Crust of the Earth: A Symposium
In a paper in 1923, Nakano, starting with equations developed by Stokes, developed the pattern of first compressions and rarefactions as a function of azimuth to be expected from various types of sources. The one of most interest to us is the quadrant distribution due to an impressed couple. Byerly et al. put the equations into displacement form (Nakano used accelerations). Byerly has also developed a technique for combining the observations with the theory to compute the nature of faulting at the source. Koning has interested himself in similar techniques. Hodgson has greatly improved Byerly’s method.
Japanese work in this field confined itself to seismograms written at Japanese stations, of earthquakes centering in or near Japan. So the interpretations were limited to two dimensions, effectively. Shida in 1917, in unpublished papers read in Japan, appears to have been the first to observe a quadrant distribution of compressions and rarefactions. Labozzetta in Italy, in 1916, had observed a shock where one straight line through the epicenter divided the regions of rarefaction and compression. There have been in Japan many investigators who have felt that faulting could explain distributions of compressions and rarefactions. However, many others, particularly Ishimoto, have definitely felt that many sources are more complicated.
In 1922 Gherzi became interested in the problem of the nature of initial P waves at Zi-ka-wei, from distant foci. But it was Somville in 1925 who published a map of the world, showing that epicenters from certain regions always sent rarefactions to Uccle, while, from other regions, the first motion was always a compression. Recently (1950) Byerly and Evernden published a similar map for circumpacific epicenters, showing the nature of first P wave at Berkeley. They differentiated the epicenters as to the depth of focus. Particularly to the south of Berkeley, the deep-focus shocks tend to send dilatations where the shallow shocks send compressions, suggesting a difference in fault orientation with depth. Båth, in 1952, made a similar study for the Pasadena and Huancayo stations.
Byerly, in a series of papers, 1926 to 1938, gradually developed a method for deducing nodal planes at the focus, from recordings of compressions and rarefactions from stations all over the earth. Observations alone do not distinguish between the fault plane and that plane which bisects it perpendicularly and is also normal to the fault motion. He applied the method to a few shocks. Hodgson and Milne have applied it to many. Koning and his group in Holland have also studied the initial motion of a number of shocks.
Some writers have tried to combine the first motion of S in addition, to solve the ambiguity of the P solution. Gutenberg seems to have been successful in this, for the Tehachapi earthquake.
The preponderance of transcurrent faulting is remarkable.