# Volcano Watch — Kīlauea has its faults

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Visitors to the lava viewing area at the end of the Chain of Craters road in Hawaii Volcanoes National Park travel over one of the largest fault systems on the island on Hawaii. The Hilina fault system, located within the south flank of Kīlauea Volcano, consists of a series of subparallel and en echelon normal faults.

Visitors to the lava viewing area at the end of the Chain of Craters road in Hawaii Volcanoes National Park travel over one of the largest fault systems on the island on Hawaii. The Hilina fault system, located within the south flank of Kīlauea Volcano, consists of a series of subparallel and en echelon normal faults.

Normal faults are fractures in the Earth's surface where one block appears to be displaced downward relative to an adjacent block. They occur in areas subject to tensional stress or extension.

The gravitationally induced seaward sliding of the unbuttressed south flank of Kīlauea Volcano produces the tensional conditions. Periodic measurements by the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) indicate that the coastal areas are moving at a rate of 2.5 inches per year, about as fast as your fingernails grow, while inland areas are moving at slower rates.

Occasionally, the entire flank will move rapidly and far, as it did during the early morning hours of November 29, 1975, when a magnitude-7.2 earthquake occurred. Parts of the Hilina fault system were displaced over 10 feet seaward and another 10 feet downward during the earthquake. The dynamic nature of the region makes it a natural laboratory for surface deformation studies.

Through cooperative investigations with the University of Hawaii and Stanford University, HVO receives data from a network of 13 continuously recording GPS stations located from the summit of Kīlauea to the south coast. These data allow us to constantly monitor the movements of the summit and south flank regions of Kīlauea and to determine the temporal relationship between the movements. The relevance of this study is the possible recognition of precursory surface deformation phenomena of large earthquakes from the south flank.

Knowing that the region is moving relatively fast (for earth surface motion), NASA scientists use the area to test and calibrate their technique of measuring surface deformation from space with radar interferometry. The GPS measurements by HVO provide ground-truth to these airborne measurements which someday may be used to routinely monitor the surface deformation of Mauna Loa Volcano.

When you visit the lava viewing area again, you may recognize that the steep, 2,500-foot descent down to sea level is along the fault scarps of the Hilina fault system. The most prominent fault scarps in the system are the Hilina, Poliokeawe, and Holei Palis.

### Volcano Activity Update

The current eruptive activity of Kīlauea Volcano continues unabated. Lava flows from the vent at the base of Puu Oo through a well-established tube system to the sea. Sporadic explosive activity was observed at the coastal entry during the week.

One earthquake was reported felt in the past week. At 6:29 a.m. on November 11, residents of the Volcano Golf Course subdivision were shaken by a magnitude-3.0 temblor. The earthquake was located 3 miles northwest of the summit of Kīlauea Volcano at a depth of about 1 mile.