TIDAL WAVES.

Hokitika Guardian, 14 September 1923, Page 4

TIDAL WAVES.

HOW THEY ORIGINATE. OLD DISTURBANCES RECALLED. WELLINGTON, Sep. 0. The Dominion Nautical Adviser, Captain (I. S. Hooper, lias given the “ Post some interesting information regarding what are called “'tidal waves," hut more scientifically seismic

waves, At Lyttelton, on August loth. Rfi?. at 8.30 a.in., the water receded limit the harbour, rushing past shipping in the stream and at anchor neir the entrance at a great velocity. .011 hotu later tlm end of the breakwater, v. lu re the depth usually was fifteen feet, became dry. At 1.30 n.m.. the water Returned in the form of a lotid-roaimg wave eight feet high, rushing up the harbour, lu twenty minutes it rose to within three feet of the railway level, three feet above the highest springs. Lyttelton harbour was quite dry from wharf to Officer’s Point, and vessels were lying 011 their sides. The immense incoming wave tore vessels from the wharves and did much damage. Similarly. though diminishing, . oscillations continued for two days, normal conditions reappearing on the fourth day.

Simultaneously, tidal disturbances occurred at Bluff, Otago Harbour, Oanniru. Timarii, Nelson, Wellington, Napier, Opotiki. and Mongonui. At Oamaru, -the tide fell eight feet below low water, and changed its perpendicular height fifteen feet at intervals of Id minutes. Timarii experienced a sudden rise .of six feet, and a fall four minutes later to an abnormally lowlevel. At Nelson the tide was reported to have entered over the Boulder Bank, and at Wellington. Napier, and Opotiki a wave six feet high entered the harbour. The disturbance was also felt at Sydney, and some damage resulted at Chatham Islands. HOW THE WALES ORIGINATE. There are two general classes of seismic seva waves. The first is due to the sinking of the sea bottom, anil is characterised hv a withdrawal of the water after the earthquake, to lie followed later by a return of a great wave, and the second is due to the uplift of the bottom, and is characterised by the sudden rise ol the sea without any previous withdrawal from tiie .shore. Most of the historical inundations nf the sea have been due to waves of the first class. Ihe phenomena usually noted are: (1) An earthquake. (O) After a short interval the sea is noticed to be draining away, laying bare the bottom where it is ordinarily

deep enougu uir snips to inn ■ • (3) After an interval of an hour or so. the sea is seen to bo returning as a mightv wave, washing everything before it, and thus carrying ships shoreward and stranding them. (!) Having once swept the shore, the sea again withdraws and lays bare t}>o harbour as before, and, after about the same interval, again returns as a second tidal wave. This periodic movement of the sea may be kept up for quite a while, and sometimes quiet is not restored lor a dav of two. Historical sea waves ~re those which overwhelmed He!ike in 137:5 8.U., the waves of Callao in 1724 and I Mb. the wave following the Lisbon earthquake in (Too. the waves of! iii-i. .. in laitss li.ii'itiKt in Is. t nnil

the Japanese wave in l8!Ki. In all 1 here cases 11 10 water lirsl withdrew froli; the shore —not suddenly. hut lowly, ns in llio drawing away of a titlp, though somewhat more rapiillv. The must disastrous of the waves of the second ciass (due to an uplift in the sea bottom) was that u Inch occurred in connection wit It the Lisbon earthquake of lion. This caused the death of 100.01)!) persons and enormous damage to property. A I.KSSOX TO MAUTXKICS. The lesson to he impressed upon mariners in the light of such happenings as have been referred to above is that the chances of safety to shipping arc much increased by proceeding to sea. if practicable, immediately after the occurrence of an earthquake, and this, according to the I’nitod States ! lydmgrapher, i- especially to he enjoined if the water of the harbour is found to lie draining away after the shocks of the earthquakes have been felt. In the open ocean, seismic sea waves are not generally dangerous to vessels, because their very great length ami moderate height give them a gentle slope, and their period is very much longer than that of the ordinary sea waves, i’y a study of the charts o! deep-sea soundings throughout- the world, it can lie told quite closely what the expected state of seismic activity of anv region should he. Although too Mediterranean Sea. the Azores, the West, Indies, and the West Coast of Sont.li America are districts of seismic activity, it is generally recognised by investigators that the most active regions of seismic disturbances are along the Aleutian, Kurile, and Japanese islands, and in the Hast Indies.