P wave amplitude measurements at the Kaapvaal network: do they help in resolving deep earth structure? |
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| Authors: | C Wright |
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| Institution: | (1) Institute of Earth Sciences, Academia Sinica, P.O. Box 1-55, Nankang, Taipei, 11529, Taiwan |
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| Abstract: | Measurements of the amplitudes of seismic body waves at teleseismic distances have rarely resolved significant features mainly
because of the large scatter of the data. However, amplitudes are easy to measure and may provide additional constraints on
structure to supplement times and waveforms. A new approach to analysing body wave amplitudes at a regional network of similar
instruments seeks to minimize scatter by first deriving amplitude station corrections analogous to station corrections for
times. After correction for station effects, amplitudes from several events can be combined to give regional amplitude–distance
curves without using information on event magnitudes. However, the earthquakes providing the observations must lie in a restricted
range of azimuths from the stations of the network and provide considerable overlap in the range of distances between adjacent
events, with no gaps in distance coverage. The advantages of the method are explored using P wave amplitudes from two sets
of earthquakes in the Indonesian and South American regions recorded by the Kaapvaal network deployed across southern Africa.
In the first example, high amplitudes near 88° distance suggest the presence of a small discontinuity at the top of D″ that
causes constructive interference between the closely separated arrivals of a small triplication in the travel times. The second
example, supplemented by calculations using synthetic data, shows how long-wavelength regional variations in amplitudes can
be resolved to assist the interpretation of times and waveforms. However, the limited range of distances in the observations
and lateral heterogeneities at any depths can result in bias or tilt of the amplitude–distance relationships. Constraining
the depths of the structure causing the long-wavelength variations is a subject for future research. |
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| Keywords: | Amplitudes of seismic body waves Regional seismic networks Structure of the lowermost mantle |
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