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Volcano Watch — Episode 51 vents now mostly inactive

April 24, 1992

The episode 51 vents on the west flank of Pu`u `O`o were inactive most of this past week. Last Sunday afternoon, the flows had advanced about one mile south of the vents and burned about 150 acres of forest. 

Episode 51 vents now mostly inactive

The episode 51 vents on the west flank of Pu`u `O`o were inactive most of this past week. Last Sunday afternoon, the flows had advanced about one mile south of the vents and burned about 150 acres of forest. However, by Sunday evening, the tremor decreased sharply near the vent, and we infer that the activity at the vents stopped about 8 p.m. The vents remained inactive throughout the week, with the exception of several small sluggish `a`a flows adjacent to them at several times during the week. On Thursday morning between 10 and 11 a.m., the vent began erupting again and a fast-moving flow reoccupied the channel formed last week. The new flow had already progressed about one mile to the south and was burning the forest by 9:30 a.m. on Friday. The eruptive pause this week, lasting from Sunday night to Thursday morning, was the longest since episode 51 on March 7. The map shows the location of the active flow where it enters the forest.

The Pu`u `O`o cone still contains an active lava lake, which is about 130 feet in diameter and about 125 feet below the spillway on the east side of the cone. On Thursday afternoon, the lava lake overflowed the small pit inside Pu`u `O`o and covered the entire floor of the cone with active lava. The level of the lava lake has remained high throughout the week.

An interesting aspect of the eruptive pause this week was that the summit did not reinflate with magma as it has during each of the earlier pauses. The tilt at the summit ranged from an unchanged to a slightly inflated state during most of the week. The number of small earthquakes beneath the summit also increased from background levels of one or two per hour to greater than 4 per hour starting at about 9 p.m. last Sunday night. Throughout the week, the number of earthquakes near the summit remained high, averaging six to seven events per hour, then declined to near-background levels Thursday evening after the episode 51 eruption resumed. All of these earthquakes were too small to be felt, but their abundance indicates that the summit and upper-most East Rift Zone were readjusting to changes in the pressure of magma in the rift zone. We suspect that the combination of abundant earthquakes and nearly unchanged tilt at the summit indicates that magma was continuing to migrate out from beneath the summit, but instead of being erupted at the episode 51 vent, it was being stored beneath the uppermost East Rift Zone during the week.

There were several earthquakes this week with magnitudes greater than 3.0. The first occurred Saturday a week ago at 10:26 p.m., too late to be included in this column. It was located about 115 miles west of Kailua and about 25 miles beneath the sea floor. Although it had a magnitude of about 3.4, we received no reports that it was felt along the Kona coast, probably because it was so far away and so deep. The second earthquakethis week occurred last Sunday at 5:36 p.m. It was located just east of the summit on Mauna Loa Volcano at a depth of about 7 miles and had a magnitude of 3.0.

The episodic nature of episode 51 suggests that this eruption probably will not evolve into a long-lived steady-state one like the Kupaianha vent from July 1986 to February 1992. In addition, the gradual accumulation and storage of magma in the upper East Rift Zone that occurred this past week, and the earlier abrupt intrusion and storage of magma near Mauna Ulu that occurred on March 3, indicate that the underground conduit carrying magma from the summit to the vent is becoming increasingly unstable. At some point, probably within the next six months or so, we expect this instability to lead to an eruption in the upper East Rift Zone.