Ground vibration near explosions.*

Abstract

Summary In summary, the following pulses were recognized on many or all of the records taken in the Los Alamitos region: P The first pulse to arrive. P3 A compressional pulse arriving later than P. X1, X2, X3 Three pulses assumed to be body waves traveling along deeper paths than P and P3. C A strongly dispersed pulse largely confined to the longitudinal component. T Motion on the transverse component arriving largely coincident with C. H A direct elliptical motion in a vertical plane. R A Rayleigh-type motion. P, C, H, and R are prominent on all the records, though at distances less than 300 meters from the explosions C, H and R overlap each other, and are not separable. They are strongly dispersed, and their beginnings are therefore difficult to identify. It is the times of arrival of their maxima which are plotted in the travel-time curves. The recorded pulses of energy are all of types previously reported. However, no completely satisfactory theory explaining C, H, or R exists. Any theory describing these pulses must be based on a knowledge of the fundamental properties of the first few meters of the earth's crust, a complicated medium which is plainly neither homogeneous nor elastic. The fact that such a medium can transmit both compressional and Rayleigh-type waves is an encouraging sign, since it means that its behavior can not be radically different from that of elastic substances. It is to be hoped that the mathematical physicists will soon develop equations describing wave transmission through such media. In closing, the author would like to thank the many persons, especially Dr. Beno Gutenberg of the California Institute of Technology and Dr. R. A. Peterson of United Geophysical Company, for their advice and assistance, without which this research could not have been done.