1933.] ADAMS, BARNETT, AND HAYES, – SEISMOLOGICAL REPORT. 93
SEISMOLOGICAL REPORT OF THE HAWKE’S BAY EARTHQUAKE OF 3rd FEBRUARY, 1931.
By C. E. ADAMS, Dominion Astronomer and Seismologist ; M. A. F. BARNETT, Department of Scientific and Industrial Research; and R. C. HAYES, Dominion Observatory.
PART I. – GENERAL INTRODUCTION.
RELATION OF NEW ZEALAND SEISMOLOGY TO THAT OF THE PACIFIC.
THE Islands of New Zealand constitute the visible southern portions of a great submarine plateau, the ledge of which runs parallel to the east coast of the North Island, and then strikes eastward, terminating in the Chatham Islands, Bounty Island, and Antipodes Island. From New Zealand, the submarine scarp extends in a north-north-east direction through the Kermadec Islands and Tonga Islands, where it borders on two great ocean deeps; finally terminating just south of Samoa. This scarp forms part of the real western border of the great basin of the Pacific, and, like other parts of the Pacific border, it is a region of incessant seismic activity.
The concentrated seismic activity round the border of the Pacific is well shown in the map (fig. 1), which represents the earthquake epicentres from the International Seismological Summary for the years 1922, 1923, 1924, 1925.
An explanation of the intense seismic activity experienced in the countries bordering on the Pacific Ocean, of which New Zealand forms part, is given by Sir Edgeworth David in a statement published shortly after the Hawke’s Bay earthquake. The writer says :-
Most of the great earthquakes of the world owe their origin to . . . the sinking of ocean floors, and the consequential thrust which they deliver against their shores. One is apt to think of oceans as hollows or cavities in the earth’s crust, and so they are in respect to their margins. But on. the whole the floors of all the great oceans are convex. This might be illustrated with an orange. If we peel off, say, a third of its skin, we leave a depressed area where the skin was removed, but while there is a small drop down into this miniature Pacific Ocean just around its margins, its floor is convex. Any attempt to flatten such a floor must exert pressures around the margin, as the “arc is greater than its chord,” the bent bow is longer than its string. Thus, if we take a bow bent, and string, place it on a table, and then allow the bow to flatten itself by cutting the string, the ends move outwards as the bow flattens itself.
Similarly, if the floor of the Pacific Ocean is slightly flattened, the margins move outwards, but as they meet with resistant rock they have either to bend or break the rock in order to make room for this adjustment.
Now, the floor of the Pacific is made of rock material heavier than that of the rock in the surrounding lands, so the tendency is for this heavy material to slowly sink deeper, and so deepen the Pacific Ocean, while the thrust from the Pacific floor bulges up the surrounding lands and crumples them up into great rock folds.
Sooner or later this stupendous folding force exceeds the elastic limits of the rocks, and they are suddenly fractured with an awe-inspiring roar and rumble, as of some mighty war barrage. . . .
That New Zealand has been subjected to intense thrusts from the Pacific for ages in the past is proved by the fact that the strata in the New Zealand Alps, originally deposited as flat beds of clay and sand on a sea-floor, have been raised thousands of feet above sea-level, and have been rucked up into folds which have been overturned towards the west. That this great thrust from the Pacific is still operating is proved by the recent raised beaches, from 200 to 500 ft. above sea-level, of the east coast of New Zealand. These extend from Timaru in the South Island to considerably beyond Cape Palliser in the North Island.