Art. XLV.—Notes on the East Coast Earthquake of 9th August, 1904.
[Read before the Wellington Philosophical Society, November, 1904.]
This earthquake was felt nearly all over the colony, from Auckland to Queenstown. Its effects were most marked in the Hawke's Bay and Wellington Districts, especially on and near the coast from Porangahau to Castlepoint, where rockfalls occurred from cliffs, and fissures were formed in the surface-crust, indicating an intensity of IX. (or nearly so) on the Rossi-Forel scale. In the area affected and in its intensity it closely resembled the earthquake of the 17th February, 1863, which proceeded from the same region of disturbance.
Memoranda or notices of the shock were received from the following places, the Roman numbers denoting the degree of intensity (in some cases only approximately):—
IX.: Castlepoint, Motuotaraia, Porangahau.
VIII.-IX.: Napier, Hastings, Te Aute, Kopua, Dannevirke, Pahiatua, Wellington.
VIII.: Woodville, Masterton, Featherston, Carterton.
VII.—VIII.: Wairoa, Palmerston North.
VII.: Gisborne, Feilding.
VI.: Opunake, Aramoho, Marton, Nelson, Blenheim, Taupo.
V.-VI.: New Plymouth, Hawera, Kaikoura, Motueka Collingwood, Wakapuaka.
V.: Greymouth, Hokitika, Westport, Christchurch.
IV.-V.: Auckland, Rotorua, Ashburton, Timaru.
IV.: Dunedin, Queenstown.
The significance of this grouping will appear from the Rossi-Forel scale, which I make no apology for quoting at length, as I believe it has never yet appeared with the absolute equivalents in any New Zealand publication. The Arabic figures express the equivalents on the absolute scale—that is, the maximum acceleration of the earth's surface in millimeters per second per second, the acceleration due to gravity being about 9,600 mm./sec.2
|Rossi-Forel Scale of Intensity.||Absolute Scale. mm./sec2|
|I.||Recorded by a single seismograph, or by some seismographs of the same model, but not by several seismographs of different kinds; the shock felt by an experienced observer||20|
|II.||Recorded by seismographs of different kinds; felt by a small number of persons at rest||40|
|III.||Felt by several persons at rest; strong enough for the duration or the direction to be appreciable||60|
|IV.||Felt by persons in motion; disturbance of movable objects, doors, windows, cracking of ceilings||80|
|V.||Felt generally by every one; disturbance of furniture and beds, ringing of some bells||110|
|VI.||General awakening of those asleep; general ringing of bells, oscillation of chandeliers, stopping of clocks; visible disturbance of trees and shrubs; some startled persons leave their dwellings||150|
|VII.||Overthrow of movable objects, fall of plaster, ringing of church bells, general panic, without damage to buildings||300|
|VIII.||Falls of chimneys, cracks in the walls of buildings||500|
|IX.||Partial or total destruction of some buildings||1200|
|X.||Great disasters, ruins, disturbance of strata, fissures in the earth's crust, rock-falls from mountains||?|
The Milne horizontal pendulum seismograph at Wellington gave a good record, and nearly all the phases were also recorded on the Milne instrument at the Perth Observatory, West Australia, to the Director of which I am indebted for a copy of the seismogram.
The best time-observations in New Zealand were—
|Beginning of Shock.|
These, by the method of circles or the method of equations, gave the position of the origin as 42° 23½′ S. lat., 178° 58′ E. long.—that is, the epicentral area was situated about a point 215 miles S.E. by S. from Napier, and 227 miles E.S.E. from Wellington. The depth of the origin is uncertain, but it was probably about 15 miles (24 kilometers), or a very little less.
The transit-velocity of the large waves (longitudinal waves) was 129.6 miles per minute, or 3.48 kilom. per second.
The transit-velocity of the preliminary tremors (rapid fine waves), as determined from the seismographic records, was about 478 miles per minute, or 12.8 kilom. per second. The speed of the transverse waves, similarly determined, was 2.2 kilom. per second.
The averages of the corresponding numbers for nineteen large earthquakes, as calculated by Professor Omori and Mr. Imamura, of the Hongo Observatory, Tokyo, are—
|Kilom. per Second.|
|Preliminary tremors, velocity (V1, Imamura)||13.2|
|Large waves (V5)||3.3|
|Transverse waves (V8)||2.1|
|[Earthquake Investigation Committee of Japan.]|
The theoretical value of V5 for hard granite is 3.95 kilom. per second: the value 3.48 kilom. per second would agree with a hypothesis that these waves passed for two-fifths of their path through granite, and for the other three-fifths through some such rock as limestone.
The seismogram taken at Wellington shows a very slight tilting towards the west, probably evidence of slipping or accelerated folding of the underlying rocks.
The main earthquake was preceded by slight shocks in July and August, and was followed by after-shocks (twenty or more) until October.
The origins of these and other earthquakes, notably that of 1863 already mentioned, and that of the 9th March, 1890, seem to be situated in or near a strip or region of the earth's crust shown on the map as EE (Plate XLIX.), which is parallel
to the axis of folding of the older Pliocene rocks in Hawke's Bay, and to the general axis of New Zealand. They are also just on the outside of the sloping plateau which about 250 miles to the E.S.E. rises above the sea as the Chatham Islands. Between this line of origins and the south-east coast of the North Island is a trough in the ocean-bed with a depth of 1,000 to 2,000 fathoms (AA).
It is probable that the earthquake of the 9th August was due to sudden slipping along a fault-plane, or other similar movement that occurred in the process of the “repacking” of the deeper rocks, consequent upon the continuance of the folding which the geological evidence shows to have been going on for many ages.
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The maximum displacement or amplitude of an earthparticle may be calculated from the other elements of the earthquake: the data in this case give 2–2.5 mm. as the maximum horizonal displacement; the vertical displacement would consequently be not greater than ½ mm. or 1/50 in.
The isoseismals, or lines of equal earthquake-intensity, marked on the map confirm in a general way the conclusions already established in regard to the region of disturbance.
[I am indebted for valuable observations and notes of the earthquake to the Secretary of the Post and Telegraph Department and his officers, especially to Mr. Keys of Napier, and to many private persons, the careful notes collected by Mr. R. P. Soundy, of Dannevirke North School, being of great interest.]