The Earth's free oscillations observed on Earth tide instruments at Tiefenort, East Germany

Abstract

The great Chilean earthquake of May 22, 1960, wrote unusual long-period records on horizontal pendulum seismographs designed for recording earth tides at Tiefenort, East Germany (50°52′N, 10°57′E). A section of the EW record is reproduced in Figure 1. It shows the foreshock at 1032 GCT (Mag 712) and the main shock at 1911 GCT (Mag 812) along with the semidiurnal earth tide. Spectral analysis of the continuing long-period disturbance on this record showed a typical line spectrum characteristic of the earth (Fig. 2). The two horizontal components respond to both horizontal acceleration and tilt; thus there is no way of separating spheroidal from toroidal modes at this station except on the basis of frequency. For all but the lowest modes the spheroidal and toroidal modes are so close in frequency that even this criterion cannot be used. Uncertainties in timing plus the fact that only 22 hours of record could be digitized prevented good frequency resolution. In spite of these difficulties, the cross spectrum of the EW component and a comparable EW pendulum seismograph located at Pasadena (Fig. 3) leaves no doubt that the spectral peaks observed at Tiefenort do represent coherent vibrations of the entire earth.