Ground movements in the Koa`e fault system

Kilauea has one of the most active fault systems in the world. The Koa`e fault system is 2-3 km (1.2-1.8 miles) wide and extends about 17 km (10 miles) between the east and southwest rift zones, south of Kilauea’s caldera. Few if any eruptions take place within the fault system, and so there is little reason for the general public to hike across this remarkable country. But remarkable it is.

The Koa`e fault system is an infrequently visited area, crossed only by the Hilina Pali Road between the Chain of Craters Road and Kulanaokuaiki Pali. The Mauna Iki Trail, heavily used only during the Kilauea marathon, takes an easy route across the Koa`e fault system between Kulanaokuaiki Pali and Cone Crater.

Creased by deep cracks and corrugated by vertical pali as high as 15 m (50 feet), measurements of open cracks show that the fault system has opened by at least 32 m (100 feet) since the latest lava flow covered the area 600-700 years ago. The most recent major faulting event, in December 1965, offset the Hilina Pali Road 2.4 m (8 feet) vertically where it crosses the fault that forms Kulanaokuaiki Pali.

The faults trend east-northeast, parallel to the east rift zone. In fact, the fault system can be viewed as a noneruptive continuation of the east rift zone. One interpretation is that the east rift zone, Koa`e fault system, and lower southwest rift zone form a “breakaway zone” separating the relatively stable part of Kilauea to the north from the mobile part of the volcano to the south.

The big earthquake swarm and faulting event in December 1965 piqued the interest of HVO scientists. Within several years, a detailed network of surveying stations (benchmarks) was established and measured. The surveys enable both vertical and horizontal changes to be detected. Small changes were measured as a result of an intrusion into the eastern part of the fault system in May 1973 and the Kalapana earthquake in November 1975.

Additional benchmarks were added during the early 1990s, bringing the total number of survey points to nearly 300. Three leveling lines, used to measure vertical changes, cross the Koa`e fault system at right angles, and two of the lines are connected by a line parallel to the faults. These lines, each consisting of benchmarks 90 m (300 feet) apart, are located between the Mauna Iki Trail and the boundary between the Keauhou and Kapapala ahupua`a.

Horizontal changes can be measured at 50 benchmarks forming a network that spans the fault system from Pu`u Koa`e eastward to the ahupua`a boundary. The network used to reach still farther east, but the incursion of the faya bush has made the forest into a jungle that inhibits surveying.

Later this month, scientists from the Smithsonian Institution and the Hawaiian Volcano Observatory will join forces to resurvey the complete network, which was last done in 1998. This enterprise, estimated to take 7-10 very full days, may be the last time that the originators of the networks in the late 1960s are able to take part in the rigorous surveys.

This will also be the time to change the instrument for making the horizontal surveys. The network has always been measured with an Electronic Distance Meter (EDM), which requires a clear line of sight between the end points of each line-something increasingly difficult to maintain as the forest grows taller. This year, the EDM will be used for the last time, and a nearly contemporaneous Global Positioning System (GPS) survey will be made at many of the same benchmarks. The GPS survey will take more time but does not require line-of-sight visibility.

The surveying puts money in the bank. The account may not be drawn upon for some time, but the interest accumulates in the form of new ideas that can be tested when a big event-such as an earthquake swarm in the Koa`e fault system-takes place. Then, the banked information can be compared with new surveying results to provide a better understanding of the process that caused the earthquakes and faulting.

Activity Update

Eruptive activity at the Pu`u `O`o vent of Kilauea Volcano continued unabated during the past week. Scattered surface breakouts from the western “Kohola” lobe of the Mother’s Day flow are seen throughout the inflating flow. Streaks, patches, and spots of incandescence are visible from the coast to Paliuli. Two fingers of lava from the eastern side of the Kohola flow made it over the sea cliff onto the Wilipe`a delta but stagnated before reaching the ocean. A western finger burned more asphalt, then stopped after crossing the road. Several tongues of lava from breakouts of the main Mother’s Day tube system are wending their way down Pulama pali and giving visitors a great view. The only lava entering the ocean is at the West Highcastle delta.

The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

One earthquake was reported felt during the past week. Residents of Glenwood and Volcano felt an earthquake at 4:34 a.m. on April 1. The magnitude-2.8 event was located 10 km (6 mi) southeast of Kilauea Summit at a depth of 9 km (5.4 mi).

Mauna Loa is not erupting. The summit region continues to inflate, but seismic activity remains low, with only one earthquake located in the summit area during the last seven days.

This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory and is republished by HawaiiNews.com with permission.