COAST ARCHED

Hokitika Guardian, 6 March 1931, Page 5

COAST ARCHED

LAND MOVED IN QUAKE. GEOLOGISTS’ REPORT. CENTRE OF DISTURBANCE. ■After the earthquake in Hawkes La v ’oil i toniar y isx . rieiuiciswj amt Messrs Al. Ongity and L. l. Giange, oi ;he »>ew /jwiunu. c.a w-j iv.,, sOiihj time in area man mg an invCsugat-.un as iu "n H L geoing.cai ueauci-.0.,s were to i>o oouuu-od iroiit cue enecis of uie pneiiv*. iiienun. J lieu- oysti vutions itnoy nave enmouied I.i a report to cue Prime -..juis.ei, x»t. non. u. \v. jrorbes anti iota ms latter lias released, lor publication. "rroin wnat is known of the cause of eaiLiiqutikes geuerai.y and from the facts deserved it seems reasonable lo suppose that the pressure miges main tne surface trace or an important eaith lracLiue along winch the mb\m-nm-c o..u.nig itiie *vapicr earuiquaite took place, .states the report, “oince the ground on tne north-west side of the rraeture is assured ana swollen anu that on the south-east- side unaifccted it appears probable that the - caiifi block on the north-west side moved, j The movement was toward the -easit, the horizontal component in this area being aoout pic. aim uie vertical auout 2tt. Probably also movement- occurred along several of the weak' layers, for the pressure ridge on the Turamoe estate, though parallel with, is certainly different from that aloug the Poukawa, This in turn suggests that the , movement was distributed over a zone perhaps several miles wide.

‘•Undoubtedly the main fracture passed east- of Napier, possibly a subsidiary one extends past Taradale and along tile coast- near Petane ana Tango io, localities where the shocks were severe. Certainly the line of fracture if produced passes through or very close to the principal earthquake centres in Hawkes Bay ’ determined by seismographs, and there is pronounced uplift of the sea floor and of the coast, west, of' the produced line. CHANGE IN COAST- LEVEL. “So many factors influence the height of the tide that it is difficult to determine a change of a- few feet in the level of the coast. No change was detected at the mouth of * the Ngarurore. At Napier the rise is oft. to 6ft., and this decreases southward. At Petane it is of the order of lift to Bft. and at Wliakaari, a few miles north, perhaps 7ft. Residents at Mohnka mouth estimate the rise at about a foot., and.the unusual amount of fine material on the beach supports this conclusion. The harbourmaster at Wairoa is sure there is no change in level. These scanty observations make it appear that the coast has arched gently upon a north-east line, the crest of the arch being north of Napier. Assuming that the amount of coastal uplift is a measure of the violence of th e earthquake, the centre of the disturbance was some runes north-east of Napier, below the floor of Hawkes Bay.

“ft is generally assumed,” concludes the report, “that a study of the apparent intensities of an earthquake shock as indicated by the degree of damage suffered at different points within the area of vigorous activity will define a central track of maximum effect, surrounded by zones of 1 less and less vigorous action, diminishing outward. The central track is assumed to contain the epicentre. Man-built structures are not evenly distributed over the shaken area, but an approximate idea of the position of the principal focus may be formed from the degree of damage done *n different localities. “Napier suffered greater damage from the earthquake than Hastings, which, in turn, was much more in jured that Wnfpawa. and the mst. mentioned town suffered less than Wairoa. If it be assumed that the intensity at Wairoa and at Te Aute halfway between Hastings and Wains wa. was approximately equal, and if it be assumed that the earthquake was due to movement along a fracture in the earth’s crust extending northeast from Te Aute. the centre of the disturbance was probably on this fracture at a point about ten miles northeast of Napier.”