Earthquake Precursors

Abstract

IN NATURE, February 16 (p. 368), under the title of “Earthquake Echoes,” I described the more or less rhythmical series of fading resultants which are seen to succeed many large earthquakes. These earthquake followers, the first of which may sometimes represent the Uri Kaishi, or return shaking of the Japanese, are repeated in groups with a decreasing amplitude, an increasing period, and with a smoother and smoother contour. The last of the series may be so small that it is usually difficult to say with certainty when a large earthquake has ceased to exist. As pointed out by Mr. R. D. Oldham, it seems quite possible that certain of the terminal vibrations may have travelled round the world in a direction opposite to that taken by the larger members of the series. The movements to which I now refer are the procession of vibrational groups which run before the main disturbance, with the smaller of which, under the name of preliminary tremors, we are already more or less familiar. These precursors have in several respects characteristics which are exactly the opposite to those of the earthquake followers. They have a definite commencement, and with large earthquakes group after group usually increases suddenly in amplitude and period.