Volcanic Fault
Volcano Terms and Definition


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Aerial view toward the NE of the Pu`u Kapukapu fault scarp (maximum height about 320 m) in the Hilina fault system, south flank of Kilauea Volcano, Hawai`i. In Hawai`i, these tall cliffs are called "pali's".
Fault


Faults are fractures or fracture zones in the Earth's crust along which one side moves with respect to the other. A fault scarp is a cliff or steep slope that sometimes forms along the fault at the surface. There are many types of faults (for example, strike-slip, normal, reverse, and thrust faults) ranging in size from a few tens of meters to hundreds of kilometers in dimension.




Mount St. Helens, Washington

Scientists measure the distance between two benchmarks spanning the fault scarp of a thrust fault on the crater floor of Mount St. Helens. This scarp developed on the crater floor in 1981 as magma rose into the lava dome (backgound) before erupting onto its surface. The pressure exerted by the rising magma against rocks surounding the conduit caused the crater floor to fracture along a plane gently inclined toward the dome.

Material above the fault (person kneeling on upper surface) was pushed over material below the fault (person in lower right); in this view the fault scarp advances toward the lower right; arrow indicates direction of movement. Scientists measured an increasing rate of movement of this thrust fault before two eruptions in June and October 1981, which helped them to predict both eruptions accurately.


Kilauea Volcano, Hawaii

Scientist measures the vertical offset along a fault that moved as a consequence of a 7.2 magnitude earthquake on the south flank of Kilauea on November 29, 1975. The total observed displacement along several similar faults of the Hilina fault system was at least 2.5 m. This earthquake resulted in dramatic ground movements along the south coast of Kilauea.


Ground Movement
Associated with the Magnitude 7.2 Earthquake on November 29, 1975

When the ground stopped shaking from the magnitude 7.2 earthquake located beneath the south flank of Kilauea, people began noticing dramatic changes to familiar landmarks along the volcano's shoreline. At Halape (see photos below), 30 km southwest of the epicenter, the ground subsided by as much as 3.5 m, which left a grove of coconut palms standing in water about 1.2 m deep and the new shoreline about 100 to 150 m inland from the presubsidence shoreline.

Aerial view southeast toward Halape, Hawai`i
Pre-earthquake view of Halape SE toward coconut grove
Photograph by Don Reeser, National Park Service

 
Aerial view northeast toward Halape, Hawai`i
Post-earthquake view of Halape NE toward submerged coconut grove
Inland of Halape, a nearly continuous zone of ground cracking and faulting occurred for about 25 km along the large Hilina fault system, which consists of a series of normal faults with scarps as high as 500 m on Kilauea's south flank. Scientists measured vertical offsets along some faults as much as 1.5 m (note brown scarp in photo, right).
The magnitude of the horizontal displacements became apparent only when the large HVO survey network on Kilauea was measured after the earthquake. Coastal areas between Kalu`e and Keauhou Landing moved seaward between 4 and 8 m, among the largest observed displacements caused by any earthquake (photo left).

Subsidence Associated with Earthquake


Subsidence of the ground shown by contours (m) on Kilauea Volcano associated with the magnitude 7.2 earthquake on November 29, 1975 (epicenter shown by star).Horizontal Ground Movement Associated with Earthquake

Horizontal displacments of benchmarks surveyed before and after the magnitude 7.2 earthquake on November 29, 1975 (epicenter shown by star). Use the vector scale (displacement, above) to determine the horizontal movement. For example, Kilauea's caldera moved southeast about 1 m and its south flank moved southeast between 4 and 8 m.

Disappearing Evidence of 1975 Earthquake

The physical evidence that such large movements occurred on Kilauea Volcano during the 1975 earthquake is becoming increasingly difficult to recognize. The famous submerged coconut grove at Halape is now barely recognizable.

Coconut grove at Halape, Hawai`i
Halape coconut grove in 1975
Coconut grove at Halape, Hawai`i
Halape coconut grove in 1987

Similar earthquakes and associated ground movements and tsunamis can be expected to occur on Hawai`i as long as Kilauea and Mauna Loa volcanoes remain active. Because the number of residents of the Big Island has greatly increased since 1975, with most having no memory of the earthquake or tsunami, people need to periodically remind themselves of the safety steps to take before, during, and immediately after the next large earthquake.


Hilina Fault System, Kilauea Volcano, Hawaii
Aerial view to the north of the Hilina fault system, which consists of a series of normal faults with scarps as high as 500 m on Kilauea's south flank. The large-scale offsets and associated seismicity in the south flank are thought to result from gravitational instability of this part of the volcano and from the repeated injection of magma into Kilauea's east rift zone. In contrast, the northern flank of Kilauea is relatively stable because it is buttressed by the mass of Mauna Loa Volcano.


Hilton Creek fault, Long Valley Caldera, California

Aerial view of a glacial moraine along McGee Creek that has been offset about 15 m by part of the Hilton Creek fault system. Note cars in parking lot (lower left) for scale. The Hilton Creek fault is much larger than suggested by this relatively small offset--its scarp in places is about 1,100 m tall and forms part of the eastern front of the Sierra Nevada. The northern part of the fault terminates in Long Valley caldera, about 5 km NNE from this site.

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Source:
U.S. Department of the Interior