EARTHQUAKE  . Although the terrible effects which often accompany earthquakes have in all ages forced themselves upon. the See also:

• ATTENTION (from Lat. ad-tendo, await, expect; the condition of being " stretched " or " tense ")

attention of See also:

• MAN
• MAN, ISLE OF (anc. Mona)

man, the exact investigation of seismic phenomena See also:

• DATES

dates only from the See also:

• MIDDLE

middle of the 19th See also:

• CENTURY (from Lat. centuria, a division of a hundred men)

century . A new See also:

• SCIENCE (Lat. scientia, from scire; to learn, know)

science has been thus established under the name of seismology (Gr. o-ecoµos, an earthquake) . See also:

• HISTORY

History.—Accounts of earthquakes are to be found scattered through the writings of many See also:

• ANCIENT
• ANCIENT (also spelt ANTIENT; derived, through the Fr. ancien, old, from the late Lat. antianum, from ante, before)

ancient authors, but they are, for the most See also:

• PART

part, of little value to the seismologist . There is a natural tendency to exaggeration in describing such phenomena, sometimes indeed to the extent of importing a supernatural See also:

• ELEMENT (Lat. elementum)

element into the description . It is true that attempts were made by some ancient writers on natural See also:

• PHILOSOPHY (Gr. gthos, fond of, and vo4 (a, wisdom)

philosophy to offer a rational explanation of earthquake phenomena, but the hypotheses which their explanations involved are, as a See also:

• RULE

rule, too fanciful to be See also:

• WORTH
• WORTH, CHARLES FREDERICK (1825-1895)

worth reproducing at the See also:

• PRESENT

present See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

day . It is therefore unnecessary to dwell upon the references to seismic phenomena which have come down to us in the writings of such historians and philosophers as See also:

• THUCYDIDES (OovavMns)

Thucydides, See also:

• ARISTOTLE
• ARISTOTLE (384-322 B.C.)

Aristotle and See also:

• STRABO (born c. 63 n.c.)

Strabo, See also:

• SENECA

Seneca, See also:

• LIVY [Thus L1v1us] (59 B.C.—A.D: 17)

Livy and See also:

• PLINY, THE ELDER
• PLINY, THE YOUNGER

Pliny . Nor is much to be gleaned from the pages of See also:

• MEDIEVAL

medieval and later writers on earthquakes, of whom the most notable are Fromondi (x527), Maggio (1571) and Travagini (1679) . In See also:

• ENGLAND
• ENGLAND, THE CHURCH OF

England, the earliest See also:

• WORK

work worthy of mention is See also:

• ROBERT
• ROBERT (1275—1343)
• ROBERT, HUBERT (1753-1808)
• ROBERT, LOUIS LEOPOLD (1794-1835)

Robert See also:

• HOOKE, ROBERT (1635–1703)

Hooke's Discourse on Earthquakes, written in 1668, and read at a later date before the Royal Society . This discourse, though containing many passages of considerable merit, tended but little to a correct See also:

• INTERPRETATION (from Lat. interpretari, to expound, explain, inter pres, an agent, go-between, interpreter; inter, between, and the root pret-, possibly connected with that seen either in Greek 4 p4'ew, to speak, or irpa-rrecv, to do)

interpretation of the phenomena in question . Equally unsatisfactory were the attempts of See also:

• JOSEPH
• JOSEPH, COMTE DE (1785-1870)
• JOSEPH, FATHER (FRANCOIS LECLERC DU TREMBLAY) (1577-1638)

Joseph See also:

• PRIESTLEY, JOSEPH (1733-1804)

Priestley and some other scientific writers of the 18th century to connect the cause of earthquakes with See also:

• ELECTRICAL
• ELECTRICAL (or ELECTROSTATIC) MACHINE

electrical phenomena . The See also:

• GREAT

great earthquake of See also:

• LISBON (Lisboa)

Lisbon in 1755 led the Rev .

See also:

• JOHN
• JOHN (1167–1216)
• JOHN (1290-c. 1320)
• JOHN (1296-1346)
• JOHN (1371–1419)
• JOHN (1468-1532)
• JOHN (1801-1873)
• JOHN (Heb. llni')
• JOHN (ZAPOLYA) (1487-1540)
• JOHN, 4TH MARQUESS OF TWEEDDALE (c. 1695-1762)
• JOHN, DON (1545-1578)
• JOHN, DON (1629–1679)
• JOHN, GOSPEL OF ST
• JOHN, or HAYS (1513–1571)
• JOHN, THE APOSTLE
• JOHN, THE EPISTLES OF

John See also:

• MICHELL, JOHN (1724-1793)

Michell, See also:

• PROFESSOR (the Latin noun formed from the verb profiteri, to declare publicly, to acknowledge, profess)

professor of See also:

• MINERALOGY

mineralogy at See also:

• CAMBRIDGE
• CAMBRIDGE, EARLS AND DUKES OF
• CAMBRIDGE, RICHARD OWEN (1717-1802)

Cambridge, to turn his attention to the subject; and in 176o he published in the Philosophical Transactions a remarkable See also:

• ESSAY, ESSAYIST (Fr. essai, Late Lat. exagium, a weighing or balance; exigere, to examine; the term in general meaning any trial or effort)

essay on the Cause and Phenomena of Earthquakes . A See also:

• SUGGESTION

suggestion of much scientific See also:

• INTEREST

interest was made by See also:

• THOMAS
• THOMAS (c. 1654-1720)
• THOMAS (d. 110o)
• THOMAS, ARTHUR GORING (1850-1892)
• THOMAS, CHARLES LOUIS AMBROISE (1811-1896)
• THOMAS, GEORGE (c. 1756-1802)
• THOMAS, GEORGE HENRY (1816-187o)
• THOMAS, ISAIAH (1749-1831)
• THOMAS, PIERRE (1634-1698)
• THOMAS, SIDNEY GILCHRIST (1850-1885)
• THOMAS, ST
• THOMAS, THEODORE (1835-1905)
• THOMAS, WILLIAM (d. 1554)

Thomas See also:

• YOUNG
• YOUNG, A
• YOUNG, ARTHUR (1741-1820)
• YOUNG, BRIGHAM (1801-1877)
• YOUNG, CHARLES MAYNE (1777–1856)
• YOUNG, EDWARD (1683–1765)
• YOUNG, JAMES (1811-1883)
• YOUNG, THOMAS (1773-1829)

Young, when in his Lectures on Natural Philosophy, published in 1807, he remarked that an earthquake "is probably propagated through the See also:

