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Ground Movement
Associated with the Magnitude 7.7 Earthquake on November 29, 1975

When the ground stopped shaking from the magnitude 7.7 earthquake located beneath the south flank of Kīlauea, people began noticing dramatic changes to familiar landmarks along the volcano's shoreline. At Halapē (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 (see photo from ground).

Aerial view southeast toward Halapē, Hawaiʻi
Pre-earthquake view of Halapē SE toward coconut grove
Photograph by Don Reeser, National Park Service

 

Aerial view northeast toward Halapē, Hawaiʻi
Post-earthquake view of Halapē NE toward submerged coconut grove

Inland of Halapē, 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 Kīlauea'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 Kīlauea was measured after the earthquake. Coastal areas between Kaluʻe 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 Kīlauea Volcano associated with the magnitude 7.7 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 7.7 earthquake on November 29, 1975 (epicenter shown by star). Use the vector scale (displacement, above) to determine the horizontal movement. For example, Kīlauea'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 Kīlauea Volcano during the 1975 earthquake is becoming increasingly difficult to recognize. The famous submerged coconut grove at Halapē is now barely recognizable.

Coconut grove at Halapē, Hawaiʻi
Halapē coconut grove in 1975
Coconut grove at Halapē, Hawaiʻi
Halapē coconut grove in 1987

Similar earthquakes and associated ground movements and tsunamis can be expected to occur on Hawaiʻi as long as Kīlauea 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.

Reference

Lipman, P.W., Lockwood, J.P., Okamura, R.T., Swanson, D.A., and Yamashita, K.M., 1985, Ground deformation associated with the 1975 magnitude-7.7 earthquake and resulting changes in activity of Kīlauea Volcano, Hawaii: U.S. Geological Survey Professional Paper 1276, 45 p.


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The URL of this page is: http://hvo.wr.usgs.gov/earthquakes/destruct/1975Nov29/deformation.html
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Updated: 17 December 1998 (pnf)