Determination of P Wave Onset Times by a Digital Computer

Abstract

Programs for automatic reading of seismic P wave onset by a digital computer are introduced. These programs are composed of a series of simple processes as follows. A low cut recursive filter is used to increase S/N amplitude ratio. The arrival times of P waves are read by means of comparison with a threshold level corresponding to noise level before the P waves. Discrimination between P arrivals and noises are made by counting the number of points where amplitude of the wave exceeds the threshold level during a pre-set time interval. Then, the earlier of the closest two reading times among the three independently obtained from the three component data, is selected as the best P wave onset time. As the result, 66% of microearthquakes observed at the Iwatsuki deep borehole observatory could be read within the time difference of 0.1sec between the automatic readings and ordinary visual readings. Furthermore, an effective indicator of reliability of the finally adopted P onset times is discussed.

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