• EARTH
• EARTH (a word common to Teutonic languages, cf. Ger. Erde, Dutch aarde, Swed. and Dan. jord; outside Teutonic it appears only in the Gr. 'pq'e, on the ground; it has been connected by some etymologists with the Aryan root ar-, to plough, which is seen in
• EARTH, FIGURE OF
• EARTH, FIGURE OF THE

earth nearly in the same manner as a See also:

• NOISE (a word of doubtful origin; O. Fr. nogse or nose; Prov. nausa, which points to Lat. nausea, sickness, as the origin; others take Lat. noxia, harm, as the source)

noise is conveyed through the See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air." The recognition of the fact that the seismologist has to See also:

• DEAL

deal with the investigation of See also:

• WAVE

wave-See also:

• MOTION (Lat. motio, from movere, to move)
• MOTION, LAWS OF

motion in solids lies at the very See also:

• BASE

base of his science . In 1846 Robert See also:

• MALLET (or MALLOCH), DAVID (?17o5–1765)
• MALLET, PAUL HENRI (173o–18o7)

Mallet communicated to the Royal Irish See also:

• ACADEMY, GREEK
• ACADEMY, ROYAL

Academy his first See also:

• PAPER
• PAPER (Fr. papier, from Lat. papyrus)

paper " On the See also:

• DYNAMICS
• DYNAMICS (from Gr. bbvayts, strength)

Dynamics of Earthquakes "; and in the following See also:

• YEAR

year W . See also:

• HOPKINS, EDWARD WASHBURN (1857— )
• HOPKINS, ESEK (1718-18oz)
• HOPKINS, MARK (1802—1887)
• HOPKINS, SAMUEL (1721–1803)
• HOPKINS, WILLIAM (1793–1866)

Hopkins, of Cambridge, presented to the See also:

• BRITISH

British Association a valuable See also:

• REPORT (O.Fr. report or raport, modern rapport, from O.Fr. reporter, mod. rap porter, Lat. reportare, to bring back, in poetical use only, of bringing back an account, news, &c.)

report in which earthquake phenomena were discussed in some detail . Mallet's labours were continued for many years chiefly in the See also:

• FORM (Lat. forma)

form of Reports to the British Association, and culminated in his great work on the Neapolitan earthquake of 1857 . An entirely new impetus, however, was given to the study of earthquakes by an energetic See also:

• BODY

body of observers in See also:

• JAPAN

Japan, who commenced their investigations about the year 1880, mainly through the See also:

• INFLUENCE (Late Lat. influentia, from influere, to flow in)

influence of Prof . John Milne, then of See also:

• TOKYO (or TOKIO, formerly called Yedo)

Tokyo . Their work, carried on by means of new See also:

• INSTRUMENTS

instruments of precision, and since taken up by observersin many parts of the See also:

• WORLD

world, has so extended our knowledge of earthquake-motion that seismology has now become practically a new See also:

• DEPARTMENT (Fr. departement, from departir, to separate into parts)

department of See also:

• PHYSICAL

physical science . It is hardly too much to say, however, that the earliest systematic application of scientific principles to the study of the effects of an earthquake was made by Mallet in his investigation of the Neapolitan earthquake mentioned above . It is true, the great Calabrian earthquake of 1783 had been the subject of careful inquiry by the Royal Academy of See also:

• NAPLES
• NAPLES (Ital. Napoli, and Lat. Neapolis)
• NAPLES, KINGDOM OF

Naples, as also by Deodat See also:

• DOLOMIEU

Dolomieu and some other scientific authorities; but in consequence of the misconception which at that See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time prevailed with regard to the nature of seismic activity, the results of the inquiry, though in many ways interesting, were of very limited scientific value . It was reserved for Mallet to undertake for the first time an extensive See also:

• SERIES (a Latin word from serere, to join)

series of systematic observations in an See also:

• AREA

area of great seismic disturbance, with the view of explaining the phenomena by the application of the See also:

• LAWS

laws of wave-motion . The " Great Neapolitan Earthquake," by which more than 12,300 lives were lost, was See also:

• FELT (cognate with Ger. Filz, Du. vilt, Swed. and Dan. fill; the root is unknown; the word has given Med. Lat. filtrum, " filter ")

felt in greater or less degree over all See also:

• ITALY
• ITALY (Italia)

Italy See also:

• SOUTH
• SOUTH, ROBERT (1634–1716)

south of the parallel of 42°, and has been Neapollregarded as ranking third in See also:

• ORDER
• ORDER (through Fr. ordre, for earlier ordene, from Lat. ordo, ordinis, rank, service, arrangement; the ultimate source is generally taken to be the root seen in Lat. oriri, rise, arise, begin; cf. " origin ")
• ORDER, HOLY

order of severity among the tan earth-recorded earthquakes of See also:

• EUROPE

Europe .

The See also:

• PRINCIPAL

principal See also:

• SHOCK, or COLLAPSE

shock quake, occurred at about 10 P.m. on the 16th of See also:

• DECEMBER (Lat. decent, ten)

December lases 1857; but, as is usually the See also:

• CASE
• CASE, JOHN (d. 1600)

case, it had been preceded by See also:

• MINOR
• MINOR (Lat. for smaller, lesser)
• MINOR, ROBERT CRANNELL (1839-1904)

minor disturbances and was followed by numerous after-shocks which continued for many months . See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

Early in 1858, aided by a See also:

• GRANT (from A.-Fr. graunter, O. Fr. greanter for creanter, popular Lat. creantare, for credentare, to entrust, Lat. credere, to believe, trust)
• GRANT, ANNE (1755-1838)
• GRANT, CHARLES (1746-1823)
• GRANT, GEORGE MONRO (1835–1902)
• GRANT, JAMES (1822–1887)
• GRANT, JAMES AUGUSTUS (1827–1892)
• GRANT, ROBERT (1814-1892)
• GRANT, SIR ALEXANDER
• GRANT, SIR FRANCIS (1803-1878)
• GRANT, SIR JAMES HOPE (1808–1895)
• GRANT, SIR PATRICK (1804-1895)
• GRANT, U
• GRANT, ULYSSES SIMPSON (1822-1885)

grant from the Royal Society, Mallet visited the devastated districts, and spent more than two months in studying the effects of the See also:

• CATASTROPHE (Gr. Karaurpoch, from KaraQrp4ecv, to over-turn)

catastrophe, especially examining, with the See also:

• EYE
• EYE (O. Eng. edge, Ger. Auge; derived from an Indo-European root also seen in Lat. oc-ulus, the organ of vision (q.v.)

eye of an engineer, the cracks and ruins of the buildings . His voluminous report was published in 1862, and though his methods of See also:

