Magma was intruded into the upper E rift zone on 27 August, the first intrusive activity there since March. As in the two March intrusions, no eruption took place. An earthquake swarm began near Puhimau Crater (about 1.5 km SE of the caldera rim) at 1425 on 27 August. Within 6 minutes, the number of microearthquakes had increased to several per minute. Summit deflation started 30 minutes after the swarm, at 1455.

The earthquakes migrated generally downrift at about 1 km/hour. Several were felt nearby, with the three highest magnitudes ranging from 3.6 to 4.0. Hundreds of magnitude 1-3 events occurred at depths of 1-4 km. The seismographs closest to the swarm apparently registered some shallow volcanic tremor, but microearthquakes occurred so rapidly that tremor was obscured on the records. The number of microearthquakes per minute started to decrease at about 1830 and the swarm ended early the next morning. Summit deflation had stopped at about 1930 on 27 August, after 7.5 µrad of tilt had been recorded.

The USGS interpreted the activity as the formation of a dike estimated to be about 3 km long, 1-2 km high, and l m wide, located 1-3 km beneath the surface. About 4 x 10⁶ m³ of magma were calculated to have been injected into the dike between 1500 and 1930 on 27 August.

SO₂ emission was detected in the Puhimau thermal area (where the earthquake swarm began) on 28 August. The CO₂/SO₂ ratios of gases emitted by the summit fumaroles before and after the intrusion remained about the same as the previous week.

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: Hawaiian Volcano Observatory, USGS.