Earthquakes and Faults
New Zealand provided one of the earliest well-substantiated occurrences of movement on a geological fault accompanying an earthquake, when the Wairarapa Fault moved at the time of the large Wellington earthquake of 1855. Several dozen other coincidences of this kind, in various countries, have since been reported, and hence has arisen the idea that it is fault movement that causes earthquakes. This hypothesis has been widely publicised, especially by English and American writers, but the difficulties that confront its acceptance are often overlooked.
Movement on the great San Andreas Fault in California has been the subject of much careful study. At the time of the disastrous San Francisco earthquake of 1906, movement of up to 15 ft occurred on this fault, over a distance of some 200 miles. A comparison of triangulation surveys made before and after the event showed that elastic strain had been released over a wide area of ground. It is commonly supposed that this strain release supplied the energy that was radiated as earthquake waves, but we still know too little about either class of phenomenon to be sure that the strain energy would be sufficient. Sudden fault movements may yet turn out to be but part of the damage wrought by large earthquakes. The most serious effects of the 1906 disaster did not occur in the neighbourhood of the fault.
The New Zealand Alpine Fault, which extends from Milford Sound to Lake Rotoiti, where it joins the Wairau Fault, is comparable to the San Andreas Fault in length, and from geological evidence appears to have undergone lateral movement totalling 300 miles. This movement has continued into geologically recent times, yet the Alpine Fault is not marked by earthquake activity, and in central Westland it traverses a region where even small earthquakes seldom occur.
Recent observations on the San Andreas Fault have revealed a gradual creep movement of about ½ in. a year. This discovery appears to invalidate the commonly held view that all displacement on geological faults is evidence of past earthquake activity. A similar rate of creep would be sufficient to have given the total movement of the Alpine Fault, and it seems possible that gradual creep, which would not ordinarily be noticed, may be a regular mode of fault movement.
Physicists have pointed out that fault movement as usually conceived, involving shear fracture with sudden release of elastic strain, cannot be presumed to occur at depths greater than a few miles, because of the inhibiting effect of frictional forces. There is, moreover, no compelling geological evidence that fault movement extends to great depths. Thus it is of interest that the study of earthquake waves has given no suggestion that the shallowest earthquakes might have a different source mechanism from deeper ones.
The relation between earthquakes and faults is part of the larger problem of earthquake mechanism, the solution of which may eventually lead to methods of predicting earthquakes, perhaps even of controlling them. New Zealand as a region of moderate though varied earthquake activity has much to offer towards the solution of these challenging problems.
by Frank Foster Evison, M.A., B.SC., PH.D.(LOND.), D.I.C., Director, Geophysics Division, Department of Scientific and Industrial Research, Wellington.
- Earthquakes, Eiby, G. A. (1957)
- Elementary Seismology, Richter, C. F. (1958)
- Seismicity of the Earth, Gutenberg, B., and Richter, C. F. (2nd ed. 1954)
- New Zealand Seismological Reports, Department of Scientific and Industrial Research
- New Zealand Journal of Science and Technology, Department of Scientific and Industrial Research
- New Zealand Journal of Geology and Geophysics, Department of Scientific and Industrial Research.