• RESEARCH (O. Fr. recerche, from recercher, re- and cercer, mod. chercher, to search; Late Lat. circare, to go round in a circle, to explore)

research and his deductions have in many cases been superseded by the advance of knowledge, the report still remains a memorable work in the history of seismology . Much of Mallet's labour was directed to the determination of the position and magnitude of the subterranean source from which the vibratory impulses originated . This is known variously as the seismic centre, centrum, hypocentre, origin or See also:

• FOCUS (Latin for " hearth " or " fireplace ")

focus . It is often convenient to regard this centre theoretically as a point, but practically it must be a See also:

• LOCUS (Lat. for " place "; in Gr. rinros)

locus or space of three dimensions, which in different cases varies much in See also:

• SIZE

size and shape, and may be of great magnitude . That part of the See also:

• SURFACE

surface of the earth which is vertically above the centre is called the epicentre; or, if of considerable area, the epicentral or epifocal See also:

• TRACT (from Lat. tractare, to treat of a matter, through Provencal tractat and Ital. trattato)

tract . A See also:

• VERTICAL (from Lat. vertex, highest point)

vertical See also:

• LINE

line joining the epicentre and the focus was termed by Mallet the seismic vertical . He calculated that in the case of the Neapolitan earthquake the See also:

• FOCAL

focal cavity was a curved lamelliform fissure, having a length of about to m. and a height of about 32 m., whilst its width was inconsiderable . The central point of this fissure, the theoretical seismic centre, he estimated to have been at a See also:

• DEPTH

depth of about 62 m. from the surface . Dr C . See also:

• DAVISON, WILLIAM (c. 1541-1608)

Davison, in discussing Mallet's data, was led to the conclusion that there were two distinct foci, possibly situated on a See also:

• FAULT (Mid. Eng. faute, through the French, from the popular Latin use of fallere, to fail; the originallof the Latin being replaced in English in the 15th century)

fault, or See also:

• PLANE

plane of dislocation, See also:

• RUNNING

running in a See also:

• NORTH
• NORTH, BARONS
• NORTH, MARIANNE (1830—1890)
• NORTH, ROGER (1653-1734)
• NORTH, SIR THOMAS (1535?-16o1?)

north-See also:

• WEST
• WEST, BENJAMIN (1738-1820)
• WEST, NICHOLAS (1461-1533)

west and south-See also:

• EAST
• EAST, ALFRED (1849- )

east direction .

Mallet located his epicentre near the See also:

• VILLAGE

village of Caggiano, not far from Polla, while the other seems to have been in the neighbourhood of Montemurro, about 25 M. to the south-east . The intensity, or violence, of an earthquake is greatest in or near the epicentre, whence it decreases in all directions . A line See also:

• DRAWN

drawn through points of equal intensity forms a See also:

• CURVE (Lat. curvus, bent)

curve See also:

• ROUND (O. Fr. rond, Lat. rotundus, the Fr. is the source also of Du. rond; Ger., Swed., Dan. and Nor. rond)

round the epicentre known as an isoseist, an isoseismal or an isoseismic line . If the intensity declined equally in all directions the isoseismals would be circles, but as this is rarely if ever the case in nature they usually become ellipses and other closed curves . The tract which is most violently shaken was termed by Mallet the meizoseismic area, whilst the line of maximum destruction is known as the meizoseismic line . That isoseismal along which the decline of See also:

• ENERGY (from the Gr. ivipyela; iv, in, ipyov, work)

energy is most rapid was called by K. von See also:

• SEEBACH, MARIE (183o-1897)

Seebach a pleistoseist . In order to determine the position of the seismic centre, Mallet made much use of the cracks in damaged buildings, especially in walls of See also:

• MASONRY

masonry, holding that the direction of such fractures must generally be at right angles to that in which the normal earthquake-wave reached them . In this way he obtained the " See also:

• ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

angle of emergence " of the wave . He also assumed that See also:

• FREE

free-falling bodies would be overthrown and projected in the direction of See also:

• PROPAGATION

propagation of the wave, so that the epicentre might immediately be found from the intersection of such directions . These data are, however, subject to much See also:

• ERROR (Lat. error, from errare, to wander, to err)

error, especially through want of homogeneity in the rocks, but Mallet's work was still of great value . A different method of ascertaining the depth of the focus was adopted by See also:

• MAJOR
• MAJOR (Lat. for " greater ")
• MAJOR (or MAIR), JOHN (1470-1550)

Major C . E .

Dutton in his investigation of the See also:

• CHARLESTON

Charleston Charleston earthquake of the 31st of See also:

• AUGUST (originally Sextilis)

August 1886 earth- for the U.S . See also:

• GEOLOGICAL

Geological Survey . This catastrophe quake, was heralded by shocks of greater or less severity a 1886' few days previously at Summerville, a village 22 M. north-west of Charleston . The great earthquake occurred at 9.51 P.M., See also:

• STANDARD
• STANDARD, BATTLE OF THE

standard time of the 75th See also:

• MERIDIAN
• MERIDIAN (from the Lat. meridianus, pertaining to the south or mid-day)

meridian, and in about 70 seconds almost every See also:

• BUILDING

building in Charleston was more or less seriously damaged, while many lives were lost . The epicentral tract was mainly a See also:

• FOREST

forest region with but few buildings, and the principal records of seismological value were afforded by the lines of railway which traversed the disturbed area . In many places these rails were flexured and dislocated . Numerous fissures opened in the ground, and many of these discharged See also:

• WATER

water, mixed sometimes with See also:

• SAND
• SAND, GEORGE (1804-1876)

sand and silt, which was thrown up in jets rising in some cases to a height of 20 ft . Two epicentres were recognized—one near See also:

• WOODSTOCK

Woodstock station on the South Carolina railway, and the other, being the centre of a much smaller tract, about 14 M. south-west of the first and near the station of Ran towles on the Charleston and See also:

• SAVANNAH

Savannah line . Around these centres and far away isoseismal lines were drawn; the relative intensity at different places being roughly estimated by the effects of the catastrophe on various structures and natural See also:

• OBJECTS

objects, or, where visible records were wanting, by See also:

• PERSONAL

personal See also:

• EVIDENCE (Lat. evidentia, evideri, to appear clearly)

evidence, which is often vague and variable . The See also:

• ROSSI, LUIGI DE
• ROSSI, PELLEGRINO LUIGI EDOARDO, COUNT (1787-1848)

Rossi-Forel See also:

• SCALE
• SCALE (1) A

scale was adopted . This is an arbitrary scale formulated by Professor M . S. de Rossi, of See also:

• ROME
• ROME (Roma)

Rome, and Dr F .

