On 25 June, HVO recorded a small but rapid summit-area inflation episode accompanied by shallow microearthquakes and harmonic tremor. As rapid summit inflation started about 1500, a single seismic station in the SE part of the caldera began to record weak harmonic tremor. Six minutes later, tremor amplitude increased and scattered small individual earthquakes started to appear, still at only one seismic station. Tremor strengthened again and earthquakes became larger and more frequent at 1510, as instruments to a radius of 5 km began to register the individual events.

A magnitude 2.5 earthquake at 1517 was followed by relatively intense localized shallow activity, including tremor and more than 6 microearthquakes per minute between 1518 and 1530. Inflation ended about 1530, as harmonic tremor weakened and the number of microearthquakes started to decline. Between 1500 and 1530, 6 µrad of inflation had been registered by the E-W component of a continuously-recording tiltmeter on the W rim of the caldera. No tremor was detected after 1600, but microearthquake activity remained high, gradually decreasing over the next 2 days. After inflation had ended, reoccupation of dry tilt stations near the caldera showed a maximum net tilt for the episode of 20 µrad (toward the NW) at a station near the SW rim.

The USGS's preliminary interpretation of this episode is that a small shallow intrusion [took place] beneath the S or SE part of the caldera. None of the hypocenters was shallower than 2 km, indicating that magma came no closer than that to the surface.

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: R. Koyanagi and R. Decker, HVO.