抄録
In late September 1980 we installed two continuously recording self-potential lines high on the east flank of Mt. St. Helens volcano. We present here an 8 month record which includes the last of the major explosive eruptions of the series that began with the May 18, 1980 blast. The electric field changes we recorded were less than 0.4 volts on each of our lines for the 8 months. An unusual variation in the electric field occurred on October 12, 4 days before the first explosive eruption of the October series. The longer (1.6km) line measured a voltage change of 0.4 volts, about twice the amplitude recorded on the shorter (0.6km) line (0.2 volts). These lines shared a common reference electrode at the point of their highest elevation on the volcano, and both extended radially from the breached crater down the volcano slope. This short-term variation may have been caused by a transient, regional electric field change of scale length greater than 1.6km that developed on the volcano, having an amplitude of 0.31volts/km and a duration of about 12 hours. Electrokinetic effects or rainfall are possible explanations for this event. During the explosive eruptions no electric field changes were recorded by our self-potential lines (<0.02V/km).
In 1981 we added to the magnetic stations already on the flanks of the volcano by installing 4 magnetic stations in the breached crater. The station nearest to the dome was at a distance of 1km. A magnetic transient was recorded during a dome building eruption in 1981; since no electric field transient accompanied this event and it was associated with extreme nearby strain, we suggest that the magnetic changes were produced by the piezomagnetic effect. During three later time periods, (October-November 83, March-December 84, January August 85), both the tiltmeters and magnetometers recorded minor variations. These results suggest that the stresses associated with continued dome growth at Mt. St. Helens have become increasingly confined to the immediate vicinity (within 1km) of the dome.