A . Forel, of See also:

• GENEVA
• GENEVA (Fr. Geneve, Ger. Genf, Ital. Ginevra, Late Lat. Gebenna, though Genava in good Latin)
• GENEVA, LAKE OF

Geneva, based mostly on the See also:

• ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary phenomena observed during an earthquake, and consisting of ten degrees, of which the lowest is the feeblest, viz . I . Microseismic shock; II . Extremely feeble shock; III . Very feeble shock; IV . Feeble; V . Shock of moderate intensity; VI . Fairly strong shock; VII . Strong shock; VIII . Very strong shock; IX . Extremely strong shock; X .

Shock of extreme intensity . Other conventional scales, some being less detailed, have been drawn up by observers in such earthquake-shaken countries as Italy and Japan . A curve, or theoretical isoseismal, drawn through certain points where the decline of intensity on receding from the epicentre seems to be greatest was called by Dutton an " See also:

• INDEX

index-circle "; and it can be shown that the See also:

• RADIUS

radius of such a circle multiplied by the square See also:

• ROOT (late O.E. rot, adopted from Scand., cf. Norw. and Swed. rot, Dan. rod; the true O.E. word was wyrt, plant, represented in Ger. Wurz or Wurzel; the ultimate root is the same in both words, and is seen in Lat. radix)
• ROOT, ELIHU (1845– )

root of 3 gives the focal depth theoretically . In this way it was computed that in the Charleston earthquake the origin under Woodstock must have had a depth of about 12 M. and that near Rantowles a depth of nearly 8 m . The determination of the index-circle presents much difficulty, and the conclusions must be regarded as only approximate . It is probable, according to R . D . See also:

• OLDHAM
• OLDHAM, JOHN (1653–1683)
• OLDHAM, THOMAS (1816–1878)

Oldham, that See also:

• LOCAL

local earthquakes may originate in the " See also:

• OUTER

outer skin " of the earth, whilst a large world-shaking earthquake takes its origin in the deeper part of the " crust," whence such a disturbance is termed a balhyseism . Large earthquakes may have very extended origins, with no definite centre, or with several foci . The gigantic disaster known as the "Great See also:

• INDIAN

Indian Earthquake," which occurred on the 12th of See also:

• JUNE

June 1897, was the subject of areal careful investigation by the Geological Survey of Indian See also:

• INDIA
• INDIA, FRENCH

India and was described in detail by the super- earth- intendent, R . D . Oldham .

It is sometimes termed quake, 1897. the See also:

• ASSAM

Assam earthquake, since it was in that See also:

• PRO

pro- vince that the effects were most severe, but the shocks were felt over a large part of India, and indeed far beyond its boundaries . Much of the area which suffered most disturbancewas a See also:

• WILD, JONATHAN (c. 1682-1725)

wild See also:

• COUNTRY (from the Mid. Eng. contre or contrie, and O. Fr. cuntree; Late Lat. contrata, showing the derivation from contra, opposite, over against, thus the tract of land which fronts the sight, cf. Ger. Gegend, neighbourhood)

country, sparsely populated, with but few buildings of See also:

• BRICK (derived according to some etymologists from the Teutonic bricke, a disk or plate; but more authoritatively, through the French brique, originally a " broken piece," applied especially to bread, and so to clay, from the Teutonic brikan, to break)

brick or See also:

• STONE
• STONE (0. Eng. shin; the word is common to Teutonic languages, cf. Ger. Stein, Du. steen, Dan. and Swed. sten; the root is also seen in Gr. aria, pebble)
• STONE, CHARLES POMEROY (1824-1887)
• STONE, EDWARD JAMES (1831-1897)
• STONE, FRANK (1800-1859)
• STONE, GEORGE (1708—1764)
• STONE, LUCY [BLACKWELL] (1818-1893)
• STONE, MARCUS (184o— )
• STONE, NICHOLAS (1586-1647)

stone from which the violence of the shocks could be estimated . The epicentral tract was of great size, having an estimated area of about 6000 sq. m., but the See also:

• MISCHIEF

mischief was most severe in the neighbourhood of See also:

• SHILLONG

Shillong, where the stonework of See also:

• BRIDGES
• BRIDGES, ROBERT (1844– )

bridges, churches and other buildings was absolutely levelled to the ground . After the See also:

• MAIN (from the Aryan root which appears in " may " and " might," and Lat. magnus, great)
• MAIN (Lat. Moenus)

main disturbance, shocks of greater or less severity continued at intervals for many See also:

• WEEKS, EDWIN LORD (1849-1903)

weeks . It is supposed that this earthquake was connected with See also:

• MOVEMENT

movement of subterranean See also:

• ROCK
• ROCK (O.Fr. rake, Sp. rota, Ital. rocca; possibly from a Lat. form rupica, from rupes, rock)
• ROCK, DANIEL (1799–187t)

rock-masses of enormous magnitude along a great thrust-plane, or series of such planes, having a length of about 200 M. and a maximum breadth of not less than 5o m . It is pointed out by Oldham that this may be compared for size with the great Faille du Midi in See also:

• BELGIUM
• BELGIUM (Fr. Belgique; Flem. Belgic)

Belgium, which is known to extend for a distance of 120 M . The depth of the principal focus, though not actually capable of determination, was probably less than 5 M. from the surface . From the focus many secondary faults and fractures proceeded, some reaching the surface of the ground . Enormous landslips accompanied the .earthquake, and as an indirect effect of these slides the form of the water-courses became in certain cases modified . Permanent changes of level were also observed . Eight years after the great Assam earthquake India was visited by another earthquake, which, though less intense, resulted in the loss of about 20,000 lives . This cata- See also:

• KANGRA

Kangra See also:

• STROPHE (Gr. o-rpod>)

strophe is known as the Kangra earthquake, since its earth- centre seems to have been located in the Kangra quake, valley, in the north-west See also:

• HIMALAYA

Himalaya .

