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April 1, 1999

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Prelude to an explosion

It started innocently enough. The long-lived lava lake disappeared from Halema`uma`u on February 21. Seventy-eight earthquakes were recorded in March, many along Kilauea's east rift zone as far as 45 km (27 miles) from HVO; one on the 29th was felt in Hilo. All in all, there was nothing to get terribly alarmed about, and March went out like a lamb. But by the time April 1924 ended, residents of lower Puna had stories to tell their grandchildren. And there was more to come in May. This column and a second next month recall these events on the 75th anniversary of Kilauea's only explosive eruption of this century.

Seismicity increased in April and seemed to move down the east rift zone from Kilauea's summit area. On April 10, a "strongish" earthquake came from Puna and was felt in Hilo. Field examinations by HVO scientists found nothing unusual, and residents of the Pu`ulena area were not worried.

The Kapoho-Malama homesteads area began to shake on about April 17, and subterranean noises were heard. Henry Lyman of Kapoho felt 88 earthquakes in 24 hours on April 21-22. During the next night, Lyman's place was quaking almost continuously with several hundred felt earthquakes, and an observer at the cinder quarry in Kapoho felt 238 earthquakes in a 4-hour period.

The ground first cracked open on the night of April 22-23, during the most intense shaking. About 20 gaping cracks extended from Kapoho village uprift to Pu`u Ki`i and downrift to the east tip of the island. The zone of cracks paralleled the east rift zone and was some 6.5 km (4 miles) long and 1.5 km (1 mile) wide. One single chasm eventually grew to 4.6 m (15 feet) across.

The broken zone was bounded by faults with vertical movement. The largest was a reactivated fault along the northern boundary of the Kula ahupua`a. Land just south of the Kula fault sank 2.4-3.7 m (8-12 feet), destroying railroad tracks and the road from Kuki`i to Koa`e. A zone of cracks formed the south side of the 0.8-km-wide (0.5-mi-wide) sunken block, a "graben" geologically speaking. These cracks, extending northeast from Pu`ukea (at the northwest foot of Kapoho Cone) to Pu`u Kukae, formed a kind of hinge zone; the amount of sinking of the graben increased northward from the hinge to the Kula fault. The graben extended downrift to the coastline at a place called Kapele northwest of Cape Kumukahi, dropping the shore about 3.7 m (12 feet) and forming a new lagoon 2-2.5 m (8-12 feet) deep that reached inland 60 m (200 feet).

Many field workers were evacuated to Hilo during these frightening days. Only the most adventurous residents stayed. No one knew what was going to happen next, and fears of an eruption must have occupied the minds of all. The seismicity peaked on April 23, however, and most of the crisis was over by April 27, though some cracks continued to widen until April 29.

Little evidence remains of these remarkable events. The 1960 flow destroyed Kapoho village and covered most of the cracks. Part of the Kula fault is visible north of Halekamahina, but most of it and the 1924 graben are only memories. Store those memories. The graben is a major feature of the rift zone that will doubtless sink again, just as it did before and in 1924.

HVO scientists then and now believe that the lava which drained from Halema`uma`u in February moved down the rift zone and spread apart the Kapoho area to make room for itself. Whether an offshore eruption occurred is arguable. Next month a follow-up column traces what happened to Halema`uma`u in May 1924 as a result of the draining of lava from the lake.

Eruption Update

Lava continues to erupt from Pu`u `O`o and flow through a network of tubes from the vent to the sea near Kamokuna. A pahoehoe flow emanating from a breach in the tube system was active on the coastal flats since Friday, March 26. The flow is 1.4 km (0.8 mi) long, and the distal end is about 1.2 km (0.7 mi) from the ocean. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. A collapse occurred on Wednesday afternoon, March 31. All the new land (2.6 hectares or 6.4 acres) formed since March 8 disappeared beneath the waves. The steam clouds are also very hazardous, being highly acidic and laced with glass particles.

There were no felt earthquakes since March 18.

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Updated: 8 Apr 1999