Volcano Watch: The legacy of the 1975 earthquake, 50 years of study at Kīlauea’s south flank

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

The 1975 Kalapana earthquake brought significant changes to the lives of those affected by the fatalities and damage, as well as to the understanding of Kīlauea’s south flank.

Scientists at the U.S. Geological Survey (USGS) Hawaiian Volcano Observatory had been collecting data and hypothesizing that Kīlauea’s south flank could soon produce a large earthquake.

While awaiting the printing of their findings, the 1975 earthquake occurred on Nov. 29.

Previous large earthquakes, such as the magnitude-7.9 Great Kaʻū earthquake in 1868, were attributed to a nearly horizontal fault (décollement) at the boundary between the original oceanic crust and the erupted lavas from Mauna Loa and Kīlauea. It is deepest under the middle of the Island of Hawaiʻi and becomes slightly shallower toward the shoreline.

Seismic and deformation data for the Kalapana earthquake indicated that the primary motions were also on the décollement fault, but the 1975 rupture was more complex. Vertical offsets were also noted along the top of several pali (cliffs) on Kīlauea’s south flank.

These pali are the surface expression of the Hilina fault system. This suggests that both the Hilina fault system and the décollement fault slipped during the 1975 earthquake, causing scientists to wonder whether the Hilina fault system might connect directly to the décollement fault.

Additional data collection in the past 50 years has shown that this scenario is not the case, with one of the major lines of evidence coming from a series of offshore seismic reflection profiles published in 2000—a technique that images the inside of the crust, similar to an MRI.

The reflection profiles showed that the head scarps of the Hilina fault system curve with depth and do not continue deep enough to intersect with the décollement. This means that the Hilina fault blocks are smaller than they would be if they connected to the décollement and that the two structures do not directly connect.

After the 1975 earthquake, there was a greater emphasis on monitoring the motion of the south flank. Hawaiian Volcano Observatory and the University of Hawaiʻi at Mānoa installed continuous GPS instruments on Kīlauea’s south flank that recorded the steady seaward motion of the south flank at a rate of about 5 centimeters (2 inches) per year.

The instruments also showed that the motion of the flank was not always steady. Every two to three years, the flank would speed up for two to three days, in a phenomenon called a slow slip event. These events released fault strain energy equivalent to a magnitude-6 earthquake, but did it so slowly that no damaging shaking was generated.

Slow slip events have been observed on faults worldwide, including the San Andreas fault and the Cascadia subduction zone. The observation of slow slip events at Kīlauea was key in demonstrating that their occurrence is not unique to plate boundaries but is a fundamental result of velocity-strengthening fault friction that can occur wherever those frictional conditions exist.

The magnitude-6.9 earthquake in 2018 provided more insights, showing no evidence for motion on the pali, likely because it was smaller than the 1975 earthquake, given what has been learned about the structure of the south flank.

Strong shaking during the 1975 earthquake may have led to motion on the Hilina fault system, a process called dynamic triggering. However, even with a large earthquake, there is a limit to how much the Hilina fault zone will move.

The seismic reflection profiles published in 2000 showed that the décollement fault creates a zone of uplifted crust (“toe”) as it approaches the surface. This toe acts as a buttress on the flank, inhibiting large motions.

The improved understanding of how the south flank of Kīlauea behaves is part of the legacy of the 1975 earthquake, as is the appreciation for the sizes of earthquakes that can be generated on the décollement fault.

For information on how to prepare your home and family for an earthquake, visit  https://www.ready.gov/earthquakes. 

VOLCANO ACTIVITY UPDATES

KĪLAUEA

VOLCANO ALERT LEVEL: Watch

Kīlauea has been erupting episodically within the summit caldera since Dec. 23, 2024.

Episode 37 lava fountaining happened for 9 hours Nov. 25. Glow is visible overnight from the south vent. No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

WHAT’S NEXT? The summit is reinflating; forecasting models indicate another fountaining episode is likely, but is at least a week away.

MAUNA LOA

VOLCANO ALERT LEVEL: Normal

Mauna Loa is not erupting. Visit the U.S. Geological Survey’s Mauna Loa web page to learn more about what’s happening at Mauna Loa as it continue to show only minor inflation beneath the summit as the volcano recovers from the 2022 eruption and magma replenishes the reservoir system.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

EARTHQUAKES

Three earthquakes were reported felt in the Hawaiian Islands during the past week.

• A magnitude-3.4 earthquake 15 miles east-northeast of Halaʻula at a depth of 2 miles at 4:24 a.m. on Nov. 26
• A magnitude-4.6 earthquake 9 miles south-southeast of Fern Forest at a depth of 3 miles at 11:49 p.m. on Nov. 25
• A magnitude-3.2 earthquake 9 miles east-southeast of Waikōloa at a depth of 18 miles at 3:43 on Nov. 20.

Visit the Hawaiian Volcano Observatory website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov. Learn even more here.