Amplitude spectra of the spheroidal modes 0Sl(l=2-40) excited by the 1960 Chilean earthquake and observed at Pasadena, Los Angeles, and Isabella show distinct holes at 0S10(T=580 s) and 0S21(T=336s). These holes can be explained as an interference pattern caused by a composite source consisting of a finite propagating source (main shock) and a slow precursory source, 15 min before the main shock. A total seismic moment of 4-5¿1030 dyn cm is required to explain the observed amplitude.