Surface lava breakouts . . . continued to feed flows into the ocean during September. Activity was concentrated in the lower Royal Gardens subdivision (below Orchid street), and in the area between the Royal Gardens kipuka and Kupapau Point (figure 61). Two houses (on Queen St., below Orchid St.) were destroyed on the 28th. Lava emerged from the W tube ~500 m above Chain of Craters Road on the 24th, and reached the road on the 26th. By the end of the month, new lava had covered ~½ km of the road and was entering the ocean near the W portion of the Kailiili bench . . . . On 18 September, lava emerged from the W tube near the remains of the National Park Service residential area, covering the E portion of the Wahaula kipuka. The Heiau . . . was not affected. Within two days, the flow had reached the ocean at the W edge of the Poupou entry . . . where small volumes of lava intermittently entered the ocean throughout the month. The Kupapau Point entry, which had been stagnant since 21 July, was reactivated by a new flow that entered the ocean there on 3 September and remained active throughout the month. The entry E of Kupapau Point remained inactive, but breakouts continued to the E, with one flow (~475 m to the E) reaching the sea on the 14th. By 21 September, all activity E of Kupapau Point had ceased.

Lava in the Kupaianaha pond averaged 25-27 m below the rim through September. No lava was observed in Pu`u `O`o crater during the month.

Low-level tremor continued . . . near Pu`u `O`o and Kupaianaha. Occasional bursts of rockfall signals accompanied the steady background tremor from Pu`u `O`o. Weak signals associated with the submarine component of the oceanfront lava flow activity were detected by the Wahaula seismometer (~2 km NW). Intermediate-depth (6-15 km) long-period events occurred episodically beneath the summit region. The number of shallow (<5 km) microearthquakes was otherwise about average in the summit region and along the East rift zone. Several hundred tectonic events (M 0.5-3.6) located during September were concentrated along the S flank of Kīlauea and the SE flank of Mauna Loa at ~5-15 km depth.

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: C. Heliker and R. Koyanagi, HVO.