Centennial History of the Carnegie Institution of Washington

When seismology was first studied at DTM the only data from the explosion-induced waves were the times of first arrivals and their amplitudes. This information sufficed to resolve questions about the structure of the crust and to locate earthquake sources as well as to examine some questions about interior structure. It was well understood, of course, that the seismometer signals following the first arrival contained valuable information, but the seismograms were compounds of the poor frequency response of the receiving instruments and the complexity of arriving signals. During the last quarter of the century a continual improvement of seismometers and the remarkable expansion of computer power allowed investigators to dispose of most of these restrictions. A memorable step was the acquisition in 1982 of the Digital Equipment Corporation VAX 11/780 computer that was located at the Geophysical Laboratory and connected to the local PDP11/34 by a high-speed telephone link.

The earthquake generation of seismic waves results from the motion of the rock surfaces on faulting. Important parameters are the total energy release, the area of slipping, the amount and direction of displacement and its duration. The seismogram is the record of the motion of the rock as received at the instrument location, but this record is a composite of the response of the instrument, which is known, and the attenuation and propagation for the region between source and receiver, quantities that are generally known within limits that allow meaningful adjustment.