N.Z. EARTHQUAKES.

Hokitika Guardian, 1 December 1925, Page 4

N.Z. EARTHQUAKES.

INTEBESTING DIAGRAMS. FREQUENT TRE.AIORS SPELL ! SAFETY. I

A novel and extremely interesting series of diagrams is being prepared by tbe. Government Seismologist (Dr Adams) for the Dunedin Exhibition. The information, conveyed in a form readily assimilable by “the man in tbe street.” should have n reassuring effect upon those who look fearfully upon the thermal activities of parts of the Dominion, because they show, its |)r Adams points out, that the readjustments of this part of the earth’s crust are spread very evenly over the year, anil, for that matter, over the years also. “The theory of earthquakes,” said Dr Adams to a “Post” reporter, while explaining the diagrams iit everyday language ""for the benefit of the general public, “is that they tiro due to readjustments occasioned by changes in tiie earth’s crust. Where these iead i ust men ts are delayed Irom one cause or another, the ensuing tremor or quake is intensified. It will bo seen from the diagram that the readjustments in the ease ot New Zealand are frequent,, ami lin-reiore loss severe in their effect.” THE INTENSITY SCALE. The Bossi-Fore! scale, by which the intensity of the shock is determined by its effect upon houses and their (•(intents, and the observations of the occupants, is that used in the diagrams. Friday night's tremor is reckoned at 0 degrees of intensity. The scale runs to Iti degress, at which buildings are demolished, strata a fleeted, and general destruction of property follows: but the fact that o is half of JO does not imply that a shock of strength 5 is half as intense its one of strength 10. The fraction is much

The Bossi-Fore! scale is as lollnws: -- (l) Miseroseismie-hock : Becnrded by a single seismograph or by seismographs of tbe same model, lull not h\ seismographs oi dtllerenl. kinds: toe shock 101 l by an experienced observer. (■_!) Extremely feeble shock: Hceordei| ),y several seismographs ol diflerent, kinds; felt liv it small number of persons at rest. G.l) Aery feeble shock: Kelt by several persons at rest; strong enough lot the direction nr duration to he appreciable.

( I) Feeble shock: Fell h.v persons in million; disturbances of movable object-, doors, windows; cTenkitigs ol ceilings. (5) Shock of moderate intensity: l'cll generally bv everyone; disturbance of furniture, beds, etc.; ringing of swinging hells. ((if Fairly strong shock: General awakening of those asleep; general ringing; of house hells; oscillation ot chandeliers: stopping of pendulum clocks; visible agi tat ion of trees and shrubs'; some startled persons leave their dwellings. (7) Strong shock: Overthrow of movable objects: fall of plaster: ringing of church hells; general panic, withouf damage to buildings. (S) Yerv strong shock: 1-all ol chimneys: cracks ia walls of buildings. (It) Extremely strong shock: Partial or total destruction of some buildings.

(JO) Shock of extreme intensity: Great disaster; buildings ruined; disturbance ol the strata: fissures in the ••round- rock-falls from mountains. DISTBTBETfON OE SHOCKS.

Becords of these diagrams were first taken in 1021. and yearly records have been taken since.

The lirst diagram. showing Ihe intensity nl' the shock ami tile month in which it occurred, li.v means nl' vertical lines mi a horizontal scale (it twelve meiitlis. will he most valuable, niter a number nl years, in proving the much-discussed juiinl <1 seasonal ITof|i:oncy nl earthquakes. I i'e veals ItT’l i'l 1 022 shew greai er seismoluuical activity than their successors. In June and September cl 1021. and ill June. July and December of lll'J'J, there wcrii shocks of 8 degress c.l intensity. The years 1028, 1021 and 1825 show much less activity, except in the ease of a shock of 8 degrees severity ill April. 1025. The strength of 8 degrees, according to the scale is “very sLrdntr shock, the falling of chimneys, with cracks in the walls of buildings.” The chief impression conveyed by this diagram is of the genera! similarity of intensity id shucks, and tllcir regularity of incidence. 'I here tire spines in each year where no shocks were leeoided. but i.f small extent only. The second diagram exhibited shows the areas over which shocks were 101 l in various parts of New Zealand, by means ol circles. ! In* North Island, especially about its centre, shows the greatest activity. A not Iter diagram shows the extent to which individual shocks wore tell by the Itcssi-Koie! scale. One on June •J<). |!)21, for instance, was of eight degrees of -evenly ill a hell of country from Wanganui to Napier and 1 dshornc. lapi’iing in intensity to 8 degrees at bum-din and in the ex I rente Hurt h. WolJ I.D-SI lAK INO IWMNTS. Another diagram, taking in the u hole of the earth's surface, shows i lie points of origin of world-shaking earl Inplakes for some ten years, as recorded by the insl rumen Is ol the New Zealand Department. Records from various points of observation i.i New Zealand are collated, and comparison shows | lie exact point of the disturbance. The crust of Tibet and its vicinity is shown to he weak, hut the majority of the disturbances, indicated 'h,v small circles til the point ol greatest intensity, tire i lustercd along the coasts o| the continents bordering the I’aeilh . There is a theory that this preponderance of reatl.jusl.mmit on these great coastal lines is caused by the gradual removal of soil from the great mountain ranges and the detritus gradually being piled through the cent lilies on the seafloor in the deep water adjoining t>.csc continents; hut. they would not account, said l)r Adams, for the remarkable activity ill I lie narrow Mexican and Isthmian coastline, nor. while the theory is applicable to vertical readjustments, does it, explain the lateral readjustments which i eettr. Australia is almost immune from world-shaking shocks, while New Zealand is The place of origin of practically none. The diagrams, which will he exhibited the the Winter Show in Wellington ; after the Kxhibitiou. are the work o Mr R. ('. Kayos, assisted by Mr If Holt. Though difficult to describe they are easily read by anyone.