It occurred on I9os the 4th of See also:

• APRIL

April 1905, and the first great shocks were felt in the See also:

• CHIEF (from Fr. chef, head, Lat. caput)

chief epifocal See also:

• DISTRICT

district at about 6.9 A.M., See also:

• MADRAS

Madras time . Although the tract chiefly affected was around Kangra and See also:

• DHARMSALA

Dharmsala, there was a subordinate epifocal tract in See also:

• DEHRA

Dehra Dun and the neighbourhood of See also:

• MUSSOORIE, or MASURI

Mussoorie, whilst the effects of the earthquake extended in slight measure to See also:

• LAHORE

Lahore and other cities of the See also:

• PLAIN (O. Fr. plain, from Lat. plenum)

plain . It is estimated that the earthquake was felt over an area of about 1,625,000 M . Immediately after the calamity a scientific examination of its effects was made by the Geological Survey of India, and a report was drawn up by the See also:

• SUPERINTENDENT

superintendent, C . S . Middlemiss . The great earthquake, which, with the subsequent See also:

• FIRE (in O. Eng. f j'r; the word is common to West German languages, cf. Dutch vuur, Ger. Feuer; the pre-Teutonic form is seen in Sanskrit pu, pavaka, and Gr. irup; the ultimate origin is usually taken to be a root meaning to purify, cf. Lat. purus)

fire, wrought such terrible destruction in and around See also:

• SAN

San Francisco on the 18th of April 1906, was the most disastrous ever recorded in See also:

• CALIFORNIA
• CALIFORNIA, LOWER (Baja California)
• CALIFORNIA, UNIVERSITY OF

California California . It occurred between ro and 15 minutes earth- after 5 A.m., standard time of the 120th meridian. uake, The moment at which the disaster began and the I906 duration of the shock varied at different localities in the great area over which the earthquake was felt . At San Francisco the main shock lasted rather more than one See also:

• MINUTE
• MINUTE (Lat. minutes, small; minuere, to make less)

minute . According to the See also:

• OFFICIAL (Late Lat. QJicialis, for class. Lat. apparitor, from officium, office, duty)

official Report, the earthquake was due to rupture and movement along the plane of the San Andreas fault, one of a series which runs for several See also:

• HUNDRED

hundred See also:

• MILES, NELSON APPLETON (1839— )

miles approximately in a N.W. and S.E. direction near the See also:

• COAST (from Lat. costa, a rib, side)

coast line . Evidence of fresh movement along this plane of dislocation was traced for a distance of 190 m. from San Juan on the south to Point See also:

• ARENA (Lat. for " sand ")

Arena on the north . There the trace of the fault is lost beneath the See also:

• SEA (in O. Eng. sae, a common Teutonic word; cf. Ger. See, Dutch Zee, &c.; the ultimate source is uncertain)
• SEA, COMMAND OF THE

sea, but either the same fault or another appears 75 M. to the north at Point Delgada .

The See also:

• BELT (a word common to Teutonic languages, the Old Ger. form being bale, from which the Lat. balteus probably derived)
• BELT, THOMAS (1832-1878)

belt of disturbed country is notoriously unstable, and part of the fault had been known as the " earthquake crack." The direction is marked by lines of straight cliffs, See also:

• LONG, GEORGE (1800-1879)
• LONG, JOHN DAVIS (1838– )

long ponds and narrow depressions, forming a Rift, or old line of seismic disturbance . According to Dr G . K . See also:

• GILBERT
• GILBERT (KINGSMILL) ISLANDS
• GILBERT (or GYLBERDE), WILLIAM (1544-1603)
• GILBERT, ALFRED (1854– )
• GILBERT, ANN (1821-1904)
• GILBERT, GROVE KARL (1843– )
• GILBERT, J
• GILBERT, JOHN (1810-1889)
• GILBERT, MARIE DOLORES ELIZA ROSANNA [" LOLA MONTEZ "] (1818-1861)
• GILBERT, NICOLAS JOSEPH LAURENT (1751–1780)
• GILBERT, SIR HUMPHREY (c. 1539-1583)
• GILBERT, SIR JOSEPH HENRY (1817-1901)
• GILBERT, SIR WILLIAM SCHWENK (1836– )

Gilbert the earthquake See also:

• ZONE (Gr. 'WV77, a girdle, from 'wvvLvay to gird)

zone has a length of 300 or 400 M . The principal displacement of rock, in 1906, was See also:

• HORIZONTAL

horizontal, amounting generally to about TO ft . (maximum 21 ft.), but there was also locally a slight vertical movement, which towards the north end of the fault reached 3 ft . Movement was traced for a distance of about 270 m., and it is estimated that at least 175,000 sq. m. of country must have been disturbed . In estimating the intensity of the earthquake in San Francisco a new scale was introduced by H . O . See also:

• WOOD, ANTHONY A2 (1632-1695)
• WOOD, JOHN GEORGE (1827—1889)
• WOOD, MRS HENRY [ELLEN] (1814—1887)
• WOOD, SEARLES VALENTINE (1798—188o)

Wood . The greatest structural damage occurred on soft alluvial See also:

• SOIL

soil and "made ground." Most of the loss of See also:

• PROPERTY

property in San Francisco was due to the terrible fire which followed the earthquake and was beyond See also:

• CONTROL (Fr. contrdle, older form contre rolle, from Med. Lat. contra-rotulus, a counter roll or copy of a document used to check the original; there is no instance in English of the use of " control " in this, its literal, meaning)

control owing to the destruction of the See also:

• SYSTEM

system of water-See also:

• SUPPLY (through Fr. from Lat. supplere, to fill up)

supply . Immediately after the catastrophe a California Earthquake Investigation See also:

• COMMITTEE (from commute, an Anglo-Fr. past participle of commettre, Lat. committere, to entrust; the modern Fr. equivalent comite is derived from the Eng.)

Committee was appointed by the See also:

• GOVERNOR (from the Fr. gouverneur, from gouverner, O. Fr. governer, Lat. gubernare, to steer a ship, to direct, guide)

governor of the See also:

• STATE
• STATE, GREAT OFFICERS OF

state; and the See also:

• AMERICAN

American Association for the See also:

• ADVANCEMENT

Advancement of Science afterwards instituted a Seismological Committee .

The elaborate Report of the State Investigation Committee, by the chairman, Professor A . C . See also:

• LAWSON, CECIL GORDON (1851—1882)
• LAWSON, SIR JOHN (d. 1665)
• LAWSON, SIR WILFRID

Lawson, was published in 1908 . On the 17th of August 1906 a disastrous earthquake occurred at See also:

• VALPARAISO

Valparaiso, and the year 1906 was marked generally by exceptional seismic activity . The See also:

• JAMAICA

Jamaica earthquake of the 14th of See also:

• JANUARY

January 1907 appears to have accompanied movement of rock along an east and west fracture or series of fractures under the sea a few miles from the See also:

• CITY (through Fr. cite, from Lat. civitas)

city of See also:

• KINGSTON
• KINGSTON, ELIZABETH, DUCHESS OF (1720-1788)
• KINGSTON, WILLIAM HENRY GILES (1814-188o)

Kingston . The statue of See also:

• QUEEN
• QUEEN (O.E. cwen, wife, related to "quean," O.E. cwene, a hussy; cf. Gr. yvvi7: from root gan-, to produce; cf. genus, " kin," &c.)

