Spectrum of P and PcP in Relation to the Mantle-Core Boundary and Attenuation in the Mantle

Spectrum analyses were made of the records of the short-period vertical component of P and PcP phases on seismograms of array stations at Tonto Forest, Arizona, for twenty-one earthquakes over the range A _-47 ø to 83 ø. Generally, as has been reported by other investigators, the trace amplitude ratio of PcP to P is significantly larger than the theoretical ratio. The pulse width of PcP is narrower than that of P. Both of these facts can be explained by taking into account an appropriate attenuation distribution in the mantle. Taking Q•, Q for •q waves, which has been determined by Anderson, Ben-Menahem, and Archambeau using different methods as a standard, the Q distribution for P waves, Qa, can be determined as Qa • Q• at the period of about I sec. A matrix method is applied to calculate the complex reflection coefficient of a transitional mantle-core boundary. Impulse responses calculated therefrom and comparison of the waveforms of P and PcP lead to the conclusion that the major discontinuity at the mantle-core boundary is sharp and is probably less than I km thick. The effect of a more gradual transition region superposed upon such a sharp discontinuity is also discussed. The possibility of the existence of a soft layer terminated by a sharp boundary cannot be totally ruled out.