The following report was received before the start of the 16th episode (see addendum).

EPISODE 15

"The 15th major episode of Kīlauea's protracted series of middle E rift zone eruptions occurred on 14-15 February. As in recent previous eruptive episodes, the vent was at Pu'u O (figure 24).

"Following episode 14, the more gently sloping floor of Pu'u O steepened downward near the crater's center to a nearly vertical 20-m-diameter conduit. Lava was not visible in the conduit on 1 February but was seen during aerial reconnaissance on 3 and 5 February. On 7 February, the partly crusted surface of a magma column in the conduit was 45 m below the crest of the spillway through the crater's breached NE rim. The column rose steadily at a rate of 4-5 m/day. On the morning of 13 February its upper surface was an actively roiled lava pond 18 m below the spillway. By the next morning, the lava column had risen enough for a 100-m flow to have spilled through the breach, after which it subsided to a level about 10-20 m below the spillway. A vigorous 19-hour eruption began during the evening of 14 February.

"Time-lapse film shows that steady low fountain activity within the crater began at about 1940 on 14 February. By 2000, the fountain height began to increase dramatically, and a vigorous lava flow issued NE through the breach. The fountain reached a maximum height of about 320 m at approximately 2100. It remained high for about 6 more hours and was easily visible from the coast S of Royal Gardens. The high fountain produced a thick tephra blanket on the SW side of the vent, and heavy tephra fall was reported by campers on the uprift side of Napau, about 5 km from the vent. The fountain height declined abruptly (by approximately 50%) at about 0330 on 15 February.

"When HVO observers arrived at about 0700 on 15 February, the fountain typically rose about 100 m above the rim of the cone. However, pulsations in its height ranged 20-200 m above the rim. The major lava flow, debouching through the long-lived breach in the crater's NE rim, extended 2 km to the NE. A stagnating aa flow, which had apparently been fed during the night by spatter falling over the S rim of Pu'u O during the more vigorous fountain activity, extended E about 3 km.

"By the end of episode 15 at 1501 on 15 February, the main flow had extended about 5 km NE along the NW edge of flows erupted July-December 1983. Approximately 8 x 10⁶ m³ of new basalt was extruded; the total volume emplaced at the surface since the beginning of January 1983 is about 165 x 10⁶ m³. As in recent previous episodes, the basalt is slightly porphyritic with scattered olivine phenocrysts of about 1 mm size. Lava temperatures measured by thermocouple (in thin pahoehoe overflows from the lava river that supplied the NE flow) were 1,136-1,140°C.

"Steady, low harmonic tremor originating in the vent area was continuous after the end of episode 14. A slight increase in tremor amplitude related to the onset of episode 15 occurred from about 1939 to 1945 on 14 February. After a 1-minute decrease, increase in tremor amplitude resumed at 1946. The amplitude increased rapidly to the maxima reached in previous eruptive episodes, and high tremor continued until the end of eruptive activity. After episode 15, low harmonic tremor continued steadily through the remainder of the month.

"Approximately 7 µrad of inflationary E-W tilt change was recorded in the NW part of Kīlauea's summit region between the end of episode 14 and the onset of episode 15. Deflationary tilt change of 11 µrad, related to the extrusive activity, was recorded from 2030 on 14 February to 2100 on 15 February. Summit reinflation (of nearly 10 µrad) continued from 15 February through the end of the month.

"The morphology of Pu'u O's crater and vertical conduit was essentially unchanged by episode 15 eruptive activity. A partly crusted lava surface deep within the conduit was sighted from the air on 22 February. It was not visible again until 28 February, when it had risen to about 30 m below the surface of the spillway.

"Unlike many of the earlier repose periods, during which magmatic gases at the vent were highly oxidized or so dilute as to be practically undetectable, the repose periods preceding and following episode 15 have been characterized by the continuous emission of fresh, reduced, SO₂-rich magmatic gas. This may reflect the uninterrupted exposure of the active top of the magma column within the conduit (no plug of rubble or solidified lava above the column). Repose-period flux rates of such reduced gas are less than 1% of eruption-related flux rates."

Addendum: Episode 16 began at approximately 1500 on 3 March and ended at about 2230 on 4 March. Lava fountains rose as high as 200 m above the top of Pu'u O and produced a large amount of tephra. The main lava flow, fed by a vigorous pahoehoe river, extended 8 km ESE, reaching the NE corner of Royal Gardens subdivision atop episode 2 lava. Spatter-fed lava flows extended W and N of the vent.

Geological Summary. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: E. Wolfe, A. Okamura, and R. Koyanagi, HVO.