Queen See also:

• VICTORIA
• VICTORIA (or VITTORIA), TOMMASSO LUDOVICO DA (C. I540-C. 1613)
• VICTORIA [ALEXANDRINA VICTORIA]

Victoria at Kingston was turned upon its See also:

• PEDESTAL (Fr. piedestal, Ital. piedestallo, foot of a stall)

pedestal the eighth of a revolution . A terrible earthquake occurred in See also:

• CALABRIA

Calabria and See also:

• SICILY (Ital. Sicilia)

Sicily on December 28, 1908, practically destroying See also:

• MESSINA

Messina and Reggio . Messina According to the official returns the See also:

• TOTAL

total loss of See also:

• LIFE

life earth- was 77,283 . Whilst the principal centre seems to quake, have been in the Strait of Messina, whence the dis- 1908• turbance is generally known as the Messina earthquake, there were See also:

• INDEPENDENT

independent centres in the Calabrian See also:

• PENINSULA
• PENINSULA (Lat. paeninsula, from paene, almost, and insula, an island)

peninsula, a country which had been visited by severe earthquakes not long previously, namely on See also:

• SEPTEMBER (Lat. septem, seven)

September 8, 1905, and See also:

• OCTOBER

October 23, 1907 . The principal shock of the great Messina earthquake of 1908 occurred at 5.21 A.M . (4.21 See also:

• GREENWICH

Greenwich time), and had a duration of from 30 to 40 seconds . Neither during nor immediately before the catastrophe was there any See also:

• SPECIAL

special volcanic disturbance at See also:

• ETNA
• ETNA (Gr. AZ-rvn, from aZOw, burn; Lat. Aetna)

Etna or at Stromboli, but it is believed that there must have been movement along a great plane of weakness in the neighbourhood of the Strait of Messina, which has been studied by E .

Cortese . The sea-See also:

• FLOOR (from O. Eng. flor, a word common to many Teutonic languages, cf. Dutch vloer, and Ger. Flur, a field, in the feminine, and a floor, masculine)

floor in the strait probably suffered great disturbance, resulting in the remarkable movement of water observed on the coast . At first the sea retired, and then a great wave rolled in, followed by others generally of decreasing See also:

• AMPLITUDE (from Lat. amplus, large)

amplitude, though at See also:

• CATANIA (Gr. Katane, Rom. Catinal)

Catania the second was said to have been greater than the first . At Messina the height of the great wave was 2.70 metres, whilst at See also:

• ALI

Ali and Giardini it reached 8.4o metres and at San Alessio as much as 11.7 metres . At See also:

• MALTA
• MALTA (or MEDITERRANEAN) FEVER

Malta the See also:

• TIDE (O. Eng. lid, cf. Ger. Zeit, time or season, connected with root of Sanskrit a-diti, endless)

tide-See also:

• GAUGE, or GAGE (Med. Lat. gauja, jaugia, Fr. jauge, perhaps connected with Fr. jale, a bowl, galon, gallon)

gauge recorded a wave of 0.91 See also:

• METRE ((terpuc?, sc. rFxvf, from Gr. Or poi', measure)

metre . The depth of the chief earthquake-centre was estimated by Dr E . Oddone at about 9 kilometres . The earthquake and accompanying phenomena were studied also by Professor A . See also:

• RICCI, MATTEO (1552-1610)

Ricci), Dr M . Baratta and Professor G . Platania and by Dr F . Omori of Tokyo .

After the great disturbance, shocks continued to affect the region intermittently for several months . In certain respects the earthquake of 1908 presented much resemblance to the great Calabrian catastrophe of 1783 . It has been proposed by R . D . Oldham that the disturbance which causes the fracture and permanent displacement of the rocks during an earthquake should be called an "earthshake," leaving the See also:

• TERM

term earthquake especially for the vibratory motion . The movement of the earthquake is molecular, whilst that l,: the earthshake is molar . Subsequently he suggested the terms mochleusis and orchesis (µoXXfuw, I heave; 6p)6ouat, I See also:

• DANCE
• DANCE (Fr. danse; of obscure origin, connected with Old High Ger. danson, to stretch)

dance), to denote respectively the molar and the molecular movement, retaining the word earthquake for use in its ordinary sense . In most earthquakes the proximate cause is generally regarded as the fracture and sudden movement of underground rock-masses . Disturbances of this type are known as " tectonic " earthquakes, since they are connected with the folding and faulting of the rocks of the earth's crust . They indicate a See also:

• RELIEF

relief of the See also:

• STRAIN (through O. Fr. straindre, estraindre, mod. i treindre, from Lat. stringere, to draw tight, related to stress, stretch, string, &c.)

strain to which the rock-masses are subjected by See also:

• MOUNTAIN (0. Fr. montaigne; popular Lat. montanea, an adjectival form from the classical mons, montis, whence Eng. " mount," a form usually used along with the name of an individual mountain, e.g. Mt Everest)
• MOUNTAIN, THE (La Montagne)

mountain-making and other crustal movements, and they are consequently See also:

• APT

apt to occur along the steep See also:

• FACE (from Lat. fades, derived either from facere, to make, or from a root fa-, meaning " appear "; cf. Gr. cbatvstv)

face of a table-See also:

• LAND

land or the margin of a See also:

• CONTINENT (from Lat. continere, "to hold together "; hence " connected," " continuous ")

continent with a great slope from land to sea . In many cases the immediate seat of the originating impulse is located beneath the sea, giving rise to submarine disturbances which have been called " seaquakes." Much attention has been given to these suboceanic disturbances by Professor E . See also:

• RUDOLPH
• RUDOLPH (d. 1o8o)
• RUDOLPH, or RAOUL

Rudolph .

