Volcano Watch — Episode 51 stops, 52 begins at Kīlauea Volcano

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Episode 51 of the nine-year long eruption along Kīlauea Volcano's East Rift Zone stopped by 4:00 p.m. Thursday afternoon. As we forecast last Sunday, this episode was brief, lasting only from March 7-12. Flows from episode 51 went both north and south, but neither advanced more than about one mile from the vent on the west flank of the Pu`u `O`o cone. Most of the lava ponded above the flows erupted during episode 50, which was also short-lived, lasting only from February 17 to March 3. 

Episode 50 ended soon after an intrusion of magma occurred beneath the upper East Rift Zone starting early on the morning of March 3. The earthquake swarm caused by that intrusion consisted of nearly 2,400 separate events located less than 2 miles deep between the summit and Pauahi Crater, as recorded on the U.S. Geological Survey's seismic network. Five of these events and three additional earthquakes in the following two days had magnitudes greater than 3.0. Two of the later events were less than 2 miles deep and were located directly beneath the East Rift Zone. The third was four miles deeper and was related to southward motion of the south flank of Kīlauea Volcano.

The vigorous harmonic tremor recorded at a seismic station near Pu`u `O`o during all of episode 51 subsided early Thursday afternoon as the volume of erupting lava declined before the eruption stopped. Tremor continues at moderate levels near Pu`u `O`o, apparently caused by convection and degassing of lava in the pond inside Pu`u `O`o. The pond is now about 30 to 45 feet lower than the level before episode 49 began and about 140 to 155 feet below the rim of the cone. 

The degree of inflation or deflation of Kīlauea's summit is an important parameter in forecasting future activity and in understanding what the magmatic system is doing. Unfortunately, obtaining a true measure of inflation requires a combination of precise elevation measurements (leveling) combined with precise distance measurements. These measurements are done periodically but are labor intensive and time-consuming to perform. Another technique that is less quantitative is to continuously monitor changes in the angle, or tilt, of the surface, using electronic instrumentation. The amounts of tilting recorded are generally very small and are measured in units of micro-radians (one micro-radian is the tilt caused by raising one end of a half-mile long board by the thickness of a dime). The figure shows the changes in tilt recorded at a station near the Hawaiian Volcano Observatory for the past four and one-half months. 

The large decreases in tilt that accompanied the episode 49 and 50 eruptions and the intrusion into the upper East Rift Zone on March 3 are caused by a net loss of magma from beneath the summit region. Between episodes 49 and 50, the summit had a fairly steady increase in tilt, showing that magma was being supplied to the summit from below at a rate greater than the declining eruption rate at the Kupaianaha vent. The tilt change associated with the episode 51 eruption was small, indicating that lava erupted at the East Rift was balanced by magma supplied to the summit. Since episode 51 stopped on Thursday afternoon, the summit is once again inflating as magma is added beneath the summit.

As the summit tilt increases, the pressure in the system increases. This increase in pressure should raise the level of the pond inside Pu`u `O`o. As the pond level increases, it will increase the likelihood of a failure of the side of the cone and of renewed activity on the flank of the cone. If the cone is strong enough, then the increasing pressure in the summit could lead to another intrusion, and perhaps eruption, in the upper East Rift Zone, most probably near Pauahai Crater, where ground cracks formed during the March 3 intrusion.

At press time yesterday, commercial pilots had reported the beginning of episode 52 with lava flowing from a vent near the site of previous episodes 50 and 51. But due to inclement weather, no further information was available.