at some definite underground centre. Earthquakes are the subject of the science of seismology (from the Greek seismos, an earthquake), the earliest English work on which is Hooke's Discourse on Earthquakes, 1668. Attention was more directed to the subject by the Lisbon earthquake of 1755; but little is even yet known as to the cause of the phenomena.

Probably several distinct causes bring about similar results. Most earthquakes occur in proximity to volcanic centres, and most volcanic eruptions are preceded by earthquake shocks, suggesting a community of origin for the two sets of phenomena. The sudden flashing of percolating water into steam on reaching highly-heated rocks, is proposed as a sufficient cause for both. Landslips produce tremors in non-volcanic areas resembling earthquakes, and the falling-in of the roofs of subterranean cavities has also been suggested as a cause, but is only likely to affect a small area. The sudden snap and dislocation of rock-masses under pressure producing a "fault" (q.v.), or the crushing together of such masses under the tangential pressures produced by the secular cooling of the globe, are suggestions of wider-reaching causes.

Professor Alexis Perreyof Dijon finds earthquake shocks more numerous about the winter solstice, i.e. when we are in perihelion, at apogee, and when the moon is on the meridian, suggesting their connection with a tidal action on the earth's interior.

The Centre of origin of the concussion, whatever it be, is known as the seismic centre or focus, a point on the earth's surface vertically above it (where the shock will be first and generally most severely felt), being termed the epicentrum, whilst the line joining them, the length of which is the depth of origin of the concussion, is called the seismic vertical.

The so-called shock is in reality the culminating of several undulatory movements of increasing intensity, and is followed by a diminishing series. These shocks consist in successive "waves of elastic compression" transmitted through the solid earth, much as sound is transmitted through the air. In addition to the "longitudinal vibrations," or forward and backward movement of rock-particles in the direction of the transmission of the shock, there are "transverse vibrations," which tend to make the effects more complex, as, for instance, in rotating chimney-stacks without overturning them. The actual distance through which individual particles oscillate, the amplitude, that is, of the undulation, in spite of the enormously destructive effects of some earthquakes, is probably generally less than an inch. The rate of transmission of the shock depends upon the density and elasticity of the various rocks, etc., through which it passes. Mr. Robert Mallet, the first of our modern seismologists, and his son, Dr. J. W. Mallet, determined by experiments with gunpowder explosions that such shocks travelled through damp sand at a velocity of about 825 feet per second, and through solid granite at 1,665 feet per second. The shock seems to undergo partial reflection, and perhaps refraction, in passing from one medium to another of differing elasticity, as from one geological formation to another, from land to water, or vice versa. This may result in a local doubling of the effects by the reflected wave, and in the formation of an area of seismic shadow enjoying relative or complete immunity from harm. The points at which a wave-shell reaches the surface form a curve called a co-seismal line, along which the shock is felt simultaneously; but, owing to the varying conductivity of the heterogeneous materials of the earth's crust, this curve is not a circle, in other words the wave-shells are not concentric spheres, and the co-seismic points are not equidistant from the epicentrum.

The earthquake produces an atmospheric wave which will travel at the ordinary velocity of sound waves, viz. 1,100 feet per second. It has often been recorded as a rumbling noise like thunder or cannon. On reaching the sea or other large body of water the shock produces two waves: one slight one, travelling with the earth-wave along the bottom; the other, with less velocity and of a height dependent on the depth of the water, rolling in after the earth-wave, often with most fated effect, as at Lisbon in November, 1775, and at Arica, in Peru, in August. 1868, and in May, 1877.

Mr. Mallet was of opinion that the angle of emergence of the shock could be ascertained from cracks in masonry, the general direction of which might be expected to be at right angles to the direction of transmission; but, apart from such difficulties as the zigzag character of most of such cracks and the manifestation of pre-existent weaknesses in the structures, the transverse vibrations have been proved to render all such conclusions untrustworthy. Could we determine the angle of emergence, knowing our distance from the epicentrum, it would be easy to calculate the depth of the focus, and Mr. Mallet was of opinion that this was not likely to exceed 30 miles.

Besides overthrowing buildings, earthquakes frequently produce landslips (q.v.) or subsidences, alter lines of drainage, and affect springs. Only occasionally do fissures open; but they may engulf houses and human beings, or, remaining open, may be widened by other agencies into ravines.

The area affected by earthquake is very variable. That of Lisbon was felt over a region four times the area of Europe, Lake Ladoga and Loch Lomond rising and falling, the springs at Toplitz and at Bristol being affected, and that at Luchon in the Pyrenees becoming permanently warmer. The Arica earthquake was felt 2,000 miles off; but, on the other hand, destructive shocks on the islands of Casamicciola in the Bay of Naples, and Scios in the Archipelago, were hardly noticeable on the mainland.

Various seismometers and seismographs, or instruments for recording the duration, direction, and intensity of earthquake shocks have been devised. Mallet suggested a tub rubbed over with chalk inside and half filled with water. In Italy a vessel containing mercury is employed, eight openings, with cups to receive overflow, being disposed round its margin. A suspended helix of copper wire may, on dipping into the mercury, complete a galvanic circuit, ringing an alarm-bell, stopping a clock to indicate the commencement of the shock, and starting another so as to indicate its duration. Another instrument consists of two rows of wooden cylinders of equal height, but different in diameter, arranged on two planks at right angles to one another on a surface of sand, to prevent their rolling when overteerned. Other instruments consist of pendula vibrating in different planes over bowls lined with chalk or lamp-black, and much valuable information as to the direction of earthquake shocks may be obtained by merely noting the orientation of pendulum-clocks which may happen to be stopped by the shock striking them obliquely to their plane of vibration. Unfortunately, careful observations made in Japan, where earthquakes are frequent, have yielded most contradictory results.