Professor J . H . Jeans has pointed out that the regions of the earth's crust most affected by earthquakes See also:

• LIE, JONAS LAURITZ EDEMIL (1833—1908)
• LIE, MARIUS SOPHUS (1842–1899)

lie on a great circle corresponding with the See also:

• EQUATOR (Late Lat. aequator, from aequare, to make equal)

equator of the slightly See also:

• PEAR (Pyrus communis)

pear-shaped figure that he assigns to the earth . This would represent a belt of weakness, subject to crushing, from the tendency of'the pear to pass into a spherical or spheroidal form under the See also:

• ACTION

action of See also:

• INTERNAL

internal stresses . According to the See also:

• COMTE, AUGUSTE [ISIDORE AUGUSTE MARIE FRANCOIS XAVIER] (1798-1857)

comte de Montessus de Ballore, the regions of maximum seismic instability appear to be arranged on two great circles, inclined to each other at about 67° . These are the Circumpacific and Mediterranean zones . Maps of the world, showing the origins of large earthquakes each year, accompany the See also:

• ANNUAL

Annual Reports of the Seismological Committee of the British Association, drawn up by Professor Milne . It is important to See also:

• NOTE
• NOTE (Lat. nota, mark, sign, from noscere, to know)

note that Professor Milne has shown a relationship between earthquake-frequency and the wandering of the earth's See also:

• POLE (FAMILY)
• POLE, REGINALD (1500-1558)
• POLE, RICHARD DE LA (d. 1525)
• POLE, WILLIAM (1814—1900)

pole from its mean position . Earthquakes seem to have been most frequent when the displacement of the pole has been comparatively great, or when the See also:

• CHANGE (derived through the Fr. from the Late Lat. cambium, cambiare, to barter; the ultimate derivation is probably from the root which appears in the Gr. «a urmu', to bend)

change in the direction of movement has been marked . Valuable earthquake catalogues have been compiled at various times by See also:

• ALEXIS
• ALEXIS, WILLIBALD

Alexis Perrey, R. and J . W . Mallet, John Milne, T .

Oldham, C . W . C . See also:

• FUCHS, JOHANN NEPOMUK VON (1774-1856)
• FUCHS, LEONHARD (15o1-1566)

Fuchs, F. de Montessus de Ballore and others . Such earthquakes as are felt from time to time in Great See also:

• BRITAIN (Gr. Hperavuml vi7vot, Bperravia; Lat. Britannia, rarely Britannia)

Britain may generally be traced to the formation of faults, or rather to incidents in the growth of old faults . The East Anglian earthquake of the 22nd of April 1884—the most disastrous that had occurred in the British Isles for centuries—was investigated by Prof . R . Meldola and W . See also:

• WHITE
• WHITE, ANDREW DICKSON (1832– )
• WHITE, GILBERT (1720–1793)
• WHITE, HENRY KIRKE (1785-1806)
• WHITE, HUGH LAWSON (1773-1840)
• WHITE, JOSEPH BLANCO (1775-1841)
• WHITE, RICHARD GRANT (1822-1885)
• WHITE, ROBERT (1645-1704)
• WHITE, SIR GEORGE STUART (1835– )
• WHITE, SIR THOMAS (1492-1567)
• WHITE, SIR WILLIAM ARTHUR (1824--1891)
• WHITE, SIR WILLIAM HENRY (1845– )
• WHITE, THOMAS (1628-1698)
• WHITE, THOMAS (c. 1550-1624)

White on behalf of the See also:

• ESSEX
• ESSEX, ARTHUR CAPEL
• ESSEX, EARLS OF
• ESSEX, KINGDOM OF
• ESSEX, ROBERT DEVEREUX
• ESSEX, ROBERT DEVEREUX, 2ND2 EARL OF (1566-1601)
• ESSEX, WALTER DEVEREUX, IST2 EARL OF (1541–1576)

Essex See also:

• FIELD (a word common to many West German languages, cf. Ger. Feld, Dutch veld, possibly cognate with O.E. f olde, the earth, and ultimately with root of the Gr. irAaror, broad)
• FIELD, CYRUS WEST (1819-1892)
• FIELD, DAVID DUDLEY (18o5-1894)
• FIELD, EUGENE (1850-1895)
• FIELD, FREDERICK (18o1—1885)
• FIELD, HENRY MARTYN (1822-1907)
• FIELD, JOHN (1782—1837)
• FIELD, MARSHALL (183 1906)
• FIELD, NATHAN (1587—1633)
• FIELD, STEPHEN JOHNSON (1816-1899)
• FIELD, WILLIAM VENTRIS FIELD, BARON (1813-1907)

Field See also:

• CLUB (connected with " clump ")

Club . The shocks probably proceeded from two foci—one near the villages of Peldon and Abberton, the other near Wivenhoe and Rowhedge, in N.E . Essex . It is believed that the superficial disturbance resulted from rupture of rocks along a deep fault .

An See also:

• ATTEMPT (Lat. adtemptare, attentare, to try)

attempt has been made by H . See also:

• DARWIN, CHARLES ROBERT (1809-1882)
• DARWIN, ERASMUS (1731-1802)

Darwin, for the Seismological Committee of the British Association, to detect and measure any See also:

• GRADUAL (Med. Lat. gradualis, of or belonging to steps or degrees; gradus, step)

gradual movement of the strata along a fault, by observation at the Ridgeway fault, near Upway, in See also:

• DORSETSHIRE (DORSET)

Dorsetshire . Dr C . Davison in studying the earthquakes which have originated in Britain since 1889 finds that several have been " twins." A twin earth-quake has two See also:

• MAXIMA

maxima of intensity proceeding from two foci, whereas a See also:

• DOUBLE
• DOUBLE (from the Mid. Eng. duble, the form which gives the present pronunciation, through the Old Er. duble, from Lat. duplus, twice as much)

double earthquake has its successive impulses from what is practically a single focus . The See also:

• HEREFORD

Hereford earthquake of December 1896, which resulted in great structural damage, was a twin, having one epicentre near Hereford and the other near See also:

• ROSS
• ROSS, ALEXANDER (1699-1784)
• ROSS, GEORGE WILLIAM (1841- )
• ROSS, JOHN, or KOOESKOOWE (179o-1866)
• ROSS, ROBERT (1766-1814)
• ROSS, SIR HEW DALRYMPLE (1779-1868)
• ROSS, SIR JAMES CLARK (1800-1862)
• ROSS, SIR JOHN (1777-1856)

Ross . Davison refers it to a slip along a fault-plane between the anticlinal areas of Woolhope and May See also:

• HILL
• HILL (0. Eng. hyll; cf. Low Ger. hull, Mid. Dutch hul, allied to Lat. celsus, high, collis, hill, &c.)
• HILL, A
• HILL, AARON (1685-175o)
• HILL, AMBROSE POWELL
• HILL, DANIEL HARVEY (1821-1889)
• HILL, DAVID BENNETT (1843–1910)
• HILL, GEORGE BIRKBECK NORMAN (1835-1903)
• HILL, JAMES J
• HILL, JOHN (c. 1716-1775)
• HILL, MATTHEW DAVENPORT (1792-1872)
• HILL, OCTAVIA (1838– )
• HILL, ROWLAND (1744–1833)
• HILL, SIR ROWLAND (1795-1879)

