On the onset of P seismograms

Abstract

For more than 30 years, seismologists have been encouraged to read P onsets to 0.1 s. Evidence is presented that the actual errors are significantly greater than this. Three main factors determine how accurately any particular onset is read: (1) magnification and timescale of the display; (2) signal-to-noise ratio; and (3) the form of the P pulse radiated by the source. If too low a magnification is used, the first motion can have such a low amplitude that it is not identified and the reported onset is measured on a later arrival. Potentially, the largest errors will occur for stations for which direct P leaves the source near a node in the earthquake radiation pattern. If the signal-to-noise ratio is low, weak nodal P may not be observed and the onset picked on a later large-amplitude phase such as the surface reflection pP. The size of the first motion relative to the maximum amplitude of P depends principally on the rise time of the P pulse. For explosions where the rise time can be very short, first motion on conventional SP seismographs may be more than one-quarter of the maximum peak-to-trough deflection shown by P. For some earthquakes where the rise time is over 0.5 s, however, first motion may be only 1/20 of the maximum peak-to-trough deflection and so may be lost in the noise. Examples are shown of seismograms that display such features. Estimates of the reading error in P have been determined using the joint epicentre method to estimate the relative epicentres and origin times of groups of explosions at two test sites (Degelen Mt and Shagan River in Kazakhstan) and for a series of aftershocks (the 1976 Gazli, Uzbekistan sequence). As station terms take up deviations from standard traveltimes that are common to all disturbances in a group, the residuals after applying the joint epicentre method should be a measure of observational error, assuming that the effects of any lateral heterogeneity in the source region are negligible. The joint epicentre studies show, using P onsets measured on array seismograms, that the standard deviations of the residuals (on seven degrees of freedom) for the Degelen Mt and Shagan River explosions are 0.10 s and 0.14 s respectively, whereas for the Gazli earthquakes the standard deviation is 0.39 s, the difference between the variances for the earthquakes and explosions being significant at <0.1 per cent level. The results show that even for the onsets from explosion P waves recorded at arrays, which are some of the most reliable data available, observational errors are not consistently less than 0.1 s and for earthquakes the errors will often be 0.5 s or greater.