The Amplitudes of P Waves and Magnitude Corrections for Deep Focus Earthquakes.

摘要

Amplitudes of P waves recorded by World-Wide Standard Seismograph Network seismographs from deep focus earthquakes around the world were analyzed to determine long- and short-period amplitude-distance curves. Data from deep focus earthquakes were chosen to remove the effect of near source heterogeneity in the upper mantle. Effects of near receiver heterogeneity have been eliminated by estimating the station anomaly terms. The amplitudes were corrected for source radiation pattern also. The short-period amplitude-distance curve has a low (minimum) at a distance of 60 deg and a peak at a distance of 75 deg. The long-period amplitude-distance curve is smoother than the short-period curve. Both curves show a rapid decrease of amplitudes beyond a distance of 85 deg due to the effects of the core. Ray theoretical amplitudes using the Q model fit this amplitude data well, a result suggesting that average Q in the lower mantle is greater than about 1000. Body wave magnitudes determined separately from short-period and long-period amplitude-distance curves approximately agree with each other, a finding suggesting that at the source, the P wave amplitude/period ratio remains constant for deep focus earthquakes at least for the periods of 1 s and 10 s and for the range of magnitudes studied, approximately 5-7.