Volcano Watch — No episode 52 after all, just repeats of 51

With the wisdom of hindsight, we have concluded that the continuing episodic eruptive activity from the vent on the west flank of the Pu`u `O`o cone is a single episode with pauses, rather than a new episode with each eruption at the vent. We are therefore continuing to call the activity that began on March 7 "episode 51." The eruptive activity has been sporadic during this time and the volume has also varied greatly during the eruptive periods. We have combined direct field observations, observations of Pu`u `O`o glowing from Glenwood, and the amplitude of harmonic tremor recorded on the seismic station nearest the Pu`u `O`o cone to establish the times when the vent was in eruption and when it was not.

Harmonic tremor is an actual rhythmic vibration of the ground caused by movement of a liquid, in this case lava, through a tube or restricted opening, in this case the vent. It is similar to the sound produced by a wind instrument when a fluid, such as air, moves through a tube, such as a flute or clarinet. The only real difference is that the wind instrument makes a high-frequency vibration that we can hear whereas the lava through the vent makes a much lower-frequency vibration that we can feel. The nearby seismic station also feels this ground vibration and records it as a small-scale rhythmic movement of the ground. On the seismogram, the trace looks like a wavy line. The amplitude of the tremor is simply the height of those wavy lines and is a measure of the intensity of the ground vibration, and probably a rough measure of the amount of lava moving through the vent. The figure shows the amplitude of tremor recorded at the seismic station nearest Pu`u `O`o for the period from March 7 to the morning of March 20. The eruptive periods are also shown. 

On March 7, episode 51 began about 12:45 in the afternoon, but by 9:00 p.m. there was no glow visible from Glenwood, except the one seen directly above the lava pond inside the main cone at Pu`u `O`o. By midnight, there was strong glow originating outside the cone on the west side indicating that the eruption had restarted. The activity continued until March 12, when the volume of erupting lava began to sharply decrease at about 2:30 in the afternoon. By 9:00 that evening, once again, no glow was visible from Glenwood, (except that which was directly above the pond in the cone) and the tremor had decreased to background levels. 

On March 14, the vent again became active, starting about 11:30 in the morning and continuing until about 6:00 p.m. the next day. The level of activity gradually increased at the beginning of this activity and declined gradually at the end of the period of activity. On March 17 at about 6:00 a.m., the vent abruptly restarted and continued until about 3:00 a.m on March 18, when the activity apparently stopped briefly. By 6:00 a.m. that same day, the vent was again active, but with only a small-volume eruption. The level of activity increased until about 1:30 in the afternoon, but the volume remained at perhaps half that of the previous eruptive intervals. Since 1:30 p.m. on the 18th, activity has been continuous, although the volume may have fluctuated as seen in short periods of increased tremor.

The flows from the repeated eruptive periods in episode 51 have not progressed far from the vent, because each time the vent stops, the flows solidify. Each time the vent restarts, the new flows cover the same ground with another flow, rather than simply advancing the flows from the previous eruptive period. Most of the lava is simply accumulating near the vent on top of the previous flows from episodes 51 and 50. The maximum advance of flows has been about 1 mile towards the south.

The tilt level during episode 51, which indicates the amount of magma stored beneath the summit, has been increasing. During each eruptive interval, the tilt has been either flat or slightly decreased, but during the non-eruptive intervals, the tilt level has increased sharply. The overall addition of magma beneath the summit indicates that more magma has been added to the magma storage system than has been erupted during episode 51. The level of seismic activity at Kīlauea has been very low during both the eruptive and non-eruptive intervals of episode 51. Episode 51 will finally end when the vent adjacent to the Pu`u `O`o cone stops erupting and the subsequent eruptive or intrusive activity begins in a different location. Such activity should be accompanied by a fairly sharp drop in the level of summit tilt as magma migrates from beneath the summit to the new eruptive or intrusive site.