SEISMIC SEA-WAVES.

Dominion, Volume 2, Issue 484, 17 April 1909, Page 13

SEISMIC SEA-WAVES.

HOW CAFL'AIiNS SHOULD ACT.' '. INTERISTING GAZETTE NOTICE. " . Very interesting . to ship captains and others is an article- just published in the Gazette dealing with the Calnbrian and other earthquakes, and showing how certain perils to mariners may be avoided if only action is taken in time. Says the.-..Ga-zette : "The following information has .been, received from the U.S. Hydrographic Office, and the lesson to be learnt from it in an ocean-bound volcanic ccuntry like New Zealand might be valuable not only to mariners, but; possibly to landsmen also living near to the coast. On-Monday morning, December 28, • between 5 and 6 o'dock, all Italy, was;; shaken by severe earthquakes. : The 3eismic disturbance was most violent in Calabria and Sicily, and the great loss of life and destruction of property which occurred were accompanied by marine : disasters caused -, by seismic sea-waves. :According to the descriptions that have been given, after the earthquake the eea at first receded for some distance from tho shores of Calabria' and Eastern Sicily,'and then a great wave 1 32ft. high advanced upon the shore with appalling power ,and destructiveness. This is- the usual march of. events, in connection with far the greatest number and most dangerous kind of seismic sea-waves, and hence it: is important that the attention of mariners should be drawn to such characteristics of the phenomena as will lead to the: taking ; of adequate precautions against their .effects. Great' multitudes of people live in; cities by the ] sea and are subjected to the dangers arising from seismic sea-waves,, and yet, if a place of refuge exists, 'they usually have ample time to escape;"and even the ships in the' harb'eur will. generally he safe if they promptly put to: sea on tho. nrst sign, after the earthquake, of the withdrawal of the water, which always indicates that the sea-bottom'has sunk. Thus by the ™ ld y,.,°f the laws of the physical world andthe pf the resulting useful know-' ledge a great.; measure of safety may be' secured, from' somo of the:'most 'dreadful forces, m' nature, and iri the courso of time the interests of civilisation "may be enor-> .mpnsly conserved; ::,,"': '•■ ~.; ; ; ■; "There are two general classes of seismic sea-waves. Tho, first is due to the ■ sinking of the sea-bottom, and is characterised by ,a withdrawal.of. the water after the earth-' quake, to be followed later by the.return of a great wave: and the second is due to the. uplift of the bottom, and is characterised by the sudden rise bf the sea'without any previous withdrawal from the shore. The phenomena usually noted are: (First, an earthquake; second, after a short interval, the sea is noticed to be draining away, laying bare ; the bottom where it is ordinarily ..deep'enough , for , ships to anchor;;third, after an interval of an hour.or so,-the-sea is seen to be returning as a mighty: wave, washing ■everything ■, before it,' and thus carrying ships shoreward and stranding them; fourth: havUig once-swept the elio , re, < the:sea again withdraws and lays bare the harbour as before; arid after about-the same interval ; again returns great wave., This periodic movement of the sea. may be kept .up. for quite awhile, and sometimes quiet is not restored ifor.a day ortivo. ■■■.'■• "■■■.- '■•■ ■' ; '•■-■. ■ ■.

.■.-..-,!'ln.all historic cases'*the-water first withdrew from the shore-r-not but ftowly. as in the drainirig-away of a tide, •though .somewhat more rapidly. This! of course,' indicated that the sea ; bottom had sunk, and; the water was draining away tb fill up the depression in the level caused by tho'falling of the'bottom. . When-the Currents meet at the centre an"'eleyatibn is produced by their mutual, impact, and when this collapses under, gravity the.first great wave comes ashore. Ife elevation then subsides into a depression as at firsthand the ..currents again flow in and force-up'the level a second time; and, with the second collapse, another wave is sent ashore; .and' so the osculation of the sea continues, 'sometimes for a day- or two,' before it■.-' finally quiets down. ■'-: .':.'■-..' -

: , 'Seismic sea-waves of the second class are produced by the uplift of the sea-bottom into ridges -.or submarine plateaux and islands. In such cases the water rises suddenly with-' out nreviously withdrawing from the shore. _? 'The lesson to/ bo impressed upon mariners m'the light of these experiences is that the chances of safety, to shipping are inuch increased, by proceeding to sea, if practicable, immediately after the occurrence of an earthquake; and this is especially to bo enjoined if the water of.the harbour is found to be draining away after the shocks of earthquake have been felt.. In the open ocean seismic sea-waves; are not generally dangerous to vessels, because their very great length and modorate height gives them a gentle slope, and'their period is very ; much longer than that of ordinary, sea-waves. .... "By a'study of the charts of deep-sea soundings throughout the world it can'be told -quite closely what the expected state of seismic activity of any given region should be, for wherever there are very deep oceanic troughs; situated adjacent to mountainous ridges, culminating cither above or. below the'surface of the'ocean, disturbances of the crust of the earth are likely to be prevalent. In these regions the forces of nature are, iri fact, generally engaged in building up the ridges by forcing under them material" pushed from beneath the 'undermined floor of the adjacent deepening trough."