Earth-houses, a name generally given throughout Scotland to underground buildings, also known as 'Picts' houses' or 'Picts' dwellings'. The earth-house in its simplest form consists of a single irregular-shaped chamber, formed of unhewn stones, the side walls gradually converging towards the top until they can be roofed by stones of 4 or 5 feet in width, all covered in by a mound of earth rising slightly above the level of the adjacent ground. In the more advanced form of these structures two or three chambers are found. Earth-houses are frequent in the north-east of Scotland, occasionally thirty or forty being found in the same locality. Querns, bones, deers' horns, earthen vessels, cups and implements of bone, stone celts, bronze swords, and the like, are occasionally found in connection with them. Very similar structures, known as beehive-houses, occur also in Ireland and Cornwall.
Earth-nut, the Conopodium denudatum, an umbelliferous plant common in woods and fields in Britain. The leaves are ternately divided, and the small white flowers are in terminal
umbels. The tuber or nut is about 4 or 6 inches below the surface, at the termination of a long slender root. It is brown, the size of a chestnut, of a sweetish farinaceous nature, resembling in taste the common chestnut. Swine are very fond of the nuts, and fatten rapidly where they are abundant. The name is frequently applied to Carum Bulbocastănum, which has a similar tuber. See Ground-nut.
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 chasms and changes the appearance of the ground. Earthquakes originate in the crust of the earth, 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 difficult to tell the depth of the focus. Mallet estimated this by projecting backward the direction of travel of the wave at different points, as judged from the inclinations of the rents in buildings, &c., assumed to be at right angles to the line of propagation. The accuracy of this method has 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.
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 in its 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.
AB, Surface of earth. F, Focus. E, Epicentre. 1-4, Successive positions of earthquake wave. S, S, Cracked walls; the cracks being at right angles to CF and DF give some indication of the depth of focus.
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 practically always be told. With additional or particular forms of instruments, this may even be possible 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 lastly 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 classes of wave have been proved to travel through the earth, approximately along chords, but with the path slightly bent, convex towards the earth's centre. The first-phase waves are longitudinal, or waves of compression; 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 less 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 groundswell on the sea. Their time of passage 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. Great and destructive shocks are generally preceded by minor shocks in the same district, and they are always followed for months afterwards by a series of gradually lessening after-shocks. 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, Messina and Reggio in 1908. One of the greatest earthquakes of recent times was that which visited the provinces of Kansu and Shensi in North-West China, on 16th Dec., 1920. (See Seismograph.)—Bibliography: J. Milne, Earthquakes and other Earth-Movements; C. Davison, A Study of Recent Earthquakes; C. G. Knott, The Physics of Earthquake Phenomena.