EARTHQUAKE SWARMS

Wairarapa Times-Age, 27 March 1939, Page 9

EARTHQUAKE SWARMS

MONTH OF BIG SHOCKS SUNSPOT CYCLE AT WORK. STEPS IN PRESENT-DAY KNOWLEDGE. The earthquake tells us a great deal about the earth, reserving only that about itself which is more interesting than anything else, namely, when the next earthquake will occur, writes E. S. Grew, in the "Observer," London.. About the place where it will happen it is less reticent. Many years of the accountancy of earthquakes which Joint Milne set in motion have yielded precise information about the zones all over the globe where the most violent of them ail will have their centres.

The earthquakes themselves have been made to tell by the measuring instruments of observatories where in the depth of the earth's crust some breakdown has set them in motion, and how big the break is. These are the first steps on the long road to the knowledge of when and. where the next breakdown will take place. They do not seem to carry us very far, but some progress has been made. MEANS OF FORECAST. At present the only forecast of a strong earthquake to come is afforded by the occurrence of what are called earthquake swarms, comparatively inferior disturbances which have been found to precede some the disastrous earthquakes of Japan.

These swarms have sometimes gone on for several years beforehand, and point to the slow accumulation of strains in the crust which will at last declare themselves in the collapse of some faulty balanced crustal arch of rock.

Dr Charles Davison, who for fifty years of his long life has been one of the accountants of earthquakes, employs a less empirical method of forecasting, though the periodicities he endeavours to establish are of a very modest kind. He begins with an assumption something like that which explains earthquake swarms. When the earth's crust is about to give way, a slight force acting in one direction may start it. or acting in the opposite direction may stop it or delay it. If the magnitude of. this _ outside, force is itself subject to periodic variation. then the variations may be impressed on, or reflected by, the frequency of earthquakes in 'an earthquake zone. Not all earthquakes but on those which lend themselves to observation and classification.

The classification of earthquakes divides them into those which shake the whole earth and those which disturb only a part of it, but there are a number of subdivisions. Destructive earthquakes are of three grades—those which destroy towns, those which break down bridges, and those which, leaving the roofs intact, crack walls and ceilings. Dr Davison’s records show that for all-world earthquakes registered at the seismological observatories as well as for great earthquakes, the most likely month of the year is July. “MILD” EARTHQUAKES. If the earthquakes are further classified so as to take in those which exhibit themselves first on land, or arise under the sea; or those which begin in the North or Southern Hemisphere, then mid-June to the end of July is the epoch claiming the large ones, and the end of May or September the ordinary or less spectacular ones. But, except in China, the epochs of earthquakes of the highest and most destructive density usually fall in winter, December in the Northern Hemisphere and the end of June in the Southern.

Some exceptions offer themselves foi examination or criticism, especially among the mild earthquakes, of which, neglecting the 60.000 nervous quivers agitating the earth's skin, there are ye’, hundreds every year.

These, as well as the world’s seawave earthquakes (which are not tidal waves) most often fall in October; ano the deep focus earthquakes 'which originate lower down in the earth’s crust than' the granite layer of twelve to eighteen miles thickness, and sometimes far down in the basaltic foundations. exhibit a preference for the end of May. Dr Davison then examines what are the possible outside causes which might account for this periodicity.

Enlarging the scope of his periodicities at least as far as they will legitimately go, he further examines a 11year period corresponding to the ascertained sunspot cycle; and a 19-year-period closely connected with the nutation of the earth. effect of rainfall. By nutation, which is another word for nodding, is meant the wobbling of the earth’s axis, which is revealed by the movement of the pole as it describes an irregular circle in 19 years. Such a movement will be expected to put strains on the interior of the earth’s crust at more or less regular intervals as they accumulated.

They await further, examination of the earthquake records. The 11-year sunspot cycle is more responsive. In the 580 years from 1305 to 1888. there have been 53 11-year periods, in which any influence exerted would be the same for all the earth.

The records offered by. the chroniclers of China and Japan exhibit what may be taken as a periodicity of notable earthquakes, disastrous or destructive, corresponding roughly will-’, the appearance of the greater areas of sunspots. Dr Davison’s interpretation is that the sunspot cycle corresponds to an increase of earth rainfall when the sunspots are at their maximum; and that this additional fall o? moisture facilitates movements of tte earth’s crust in either direction.