608
EARTHQUAKES
single month, whilst now there are but one or two per course, be regarded as mere coincidences, but when we year. Earthquake frequency varies with time. A district consider volcanic and seismic activities as evidences of under the influence of hypogenic activities reaches a con- physical and chemical changes, together with mechanical dition of seismic strain which usually is relieved rapidly displacements of a magnetic magma, it is reasonable to suppose that they should have at least a local influence at first, but subsequently more slowly. The small shocks which follow an initial large disturb- upon magnetic needles. Another form of disturbance to ance are known as after-shocks. The first shock which in which magnetic needles are subjected is that which accom1891 devastated Central Japan was accompanied by the panies the passage of large earth-waves beneath certain formation of a large fault, and the 3364 small shocks observatories situated at great distances from earthquake which succeeded this during the following two years are origins. At Utrecht, Potsdam, and Wilhelmshaven the regarded as due to intermittent settlements of disjointed magnetographs are frequently disturbed by seismic waves, material. The decreasing frequency with which after- whilst at many other European observatories such effects shocks occur may be represented by a curve. Dr F. are absent or only barely appreciable. To explain these Omori points out that the continuation of such a curve marked differences in the behaviour of magnetic needles gives the means of determining the length of time which at different stations we are at present only in a position will probably elapse before the region to which it refers to formulate hypotheses. They may be due to the fact will return to the same seismic quiescence that it had that different needles have different periodic times of oscillation; it is possible that at one observatory the prior to the initial disturbance. The positive results that we have respecting the period- mechanical movements of the ground are much greater icity of earthquakes are but few. Generally than at others; we may speculate on the existence of Periodearthquakes are somewhat more frequent during materials beneath and around various observatories which iclty ' winter than during summer, and this applies are different in their magnetic characters; and, lastly, we to both the northern and southern hemispheres. This may picture a crust of varying thickness, which from time annual periodicity, which, however, does not show itself to time is caused to rise and fall upon a magnetic magma, if only destructive earthquakes are considered, finds an the places nearest to this being the most disturbed. A subject to which but little attention has hitherto explanation, according to Dr Knott, in the annual periodicity of long-continued stresses, as for example those due been directed is the effect which displays of to the accumulation of snow and to barometric gradients. seismic and volcanic activities have had Uma For certain earthquake regions there appears to be a dis- upon the human mind. The effects are dis- nJn(] tinct semi-annual period for which no satisfactory explana- tinctly dual and opposite in character. In tion has yet been adduced. Although the elaborate countries like England, where earthquakes are seldom registers of Japan, which have enabled us to group earth- experienced, the prevailing idea is that they are assoquakes according to their respective origins and varying ciated with all that is baneful. For certain earthquakes, intensities, and to separate after-shocks from initial dis- which fortunately are less than 1 per cent, of those turbances, have been subjected by Dr Knott to most which are annually recorded, this is partially true. A careful analysis, with the object of discovering periodicities disastrous shock may unnerve a whole community. Many connected with the ebb and flow of the tides, the lunar become hysterical, and all are seized with terror, behaving day or lunar months, nothing of marked character has for a time as if they have lost their reason. While the been found. Certainly there is slight evidence of a period- shakings continue, the “ Ora pro nobis,” or its equivalent, icity connected with the times of conjunction and opposi- will be heard on many sides. Effects of this nature, howtion of the sun and moon, and a maximum frequency near ever, differ in a marked manner with different nationalities. the time of perigee, but the effect of lunar stresses is After the shock of 1891, when Japan lost 9960 of its comparatively insignificant. Ordinary earthquakes, and inhabitants, amongst the wounded indications of mental especially after-shocks, show a diurnal period, but we can- excitement were shown in spinal and other trouble. The not say that there are more earthquakes during the night cases of tetanus which were found may be attributed to contamination of open sores. Those who had escaped than during the day. Many experiments and investigations have been made unhurt and were living in ruined buildings, on hearing to determine a possible relationship between earthquakes the sounds which preceded very many hundreds of afterand electrical phenomena, but beyond drawing attention shocks, and realizing the possibility of a repetition of what to the fact that luminous appearances may accompany the had gone before, simply rushed for the open, to return friction of moving masses of rock, and that a temporary a minute or so later laughing and joking at the continued current may be established in a line by the recurrence of so many false alarms. Notwithstanding the Magnetic disturbance of an earth plate, these inquiries lightheartedness of this particular nation, it is difficult phenojiave yieidgd little of importance. The to imagine that the long series of seismic effects chronicled inquiries respecting a possible relationship in Japanese history, which culminated in 1896 in the loss between adjustments so frequently taking place within of 29,000 lives by sea-waves, has been without some effect and beneath that region called the crust of the earth and upon its mental and moral character. Several earthmagnetic phenomena are, however, of a more promis- quakes are annually commemorated by special services at ing nature. We have seen that at or near the origin temples. In bygone times governments have recognized of earthquakes which for several hours disturb conti- earthquakes as visitations of an angry deity, whom they nents, and occasionally cause oceans to oscillate for longer have endeavoured to appease by repealing stringent laws periods, we sometimes have direct evidence of the bodily and taxes. In other countries the sermons which have displacement of many cubic miles of material. When this been preached to show that the tremblings of the world material is volcanic it is almost invariably magnetic, and were visitations consequent on impiety, and the prayers we perceive in its sudden rearrangement causes which which have been formulated to ward off disasters in the should produce magnetic effects within an epifocal district. future, far exceed in number the earthquakes which gave In Japan, where attention is being directed to phenomena rise to them. In 1755 many of the English clergy held of this description, not only have such effects been observed, the view that Lisbon was destroyed because its inhabitbut unusual magnetic disturbances have been noted prior ants were Catholics, whilst the survivors from that to the occurrence of large earthquakes. These may, of disaster attributed their misfortune to the fact that they
