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Volcano Watch — Leveling survey measures rise and fall of volcano's surface

July 13, 2000

We recently completed a leveling survey along the Ka`u trail in Hawai`i Volcanoes National Park. This 16-km (10 mi) route runs inland from the south coast of the Big Island, crosses the Great Crack, and reaches Highway 11 between Pahala and Kīlauea's summit.

We recently completed a leveling survey along the Ka`u trail in Hawai`i Volcanoes National Park. This 16-km (10 mi) route runs inland from the south coast of the Big Island, crosses the Great Crack, and reaches Highway 11 between Pahala and Kīlauea's summit. It follows a trail that existed before Cook reached Hawai`i in 1778. The Ka`u trail was first leveled in 1921, when six permanent bench marks were placed on the route. To work along the well-worn trail is a journey back in time and a means to look to the future.

Leveling is a time-honored technique to measure vertical (up and down) motion of the Earth's crust. This motion is especially pronounced on an active volcano for several reasons. Magma moving beneath the surface can cause part of the volcano to bulge or subside. The shear weight of a growing volcano can cause the underlying crust to sag. And earthquakes can cause abrupt vertical motion.

The Ka`u trail traverses part of Kīlauea's south flank, the most actively deforming part of the volcano. The south flank is capable of producing devastating earthquakes, which result when the amount of deformation exceeds the strength of the rock.

Leveling uses a telescope with its optical axis aligned to horizontal. A surveyor places the telescope between two calibrated rods standing vertically on points a few tens of meters (yards) apart. A height reading is taken on each rod. The difference between these heights is the elevation difference between the points. Leapfrogging through dozens of setups, the survey progresses from bench mark to bench mark. Adding together all the elevation differences between setups gives the elevation difference between successive bench marks. Repeated surveys of the same bench marks reveal the precise vertical changes that have occurred along the Ka`u trail, accurate to within a few millimeters (a few hundredths of a foot).

Following the 1921 survey, the Ka`u trail was surveyed in 1975 and again in 1976. The 1976 survey measured changes resulting from the November 29, 1975, Kalapana earthquake. Leveling showed that, relative to a starting point on Highway 11, subsidence increased seaward along the trail, with amounts as great as 40 cm (16 in.) near the base of Kukalau`ula Pali, in the Hilina fault system. These changes are trifling compared to the subsidence as great as 3.5 m (11.5 ft) closer to the earthquake's epicenter. The reason is because the Ka`u trail is near the western limit of the Hilina fault system.

Deformation has continued along the Ka`u trail since 1976. Relative to the Highway 11 point, the inland region has subsided gently, only about 5 cm (2 in.). In contrast, the area close to the coastal plain has risen as much as 15 cm (6 in.). Vertical deformation seemingly stops at the foot of Kukalau`ula Pali. The coastal plain south of the pali has changed little, an observation corroborated by GPS surveys of benchmarks near Nali`ikakani Point.

These findings indicate that a narrow zone forms the current boundary between moderately deformed and less deformed crustal blocks of Kīlauea volcano. This zone is coincident with faults at the base of Kukalau`ula Pali. The unanswered question is how much can the inland region deform before the crust once again ruptures along this zone? Or will earthquakes elsewhere in the region relieve the vertical changes that have accumulated in the past 25 years? The answer probably lies a few years in the future.

Volcano Activity Update

Eruptive activity of Kīlauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through a network of tubes toward the coast near the eastern boundary of Hawai`i Volcanoes National Park. A 1-km-long pahoehoe flow from a breakout of the tube system was observed on Pulama pali. Multiple areas of the flow field in the coastal flats are inflating and forming tumuli and pillow-like lobes. Lava is entering the ocean mainly at two locations: Waha`ula and Kamokuna. A third entry, located 500 m (550 yards) to the east of Waha`ula, is erratic and wispy. The public is reminded that the ocean-entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The active lava flows are hot and have places with very thin crust. The steam clouds are highly acidic and laced with glass particles.

Two earthquakes were reported felt during the week ending on July 13, 2000. A magnitude-3.1 earthquake located 9 km (5.4 mi) west of Pahala was felt in Miloli`i at 2:27 a.m. on July 6. The earthquake originated at a depth of 10 km (6 mi). Residents of Fern Forest and Aloha Estates subdivisions felt an earthquake at 5:01 p.m. on July 7. The magnitude-3.2 earthquake was located 12 km (7.2 mi) southeast of the summit of Kīlauea Volcano at a depth of 6.6 km (4 mi).

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