Volcano Watch — Forgotten faults

Most visitors to Hawai`i Volcanoes National Park have driven down the Chain of Craters Road to the coast and observed the high south-facing pali that form the Hilina fault system of Kīlauea Volcano. These active faults border huge blocks of the volcano that occasionally move downward and seaward in jumps of a meter (3 ft) or more at a time, accompanied by large earthquakes. In west Hawai`i, the pali on the north side of Kealakekua Bay is a good example of an active fault that strikingly resembles those in the Hilina system.

A lesser known and under-appreciated set of active faults and gaping ground cracks is the Koa`e fault system on Kīlauea. From the Chain of Craters Road to the hair-pin bend near Kipuka Nene, the Hilina Pali Road winds its way around fault-formed pali and across open cracks. The zone of faults is more than 3 km (2 miles) wide and 15 km (9 miles) long, reaching from near Mauna Ulu to the southwest rift zone at the Kamakai`a Hills. Most of the pali face north, toward the summit of the volcano; the most prominent is 15 m high (50 ft high) Kulanaokuaiki Pali, which forms the southern boundary of the Koa`e fault system. The faults continue into the east and southwest rift zones, where they merge with eruptive fissures.

Faults in the Koa`e are active. Parts of the fault system have cracked and been tossed up and down during big swarms of earthquakes several times during this century. The largest historical episode of cracking took place on Christmas Eve and Day 1965, when the Hilina Pali Road at its crossing of Kulanaokuaiki Pali was broken vertically more than 2.4 m (8 ft); the pali itself jumped up about 75 cm (2.5 ft), and the flat ground north of it sank about 1.8 m (6 ft). As USGS observers Dick Fiske and Bob Koyanagi wrote, "At 0840 [Christmas Day] the area was wrenched by an earthquake so violent that it nearly toppled a vehicle parked nearby, and the crack [at the foot of the pali] abruptly opened to about 1.5 m (5 feet)." Cracks and faults continued to move for another two days.

Occasionally eruptions take place from vents near both ends of the Koa`e fault system. The most recent was in 1973 next to the east rift zone. For the most part, however, the Koa`e serves as a non-eruptive "bridge" connecting the two rift zones. The rarity of eruptions is probably why this remarkable fault zone, a key to understanding the growth of Kīlauea, is so forgotten. Why go there when you can see liquid lava elsewhere?

From geologic studies we know that the Koa`e fault system has been active for at least the past 1,100 years and probably much longer. Numerous cracks totaling 18~34 m (60~110 ft) in width cut 400-year-old lava flows in the Koa`e south of Kīlauea's caldera. Other measurements suggest that similar cracking probably also took place repeatedly between 1,100 and 400 years ago. Strangely, the far western part of the fault system has not moved recently; here, unbroken beds of ash erupted in 1790 cover older cracks.

The origin of the Koa`e fault system is controversial, but many geologists believe it to be related to the splitting of Kīlauea along the east and southwest rift zones as the volcano is driven apart by a combination of gravitational collapse and magma wedging into the rift zones. That process is episodic, not continuous. At present, the Koa`e is simply sitting on the southern part of the volcano, which is rafting seaward. But eventually the rocks will break, the faults will move, and a new round of repairs will be due on the Hilina Pali Road.