SKP and related phases*

Abstract

Abstract For shallow shocks, multiple SKP phases are observed after the initial SKP motion as long as 54 seconds on short-period instruments and as long as 87 seconds on long-period instruments. Amplitude data indicate that each multiple phase has a focal point similar to that of the initial SKP phase. The focal point for waves having periods of 1 to 5 seconds occurs at 131½°, and that for waves having periods of 5 to 10 seconds is broadly defined between 130° and 131°. Short-period SKP waves extend from 129° to at least 140°; long-period SKP waves, from 125° to 145°. The long-period waves are believed to be diffracted from the caustic in accordance with Airy's hypothesis. For all types of SKP phases the energy content of the short-period waves is several times less than that of the long-period waves. For the vertical component the agreement between theoretical and observed values of energy of long-period waves is good. For the horizontal component the observation of too little energy is not satisfactorily explained. SKP″, the SKP phase associated with the inner core, is observed between 114° and 125°. It records with very short periods. The observations of SKP″ present additional support for the hypothesis of large but continuous increase of velocity at the transitional boundary of the inner core.