Hill; and according to the same authority the See also:

• INVERNESS

Inverness earthquake of the 18th of September 190I was referable to movement along a fault between See also:

• LOCH, HENRY BROUGHAM LOCH, 1ST BARON (1827-1900)

Loch Ness and Inverness . The South See also:

• WALES (Cymru, Gwalia, Cambria)

Wales earthquake of June 27, 1906, was probably due to movement connected with the Armorican system of folds, striking in an east and west direction . It may be noted than when a slip occurs along a fault, the displacement underground may be but slight and may See also:

• DIE
• DIE (Fr. de, from Lat. datum, given)

die out before reaching the surface, so that no scarp is formed . In connexion, however, with a seismic disturbance of the first magnitude the superficial features may be markedly affected . Thus, the great Japan earthquake of October 1891—known often as the Mino-Owari earthquake—was connected with the formation or development of a fault which, according to Professor B . Koto, was traced on the surface for a distance of nearly 5o m. and presented in places a scarp with a vertical throw of as much as 20 ft., while probably the maximum displacement underground was very much greater . Although most earthquakes seem to be of tectonic type, there are some which are evidently connected, directly or indirectly, with volcanic activity (see See also:

• VOLCANO

VOLCANO) .

Such, it is commonly believed, were the earthquakes which disturbed the Isle of See also:

• ISCHIA (Gr. II19r)KO"va, Lat. Aenaria, in poetry Inarime)

Ischia in 1881 and 1883, and were studied by Professor J . See also:

• JOHNSTON, ALBERT SIDNEY (1803–1862)
• JOHNSTON, ALEXANDER (1849-1888)
• JOHNSTON, ALEXANDER KEITH (1804-1871)
• JOHNSTON, ARTHUR (1587-1641)
• JOHNSTON, J
• JOHNSTON, JOSEPH EGGLESTON (1807-1891)
• JOHNSTON, SIR HENRY HAMILTON (1858- )

Johnston-Lavis and G . Mercalli . In addition to the tectonic and volcanic types, there are occasional earthquakes of minor importance which may be referred to the collapse of the roof of British earth-quakes . caverns, or other falls of rock in underground cavities at no great depth . According to Prof . T . J . J . See most earthquakes are due, directly or indirectly, to the explosive action of See also:

• STEAM
• STEAM (0. Eng. steam, vapour, smoke, cf. Du. steam; the origin is unknown)

steam, formed chiefly by the leakage of sea-water through the ocean floor . Whatever the nature of the impulse which originates the earthquake, it gives rise to a series of waves which are propagated through the earth's substance and also superficially . In Earth- one See also:

• KIND (O. E. ge-cynde, from the same root as is seen in " kin," supra)

kind, known as normal or condensational waves, quake or waves of elastic See also:

• COMPRESSION

compression, the particles vibrate waves .

to and from the centre of disturbance, moving in the direction in which the wave travels, and therefore in a way analogous to the movement of air in a See also:

• SOUND
• SOUND, THE (Danish Oresund)

sound-wave . Associated with this type are other waves termed transverse waves, or waves of elastic distortion, in which the particles vibrate across or around the direction in which the wave is propagated . The normal waves result from a temporary change of See also:

• VOLUME

volume in the See also:

• MEDIUM

medium; the transverse from a change of shape . The distance through which an earth-particle moves from its mean position of See also:

• REST (O. Eng. rest, reste, bed, cognate with other Teutonic forms, e.g. Ger. Rast, Riiste, rest, and probably Gothic Rasta, league, i.e. resting or stopping place)

rest, whether radially or transversely, is called the amplitude of the wave; whilst the double amplitude, or total distance of movement, to and fro or up and down, like the distance from See also:

• CREST
• CREST (Lat. crista, a plume or tuft)

crest to trough of a water wave, may be regarded as the range of the wave . The See also:

• PERIOD
• PERIOD (Gr. irepioSos, a going or way round, circuit, aepi, round, and &Sis, way, road)

period of a wave is the time required for the vibrating particle to See also:

• COMPLETE

complete an oscillation . As the rocks of the earth's crust are very heterogeneous, the earthquake-waves suffer See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

refraction and reflection as they pass from one rock to another differing in See also:

• DENSITY (Lat. densus, thick)

density and See also:

• ELASTICITY

elasticity . In this way the waves break up and become much modified in course of transmission, thus introducing great complexity into the phenomena . It is known that the normal waves travel more rapidly than the transverse . Measurements of the surface See also:

• SPEED, JOHN (1552–1629)

speed at which earthquake-waves travel require very accurate time-measurers, and these are not generally available in earthquake-shaken regions . Observations during the Charleston earthquake of 1886 were at that time of exceptional value, since they were made over a large area where standard time was kept . Lines drawn through places around the epicentre at which the shock arrives at the same moment are called coseismal lines . The motion of the wave is to be distinguished from the movement of the vibrating particles .

The velocity of the earth-particle is its See also:

• RATE

rate of movement, but this is constantly changing during the vibration, and the rate at which the velocity changes is technically called the See also:

• ACCELERATION (from Lat. accelerare, to hasten, celer, quick)

acceleration of the particle . Unfelt movements of the ground are registered in the earthquake records, or seismograms, obtained by the delicate instruments used by See also:

• MODERN

modern seismologists . From the study of the records of a great earthquake from a distant source, some-times termed a teleseismic disturbance, some interesting inferences have been drawn with respect to the constitution of the interior of the earth . The complete See also:

• RECORD (Lat. recordari, to recall to mind, from cor, heart or mind)

record shows two phases of " preliminary tremors " preceding the principal waves . It is believed that while the preliminary tremors pass through the body of the earth, the principal waves travel along or parallel to the surface . Probably the first phase represents condensational, and the second phase distortional, waves . Professor Milne concludes from the speed of the waves at different depths that materials having similar physical properties to those at the surface may extend to a depth of about 30 m., below which they pass into a fairly homogeneous See also:

• NUCLEUS (Lat. for the kernal of a nut, nux, the stone of fruit)

nucleus . From the different rates of propagation of the precursors it has been inferred by R . D . Oldham that below the outer crust, which is probably not everywhere of the same thickness, the earth is of practically See also:

• UNIFORM

uniform See also:

• CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character to a depth of about six-tenths of the radius, but the remaining four-tenths may represent a core differing physically and perhaps chemically from the outer part . Oldham also suggests, from his study of oceanic and See also:

• CONTINENTAL

continental wave-paths, that there is probably a difference in the constitution of the earth beneath oceans and beneath continents . The surface waves, which are waves of great length and long period and are propagated to great distances with practically a See also: