WHAT CAUSES EARTHQUAKES?

New Zealand Listener, Volume 10, Issue 255, 12 May 1944, Page 8

WHAT CAUSES EARTHQUAKES?

(From a talk broadcast from 4YA by DR.

F. J.

TURNER

Lecturer

in Geology at the University of Otago)

ENSITIVE modern §instruments distributed widely on all the continents show that the outer part of the earth trembles almost constantly, even though the majority of the tremors recorded by instruments-seismographs-are too slight to be perceived by human beings. If you watch the seismograph at the Kelburn Observatory on a windy Morning, you will see the continuous record of the earth’s vibration under the impact of heavy seas breaking on the coast some miles away. In New York the pavement vibrates continuously under the rush of wheeled traffic. But there are also tremors that arise from natural movement within the rocks of the earth’s crust. The majority are weak, but some reach destructive violence. Ail are termed earthquakes. Thousands of destructive earthquakes have been recorded during the tiny fraction of geological time comprised by the span of human history. And a considerable mass of earthquake lore — much of it illogical, inaccurate and tinged with magical or religious belief-has gradually accumulated. Human interest in earthquakes has ever been emphasised by the terrifying nature of such events-the sudden unheralded impact, the destructive effects on life and property, and the somewhat mysterious origin of the phenomena. In this evening’s talk I propose to say something of the cause and results of earthquakes, of the distribution of seismic activity in New Zealand, and of the nature of earthquake vibrations, as tevealed by the science of seismology, and the light these vibrations throw upon the nature of the earth’s interior. Origins First, then, the origin of earthquakes. Philosophers of the ancient world were content to attribute earthquakes to rather fantastic causes. Aristotle, for example, taught that they originated in the rush of winds through vast subterranean caverns. But we can pass on to more modern times, when it was found that earthquakes tend to recur in welldefined zones on the earth’s surfaceseismic zones-such as that which borders the Pacific Ocean, and incidentally passes through New Zealand. Some of these, such as The Circum-Pacific zone to which I have just alluded, are also regions of volcanic activity, and consequently there is a strong though erroneous popular belief that volcanic activity is the cause of earthquakes. Actually, the two sets of phenomena are mutually independent, but they both may be traced to a common cause. The rocks of the earth’s crust are constantly in a state of strain. This is periodically relieved along geographic zones of crustal weakness, where the rocks tend to buckle and fracture along mountain folds and fault zones. Local melting of deep-seated rock, and squeezing up of the molten ‘material so formed along great fractures leads to volcanic activity at the surface in such regions; but, quite independently, the accumulated strain in the crust may be relieved by sudden

movement of earth-blocks along faultlines, and the vibrations set up in the surrounding rocks by such movements are what we know as earthquakes. This is why earthquakes tend to occur in regions where volcanoes are active. A Typical Case Let me describe a typical instance of the events leading to an earthquake. The high mountain ranges of New Zealand, such as the alpine system of the South Island, or the Tararua and Rimutaka Ranges, are composed essentially of giant blocks of rock that have been lifted bodily to their present elevationsome thousands of feet-by vertical movements upon fault-planes whose individual outcrops on the surface can often be traced for 50 or 100 miles. This elevation has been a _ slow process, spread over a period of time reckoned in hundreds of thousands of years. Among the score or so of northwardtrending major faults and fault-zones that bound the mountain blocks of Nelson Province is the one that runs along the eastern face of the Lyell Range, a few miles west of the township of Murchison. On June 17, 1929, there was a sudden movement by which the earth block lying on the Murchison side of the fault zone was raised nearly 15 feet, and moved laterally some six feet in a north-westerly direction. For years stresses had been accumulating in the Nelson mountain systems, until at last the great rock mass had yielded along the fault of which I have spoken. Release, when it came, was instantane-ous-like the springing of a steel trap. And the resultant jolt, accompanying the sudden displacement of a whole mountain range by 15 or 20 feet, threw the surrounding sector of the earth’s crust into the violent vibration which came to be known as the Great Murchison Earthquake. As is usually the case with violent earthquakes of this kind, the main shock was followed during the rest of June 17 and the next few days by many relatively minor (continued on next page)

(continued from previous page) tremors caused by slight settling movements on the various faults in the vicinity of the Lyell Range. The Murchison Earthquake was a typical product of a sudden movement affecting earth-blocks near the earth’s surface. In just the same way, the destructive San Francisco earthquake of April 18, 1906, was caused by an abrupt horizontal movement of about 20 feet along at least 270 miles of the San Andreas fault--a major visible earth fracture bounding one of the western mountain ranges of California, In other cases, the source of the shock is a more deep-seated movement, and there is no accompanying displacement at the surface itself. But in comparison with the radius of the earth (some 4000 miles), the majority of earthquakes of destructive violence originate at very shallow depths-not more than 30 miles below the surface. There are also earthquakes of a much less spectacular nature that originate at depths of as much as 400 miles, and which are termed deep-focus earthquakes. Surface Effects An earthquake, then, is a vibration set up in the earth’s crust as a result of some abrupt dislocation of earth blocks in the outer part of the earth’s crust. What are its effects at the surface? Most obvious of all are the effects upon man-made structures such as buildings, tunnels, dams, etc. In qa rough scale of intensity designated by the numbers 1 to 10, a shock of intensity 1 is almost imperceptible, except by instruments, One of intensity 5 is felt by all persons who are awake at the time, disturbs furniture, sets the electric light bulb swinging, and may ring church beils. Intensity 8 is indicated by falling chimneys and cracking of brick buildings, while widespread destruction of buildings and disturbance of the ground eccur at the maximum intensity, 10. Structures built of steel or reinforced concrete, and purely wooden buildings are capable of withstanding the stress of vibration and hence survive, when brick or stone structures are levelled to the ground. Other things being equal, build-ings-erected on solid rock are much less vulnerable than those built on watersoaked sands and gravels. This is one of the teasons for the great damage caused to buildings at Murchison and outlying farms in that district, for the township itself and many of the farms are build upon the gravels of the Buller and tributary rivers, "Tidal Waves" In steep, hilly country, especially where a high rain-fall keeps the soil and sub-soil in a _ water-logged condition, landslips commonly occur as a result of earthquakes, as the slip-scarred hills around Murchison still testify 15 years after the great earthquake. Then, in connection with submarine earthquakes, there is the possibility that the nearby sea coast will be swept by: the destructive inundations popularly miscalled "tidal waves." These have no connection with tides. But they are due to the shoreward surging of immense wavessome 40 or 50 feet high in some casesthat are set up in ocean waters by the vibration of the sea floor beneath. Curiously enough, these waves are almost imperceptible to ships out at sea, for the distance from crest to crest may be a hundred miles or more.