A Linear Strain Seismograph. The Physical Evaluation of Seismic Destructiveness. A Method for the Instrumental Determination of the Extent of Faulting

Citation

Benioff, Victor Hugo (1935) A Linear Strain Seismograph. The Physical Evaluation of Seismic Destructiveness. A Method for the Instrumental Determination of the Extent of Faulting. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/RZNS-FS06. https://resolver.caltech.edu/CaltechTHESIS:01292018-135558372

Abstract

Part one: A linear strain seismograph:

A non-pendular seismograph is described, having a response which depends upon the linear strains between two points of the ground. Essentially, the seismometer consists of two piers separated by an interval of 20 meters, with a horizontal bar which is rigidly fastened to one pier and extending nearly to the other pier. Relative movements of the two piers thus actuate an electromechanical transducer which operates between the free end of the bar and the adjacent pier. The resulting induced currents are recorded by two galvanometers having different constants. Three different galvanometer periods being 0.2, 1.3, and 35 seconds. The equivalent pendular magnifications are 80,000, 10,000 and 100 respectively.

A theory is developed for the linear strain seismograph and related instruments.

Part two: The physical evaluation of seismic destructiveness:

The problem of designing structures to withstand destructive earthquakes is not in a very satisfactory condition. On the one hand engineers do not know what characteristics of the ground motion are responsible for destruction, and on the other hand seismologists have no measurements of seismic motion which are sufficiently adequate to serve for design, even if the destructive characteristics were known. Consequently, engineers have been forced to proceed on an empirical basis. From past experience, chiefly in Japan, it has been found that buildings which are designed to withstand a constant horizontal acceleration of 0.1 gravity are, on the whole, fairly resistant to seismic damage. It is fortunate that such a simple formula works at all, in view of its inadequacy from the point of view of precise computation. We know that seismic motions do not exhibit constant accelerations; that instead they are made up of exceedingly variable oscillatory movements. A formula based upon constant acceleration may thus lead to large errors, especially when applied to new types of structures which have not been tested in actual earthquakes. In the following paragraphs a new formula for seismic destructiveness is proposed, in the belief that it is more accurate than previous ones. In addition to providing engineers with a more rational basis for design procedure, it determines a new type of seismographic instrument for recording and measuring the destructive characteristics of seismic motion.

Part 3: A method for the instrumental determination of the extent of faulting:

When fault displacement is clearly visible at the surface of the ground, there is no difficulty in determining the extent of faulting. Thus for example, in the San Francisco earthquake of 1906, faulting was observed on land to the extent of some 180 miles. If the fault is deeply covered with sediments so that displacement can not be observed at the surface, it is necessary to employ indirect methods of measuring the extent of faulting.

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