A GEOLOGIST ON EARTHQUAKES.

Otago Witness, Issue 2719, 25 April 1906, Page 33

A GEOLOGIST ON EARTHQUAKES.

DR. P. MARSHALL'S VIEWS. Dr P. Marshall, of the Otago School of Mines, in response to a request by a member of the staff of the Otago Daily Times was good enough to supply the folowing notes :— j' The great earthquakes of the last 12 months — in Northern India, Calabria, and I California — have all occurred in regions i well known as districts of high seismic ; activity. In all of them there is a steep ' slopo in the earth's crust. The Indian earthquake was at the base of the great Himalayan Range, the Calabrian earthquake ou tho slope trom the Apennines to the floor 1 of the Mediterranean, and the terrible ; Calif ornian shake on the slope torn the i Sierras to the floor of the Pacific bed I 12,000 ft deep. In each of these areas ifc is | known that the solid ci-ust of the earth is j traversed bj a rock fracture or fault. These fractures are one result of tho immense strains that constantly arise in the earth's crust, which has to nt closely to the con- | stantly cooling and shrinking nucleus. The ' strain is sufficient to bend up with infiniie j slowness the solid rock layers into arches and troughs. From time to time the rocks break across, but the strain, still continu- ; ing-, will cause the rock on one side to slide over that on the other side of the broken ( surface. The sliding movement is not conj tinuous, fbut lasts for centuries, ihough long ! intervals separate the different slides. In ! India the fault is along the line that separates the hills from the plain of the Ganges Valley. In Italy Professor Suess has shown that a large area is gradually sinking inward, and each downward movement is the cause of an earthquake. A ' volcanic eruption often succeeds the shake, ! as this year the eruption of Vesuvius has i succeeded the Calabrian earthquake, and in 1902 in Central America the eruption of Mount Pelee succeeded the great earthquake of Guatemala, as well as in 1851, and at earlier periods. In the Californian region there are several geological faults, as in all other areas of steep coastal slopes, and in all cases these are lines of terrestrial weakness, especially liable to earthquake shakes. The effect of a sudden rock movement along a fault is in general ihn production of a series of clastic earth waves spreading out radially from tho region of movement. The waves are caused by horizontal movements of the rock particles, and as they spread outwards the movement of the particles becomes gradually less and less until it can be detected only by sensitive instruments, or seismographs specially constructed for th© purpose. The original rock movement is generally at some distance from the surface, and as the waves reach the looser and less elastic rocks near the surface they are partly transformed into waves that can actually be seen. These earth waves are 40ft or 50ft from crest to crest, and from lin to lOin in height, and their rate of movement is about 300 ft per second. It is these transformed waves that cause the great dam.iffe to buildings and cities, but they lapidly die out, and are really felt only in the ar«=>a directly over the rock fracture, or in regions closely adjacent to it. The looser and fess coherent the rock or soil the greater the intensity of these wavos. Hence an earthquake is always most severely felt where the soil is newly formed. For instance, in reclaimed land, in road embankments, and in alluvial river fiats. Thus in the Wanganui earthquake in 1897 the reclaimed land on the river bank wa« so disturbed that the railway lines laid on it were twisted and contorted, and in 1901 at Cheviot the road embankments were traI versed by g-aping- fissures. It is pro'bablv i owing to this fact that the destruction in San Francisco has been so disastrous. Much of the city is built on sandhills, which are particularly wanting in elasticity, and part was built on reclaimed land, which is not only inelastic, but in all probability has not yet settled properly. Although 254 earthquakes have been re- | corded in San Francisco since 1769 and . 514 others in the Staie of California, only I one of these (the Inyo earthquake) is classed as an earthquake of the first order of violence. In this instance subsequent shakes followed for four or five years.