QUAKE OVER

Rotorua Morning Post, Volume 2, Issue 337, 26 September 1932, Page 7

QUAKE OVER

(Special to the "Post.")

GEOLOGIST'S Vi*EWS MAIN MOVEMENT IS PROBABLY NOW ENDED. ROTORUA NOT AFFECTED.

Wellington, Sept. 23. Dr. J. Hendeson, Director of Geological Survey, has supplied the following notes upon th'e prohable origin of the .earthquake shocks and the direction of "faiilts" in the Hawke's Bay and Poverty Bay areas. The recent earthquake indicates that-k the great north-north-east-striking .earth fold stretching through the South Island and the easterh part of the Nortlr Island, hn'd 'extendihg some 1600 mile'5 across the flooi' of th'e Pacific to Tohga, is still gt'owing. The si'nking of the South Pacific basin is forcing up this great structure. The fold is not a simple regular Up-bowing of the earth's crust, for more ocr less parallel niinor f olds wrinkle its surface and the brittle rocks n.re brolcen into nuny earth-blocks. Earthquakes occur v/h'en these blocks slip on ofie another. Ili the North Island, the fold c'onsists of three majSr seCtions named, from north to south, after the inountains on its erest: The Raukumara, Huiarau-Kawelca, and Ruahine-Tara-rua seetions. Though broadly similar in. structure, some differences distinguish each section. The HuiarauUaweka yection, which includes the Hawke's feay consists of a trbugh 100 miles lohg and 20 mlles wide between the highlands on th'e west and the broad up-fold> or anticline, that forms the hills between the W aipukurau-Hastings railway and the coast. This fold can be recognised north of Hawke's. fiay between Wairoa and Mahia, and extends twenty miles north from the sliOre.

Th'e 1931 att'd 1932 Disturbances. The crustal adjustments eausing the Hawke's Bay earthquakes were along fractnres on, or near, the west fiarilc of this up-fold. The most important movements occurred on a firacture, or group of fractures, extending north-■north-east from Te Aute, across the Napier fiats, and under the floor of Hawke's Bay. Other shakes origihated from a fracture under the bay marked by a line of epicentres stretching south-sOuth-west frOm near the mouth of the Nuhaka River. Last year at Hangaroa, close to the boundary of the Huiarau-Kaweka, and Raukumara seetions, a mass of mud was forced out alpng a fault, and a similar out-squeczing of mud occurred ftt Gisborne, aiid this suggests some crustal adjustment in the Raukumara section. Ne'vertheless, few earthquakes, and none of them severe, had origins in the Raukumara section while the crustal adjustments were taking place actively in the Hawke's Bay area. The recent earthquake and the foreshocks on the "preceding day seem to be definitely connected with the Raukumara section. The relieving of th'e stresses in the Hawke's Bay region probably increased the pressures in the adjacent area to the north; henee thc present adjustments. Dr. Adams' preliminary determinations place the initial movement out to sea off Waipiro, or Open Bay, but the earthquake was most violent about Gisborne and Wairoa. This suggests that the movement extended south along a fracturezone, and was greatest 60 to 90 miles south of Waipiro Bay. Possibily, of course, the shock from near Waipiro caused an unstable block in the Hawkes Bay area to move. Main Movemient Probably Over. Probably ihe principal movement has- now taken place, but the equilibrium of the whole system of earthblocks has been disturbed, and aftershocks, produced by minor adjustments, will continue for months. The mapping of the epicentres of these may give some indication of the course of the faults on the ssa-floor or on the land. As is usual, the greatest material damage seems to have been to buildings erected on alluvium' near river channels. V/hel'ever such ground is unsupported, standing at the angle of repose for the material and the existing conditions, earthquakes open fissures and bring about slumps. In the Napier flats fissures paralleled the river channels, and were particularly noticeable along the Tutaekuri at Napier. There at some points the ground moved toward the river channel as much as twelve feet in a few chains, displacihg houses and rupturing sewer and water pipes. The latest news is particularly intehesting from a geological standpoint. A fault, much less important than many others in the region, sti'ikes north-east for teil miles from a point a niile north of Whakaki. If the earth-blOck on th'e north-west side of this fahlt is 'active the distribution of th'e dahiage would be well a'ecotmted for. The fault crosses the failway west of Whakaki, where the line is badly buekled. The Mangapoike and Tukemokihi country are on the active block, as also are Waitoa and Marumaru. Miahia is part of the stable country south-east of the fault. Nuhaka, like Whakaki, though on the "stable" side of the fault, is uncomfortably close to it. If this hypothesis is correct, re-levelling of the railway should show soine uplift west of Whakaki and depression east of it.