Analysis of short-period waveforms of P phases from deep-focus earthquakes beneath the Fiji Islands

Abstract

Summary. Analysis of seismograms from 70 deep-focus earthquakes recorded at nearby stations in the Fiji Islands has yielded estimates of the waveforms of P waves radiated from deep sources and of the seismic moments of the events. The earthquake sample consists of all well-recorded events in 1965–66 located in a limited hypocentral region within 4° of the stations and at depths of 500 to 700 km. The spatial distribution restricts the angle of incidence for the incoming direct P waves to less than about 25°, thereby reducing the extent to which the early portion of the observed P wave is contaminated by crustal effects. A study of nuclear explosions recorded by these stations indicates that the effects of the mantle, crust and seismograph on the shape of the P wave are reasonably well-accounted for in the seismogram synthesis employed in this study. The magnitudes (mb) of the events studied ranged from 3.5 to 5.9. The character of the seismograms for these events ranged from small-amplitude, high-frequency, short-duration P waves for the smallest events to large-amplitude and low-frequency or multiple events for some of the largest events. However, 60 of the 70 events, spanning nearly the entire magnitude range, have P waveforms which, during the first 1.2s, are quite similar to one another. Synthetic seismograms were computed for these events, and the best fit is obtained with a triangular farfield displacement pulse, with a 0.6 s rise time and a 0.4 s decay tune. To describe the seismograms of these 60 events, only the pulse height (proportional to seismic moment) need vary; the duration of the triangular pulse form remains nearly constant and implies a spectral ‘corner frequency’ of 0.7 Hz. Thus, although for the entire sample of earthquakes the duration of the source is found to be somewhat dependent on seismic moment, there is a range of two orders of magnitude in moment (1022 to 1024 dyne cm) over which the source duration does not significantly vary. This cannot be accounted for by variations of focal mechanism orientations but indicates large variations in stress drop among the deep events. Published studies of shallow-focus earthquakes in California have indicated that a wide range of seismic moments is associated with a small range of source durations or corner frequencies, particularly for samples of earthquake populations within limited areas. This study shows a similar variation of moment and source duration within a localized region of deep earthquakes. The similarity is in both the range and location on a moment versus source duration plot, indicating similar ranges of stress drops for the shallow California and deep Fiji events.