A volcanomagnetic observation on Mount St. Helens, Washington

Abstract

Total magnetic fields were recorded at five sites on Mt. St. Helens between 23 October and 11 November 1981, an interval which included an extrusive dome‐building eruption of the volcano. Two of the magnetometers located in the volcano's crater measured reversible magnetic changes, which correspond to fluctuations in tilt measured nearby. However, the relationship is highly nonlinear. Electric fields were measured on the east flank of the volcano near its summit to search for electrokinetic effects. They show no correlation with the magnetic changes and, in the long term, are uncorrelated with eruptive activity. Our favored interpretation of the magnetic changes is that they result from stress‐induced changes in the magnetization of the volcano. Magnetic field values returned to pre‐anomaly values. This reversibility rules out pressure‐induced magnetization as the dominant mechanism and places an upper limit of σ ∼300 bars on the stress changes. The limited spatial extent of the magnetic anomaly field places the source of stress at shallow depth beneath the crater floor consistent with models based on strain data.

Fractal dimension analysis of the magnetic time series associated with the volcanic activity of Popocatépetl

“…Changes in the local magnetic field of up to a few tens of nanoteslas (nT) have been reported during volcanic activity (Rikitake, 1951;Johnston and Stacey, 1969a, b;Pozzi et al, 1979;Johnston et al, 1981;Zlotnicki and Le Mouël, 1988;Honkura et al, 1995;Sasai et al, 1990;Zlotnicki et al, 1993;Tanaka, 1993Tanaka, , 1995. The available data indicate that small-amplitude magnetic signals of a few nT can occur at the most dangerous volcanoes, such as explosive andesitic volcanoes (Johnston et al, 1981;Davis et al, 1984;Zlotnicki, 1986;Zlotnicki et al, 1987;Tanaka, 1995;Zlotnicki and Bof, 1998), including Merapi Volcano, the most intensively monitored volcano in Indonesia.…”

Electric and magnetic field measurements on Mount St. Helens volcano at times of eruptions 1980-1985.

“…The system was powered by Air Cells and recorded onto two Russtrack recorders with coarse (2V) and fine (200mV) ranges. It survived about two years and we have records during the last of the explosive eruptions of 1980 (16-18 October) and during some extrusion events in 1981 when we also recorded magnetic fields in the crater of the volcano (DAVIS et al, 1984).…”

“…A further reason for measuring electrokinetic effects on a volcano is the possibility that the magnetic events that have been observed on a number of volcanoes (JOHNSTON and STACEY, 1969a, b;DAVIS et al, 1979DAVIS et al, , 1984 and some earthquakes (SASAI and ISHIKAWA, 1980;SHAPIRO and ABDULLABEKOV, 1983;JOHNSTON and MUELLER, 1987) are not in fact due to piezomagnetic effects in crustal rock, the favored explanation, but are due to changes in the current flow pattern in the earth. Thus, simultaneous measurement of magnetic and electric fields could, in principle, help to discriminate between "magnetokinetic" and piezomagnetic effects which can occur with or without, respectively, substantial associated electric field changes.…”

Seismo-Magnetic Studies between Dokurcun and Abant Area along the North Anatolian Fault Zone, Turkey.

“…Authors such as Kalashnikov (1954), Stacey (1963), Nagata (1970, and Stacey and Johnston (1972) have suggested the tectonomagnetic method for predicting crustal failure. Studies oftectonomagnetic effects have been and are currently being conducted in many regions of the world and magnetic field changes associated with earthquakes have been identified by several authors such as Fujita (1965), Tazima (1968), Yamazaki and Rikitake (1970), Abdullabekov et al (1972), Smith and Johnston (1976), Raleigh et al (1977), Shapiro and Abdullabekov (1982), Davis et al (1984), Coutin et al (1985), Rikitake (1987), Sasai (1989). The aim of this paper is to present a comprehensive interpretation of the relationship between local magnetic anomalies and seismic activity in the investigation area.…”