Volcano Watch — Recent earthquake activity recalls Kapoho eruption

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Earthquake activity this past week was considerably reduced from the high levels recorded the previous week. Three earthquakes occurred that had magnitudes greater than 3.0. The first, a magnitude 3.4, occurred on February 2 at 1:24 p.m. beneath the south flank of Kīlauea Volcano.

Recent earthquake activity recalls Kapoho eruption...

Recent earthquake activity recalls Kapoho eruption

(Public domain.)

Earthquake activity this past week was considerably reduced from the high levels recorded the previous week. Three earthquakes occurred that had magnitudes greater than 3.0. The first, a magnitude 3.4, occurred on February 2 at 1:24 p.m. beneath the south flank of Kīlauea Volcano. The second, a magnitude 3.0, occurred at 2 minutes past midnight on February 4; it was also beneath Kīlauea's south flank. The final earthquake had a magnitude of 3.1 and occurred at 23 minutes after midnight on February 5. It was located in the Ka`oiki fault zone between Kīlauea and Mauna Loa Volcanoes.

Lava continues to be delivered to the coast near Kamoamoa through an underground tube system in the ongoing eruption on Kīlauea's East Rift Zone. Where the lava enters the sea, numerous small explosions have occurred in the past few days. As the lava delta beyond the old shoreline is highly unstable, parts of it may collapse into the sea at any time. 

The phreatic (steam) explosions occurring where the lava enters the sea are small, compared with those generated when groundwater interacts with magma at eruptive vents. In January and early February in 1960, a major eruption took place along the lower east rift zone near Kapoho. This eruption was unusual in that shallow, saline groundwater was heated by the magma to produce steam explosions. This eruption was also the last to take place along the lowermost part of Kīlauea's East Rift Zone. 

The Kapoho eruption was preceded by the summit eruption at Kīlauea Iki, which ended in December 1959. Soon after the eruption near the summit ended, numerous small earthquakes were recorded along the east rift zone. Starting January 10, the earthquakes localized near the village of Kapoho. In the following days, stronger earthquakes occurred near the village, and ground cracks began to open. A crack that crossed the paved road in the village widened from about one inch to three feet during the day on January 13. Ground subsidence on the north side of the crack amounted to at least one foot by that afternoon, and another, parallel crack showed vertical offset of about 2.5 feet. This subsidence took place in a narrow graben in which the village of Kapoho was located. This region of subsidence was not new (the name "Kapoho" means "the sunken place"). By 7:30 p.m. that evening, lava had broken out about one-half mile northwest of the center of Kapoho village.

The fissure opened from west to east and eventually resulted in a half-mile long curtain of fire. Steam blasts accompanied - and, in some sections of the fissure, alternated with - the erupting lava. Such explosions continued sporadically throughout the eruption. The steam mixed with lava and produced a spray of molten lava, quenched lava fragments, excavated fragments of older lava, and salt. This is the same type of material ejected from the littoral explosions, where the lava is now entering the sea at Kamoamoa.

By the morning of January 14, the activity was located about one-third mile northwest of the village. Fountains frequently reached heights of 1,000 feet, and a cinder cone rapidly grew. By January 15, a flow issued from the base of the fountains had reached the ocean to the northeast. The volume of lava being extruded was estimated to be about 5,000,000 cubic meters per day (in comparison, the current eruption rate is estimated at 100,000 to perhaps 200,000 cubic meters per day). Walls were constructed to prevent the lava from flowing towards Kapoho. It was recognized that if the lava became too deep, it would overtop the 20-to-30-foot-tall walls, and the town would be lost. 

As in any attempt to save particular areas of land from lava burial, one gambles staving off or diverting the flow long enough before the eruption ends. In the case of Kapoho, the eruption did not end, and the town was buried. The eruption finally ended on February 19, after extruding about 115,000,000 cubic meters of lava. The entire east cape of the island was changed, with the new shoreline as much as a half-mile seaward from the pre-eruption shoreline. 

The accompanying map, modified from that of a 1962 report by Gordon Macdonald (Director of the Hawaiian Volcano Observatory from 1948-1955), shows the area covered by the 1960 flows and the locations of the cinder deposits along the eruptive fissure, the old shoreline, the current shoreline, and the lava-retaining walls.

Much of lower Puna is classified in lava hazard zones 1 or 2 because of the occurrence of large eruptions along the lowermost part of the East Rift Zone in 1840, 1955, and 1960 (the eruption described here). The record of eruptive activity on the East Rift Zone, particularly since 1955, suggests that the 1960 eruption will probably not be the last in this area.