Abstract

We study the nature of extremely narrow-band signals that appear in typical long-time spectra of seismic recordings. At the German Experimental Seismic System (GERESS) array, we observe several of these time-continuous spectral lines. The most prominent one has a frequency near 2.083 Hz. The close agreement of this frequency with one of the feasible rotation frequencies of synchronous machines (50 Hz /24) suggests an industrial origin of the signal. An observed signal gap of 11 h duration is thus interpreted as a temporary machine shutdown. Interestingly, the signal vanishes (or is at least strongly reduced in amplitude) in the whole eastern part of the German Regional Seismic Network (GRSN) suggesting (1) that a single source dominates the signal in that region and (2) that the narrow-band signal propagates to distances of more than 300 km. On the other hand, we study the character of the wave by performing a suitably adapted array analysis. Estimates of the propagation direction suggest a source location near the German-Czech border. We obtain an estimate of the apparent velocity near 4 km/sec. Comparison with transient arrivals from regional earthquakes suggests that the narrow-band energy propagates as regional phases of Lg and Sg type.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.