During the past few years there have been no major destructive earthquakes, and the year 1938 has been no exception. Few earthquakes have occurred outside the recognized earthquake belts. The most noteworthy earthquakes of the year were in Europe and the Near East: April 19 in Anatolia, Turkey; June 11 in Belgium; July 20 in Greece.
In the United States several minor earthquakes had some interest; those of February 12, July 29, and August 22 (near Trenton, N.J.) were felt respectively in Chicago, New York, and Philadelphia; the Blair County, Pennsylvania, earthquake of July 15 was in a region without previously known earthquakes.
There are more than one hundred seismograph stations of various kinds in the United States and the regions under its jurisdiction. The number and principal purpose are about as follows: 35, recording of distant earthquakes; 15, nearby light and moderate earthquakes; 51, strong earth motions; 5, volcanic earthquakes of all types (Hawaii).
The teleseismic stations are well distributed; the others, except as mentioned, are chiefly in the Pacific Coast and western mountain regions. The strong-motion instruments are automatic; that is, the earthquake starts the recording which lasts only during the shock. All other instruments are continuously recording; and, with few exceptions, photographic recording is used in all types in the United States.
The instrumental determinations of the earthquake epicenters do not tell the whole story, and reliance must be placed on reports of observers and eye witnesses. The collection of information from thousands of individuals is well organized. In regions of frequent earthquake the cards listing the desired information are placed in the hands of individuals in advance; elsewhere they are sent out immediately after the earthquake. Organizations taking part in these varying activities include: The Coast and Geodetic Survey and Weather Bureau; Jesuit Seismological Association (northeastern seismological association); University of California and associated institutions; California Institute of Technology; University of Alaska. There are also many scattered activities of universities and scientific institutions.
The purpose of these investigations is to find where and at what depth earthquakes occurred; the extent of area affected and the nature of surface effects; to get a better knowledge of the earth's interior and crust, including the relation of earthquakes to faults (slips at the surface); and to learn the character of the earth motion in the central region of a destructive earthquake.
The last named information is needed in designing buildings, bridges, dams, and other structures, to resist earthquake. For the same reason the periods of building and ground vibrations are measured. The question is whether earthquake damage is due to large measure to resonance; that is, whether agreement of the natural period of vibration and the principle periods in an earthquake results in excessive movement of the structure and consequent damage. Work is being done on the application of this information which is chiefly collected by the Coast and Geodesic Survey, at Massachusetts Institute of Technology, California Institute of Technology, and Stanford University. Several of these institutions operate shaking platforms which can be put into the same motion as an actual earthquake, so that models can be tested.
Complete earthquake prediction is impossible; but it may be possible to determine whether stress in the earth's crust is growing, and for this reason crustal movements are measured by geodetic means. Tilt is being measured on the Hayward fault at the University of California. On the same fault, near Oakland, the noise at the fault is recorded in a deep well, in the expectation that minute slipping preceding an earthquake, will cause slight noises. No earthquake has occurred along the fault since observations were started. See also GEOLOGY.
No comments:
Post a Comment