The San Francisco Earthquake of April 18

Abstract

THIS disastrous earthquake was remarkable for its long duration and the rotatory character of the movement. As observed at Mare Island the first sign was a very faint, gentle rustling, the waves being the merest tremors; but after about a minute's duration they had grown to such proportions as to be felt by everyone. The violent phase lasted about forty seconds, and then the shocks died out, the last feeble tremors vanishing about three and a half minutes from the time of the first perception. The writer was favourably situated for noting the slightest disturbance, and had been awake some time before the first tremors were felt, and he could see the clock face at the beginning and end of the disturbance, which read about 5h. 11m. and 5h. 14m. 30s. Two of the four astronomical clocks at the Mare Island Observatory were stopped by having their pendulums thrown upon the ledge which carries the scale for measuring the amplitude of the swing. The time of the violent oscillation thus automatically recorded was 5h. 12m. 37s., Pacific Standard Time, eight hours slow of Greenwich. The waves were mainly from the south and south-south-west, and they seemed to turn to the west, giving the movement an elliptical, clockwise rotation. The pendulums of the two clocks which kept moving had their points rubbed against the swing index Of the ledge so violently that the metal of the index was brightened by the friction of the pendulum points, and the time thereby deranged more than twenty seconds. Except for the disturbance of objects on the ground, the earthquake seemed to be essentially noiseless. Other slight shocks have continued at irregular intervals for the past five days.