The first preliminary waves of the Nevada earthquake of December 20, 1932*

Abstract

Summary The P travel-time curve of the Nevada earthquake is presented. It is drawn as a series of straight lines. It is near Δ = 28° that the data outline most clearly the sudden change in slope of the curve, but definite evidence of overlapping of the branches is lacking. At Δ = 67° (approximately) the curve branches into two parts as did the curve of the Texas earthquake. Between 4° and 12° three parallel P curves are drawn. The suggested explanation of them is that they are due to transformations of an original P or S at boundaries near the focus. This would indicate a depth of focus of about fifteen kilometers. The nature of the first motion at the various stations shows a complex distribution which does not lead to a simple explanation of the forces acting at the focus. A long-period component of P waves is observed, and it is concluded that it is present from very near the beginning of the record, although often masked by shorter-period motion. It begins in opposite phase to the shorter-period motion accompanying it.