China
Africa
Explore chapters and articles related to this topic
Volcanic activity
Published in F.G. Bell, Geological Hazards, 1999
Such uplift also means that rocks are fractured, so volcanic eruptions are generally preceded by seismic activity (Figure 2.13). However, this is not always the case. For instance, this did not happen at Heimaey, Iceland, in 1973. Conversely, earthquake swarms need not be followed by eruptions: the tremors that were felt on Guadeloupe in 1976 were not followed by an eruption. Where earthquake swarms do occur the number of tremors increases as the time of eruption approaches. For example, tremors average six per day on Kilauea but at the beginning of 1955 these increased markedly, 600 being detected on 26 February. Two days later an eruption occurred. However, tremors may continue for a few days to a year or more. A network of seismic stations is set up to monitor the tremors and from the data obtained, the position and depth of origin of the tremors can be ascertained. The earthquakes are called A-type if they have hypocentres 1 to 10 km deep, B-type if the foci are more than 10 km deep and are unclassified if they result from explosions in or just beneath the crater. They may reach magnitude 5. The harmonic tremor is a characteristic form of volcano-seismicity. It is a narrow band of nearly continuous seismic vibrations dominated by a single frequency and is associated with the rise of magma or volcanically heated fluids. Infrared techniques have been used in the prediction of volcanic eruptions since, due to the rising magma, the volcano area usually becomes hotter than its surroundings. Thermal maps of volcanoes can be produced quickly by ground-based surveys using infrared telescopes (Francis, 1979). However, consistent monitoring is necessary in order to distinguish between real and apparent thermal anomalies. Aerial surveys provide better data but are too expensive to be used for routine monitoring.
Ruapehu and Tongariro stratovolcanoes: a review of current understanding
Published in New Zealand Journal of Geology and Geophysics, 2021
Graham S. Leonard, Rosie P. Cole, Bruce W. Christenson, Chris E. Conway, Shane J. Cronin, John A. Gamble, Tony Hurst, Ben M. Kennedy, Craig A. Miller, Jonathan N. Procter, Leo R. Pure, Dougal B. Townsend, James D. L. White, Colin J. N. Wilson
Seismic recording at Ruapehu started soon after the 1945 eruption, and it was soon found that harmonic tremor, a continuous waveform with a dominant frequency, usually about 2 Hz (as opposed to individual earthquake events), was often present, whether or not there was any obvious volcanic activity (Dibble 1974; Hurst and Sherburn 1993). Latter (1981) introduced the term volcanic earthquake to describe narrow-band earthquakes, also with a dominant frequency about 2 Hz. He described how strong volcanic earthquakes and large tremor amplitudes accompanied magmatic and phreatomagmatic eruptions, but there were many occasions in which similarly strong seismicity occurred without any eruption. These studies were extended by Bryan and Sherburn (1999) and Sherburn et al. (1999), who found that during the 1995–96 eruption sequence there were significant changes in the tremor pattern, with sub-1 Hz tremor accompanying presumed magmatic intrusions during an early stage of the 1995 eruption, while wideband tremor was dominant in the later part of the 1995 eruptions and in the 1996 eruption.