96 Bulletin of the Seismological Society of America

Since the publication of our paper entitled "A Study of Blasting Recorded in Southern California ''1 one further large blast has been set off in this region, and others moderate, to comparatively small, in magnitude. Two of these especially have afforded opportunity for further direct investigation. Others have yielded additional information. 1. On the afternoon of August 4, 1931, a blast consisting of a little more than 800 pounds (say 360 kilograms) of 20 per cent dynamite, buried in scattered charges at a depth of about 16 feet (5-meters) in weathered granitic rock, was set off simultaneously as a single charge at a location on a new road under construction along the mountain side above the Arroyo Seco almost due north from the Seismological Laboratory in Pasadena. This blast was registered usefully at the Laboratory and at the seismologic station on Mount Wilson, at distances between 12.5 and 13.5 kilometers (7 .8to 8 .4miles) from the road cut. Through the courtesy of the contractors, the T. M. Morgan Paving Company, we were permitted to fire this blast and thus to control and determine the time of its detonation by a method indicated below. 2. On September 12, 1931, at 11:01 a.m., P.S.T. (19:01 G.C.T.), a blast consisting of slightly more than twenty tons of high explosive was detonated in crystalline limestone at the site of a new quarry of the Southwestern Portland Cement Company 8.5 kilometers (5 .3 miies) northeast of their plant near Victorville, California. This was registered usefully at a temporary station at this plant, and at the seismologic stations of our local network at Riverside, 5{ount Wilson, Pasadena, Haiwee, and Tinemaha, at distances ranging from 8.5 up to about 290 kilometers (5 .3to 180-miles) from the quarry. Through the cordial co-operation of Mr. L. V. Robinson, Superintendent, of the Southwestern Portland Cement Company at Victorville,