Abstract
High-amplitude, nearly monochromatic volcanic tremor was observed before, during, and after the 18 May 1980 eruption of Mount St. Helens. The tremor often remained stationary for periods of several minutes or more. For the period before and immediately following the main eruption, spectra calculated from three-component seismograms contained a nearly constant dominant frequency of 1.1 to 1.5 Hz. This contrasts with a dominant frequency of 1.7 to 2.1 Hz calculated for the tremor associated with the 25 May eruption. Amplitude decay with distance obeys the form A ∼ rn, where A is the tremor amplitude and r is the observation distance from the source. The exponent n, was determined in a least-squares sense to be −1.5 ± 0.4, −1.5 ± 0.3, and −1.7 ± 0.3, before and shortly after the main eruption on 18 May 1980 and 24 May 1980, respectively. The exponent for low-frequency volcanic earthquakes before the main eruption was −1.6 ± 0.3, a value similar to that of the volcanic tremor and suggests that the source of the volcanic tremor is shallow as is the source of the low-frequency earthquakes. The spectra of the tremor periods are very similar to those of the low-frequency volcanic earthquakes. There are cases where volcanic tremor appears as a continuation of a low-frequency earthquake. These observations suggest that the volcanic tremor and the low-frequency volcanic earthquakes are strongly interconnected and indeed may well share a common source mechanism. Based on both signal stationarity and spectral amplitude decay consistency, we suggest, in common with others, that the monochromatic nature of volcanic tremor is mainly due to a source effect, while the path and station effects are relatively minor. The tremor amplitude changes as a result of variations in the source driving force. The frequency shift for the 24 May 1980 tremor may be due to a source dimension change.
