Kilauea’s whole south flank is creeping toward the ocean about 8 centimetres a year

Kilauea’s entire south flank, the wedge of the volcano’s southeast side that reaches from the summit caldera to the Pacific floor, has been sliding toward the ocean for decades at an average pace of about 8 centimeters, roughly 3 inches, a year. The U.S. Geological Survey’s Hawaiian Volcano Observatory tracks that motion with a dense network of GPS stations bolted into the flank, and geophysicist Sarah Conway, writing in the observatory’s Volcano Watch column, traces the mechanism to a fault buried far beneath the surface rather than to anything visible on the ground.

A detachment fault, also called a décollement, sits roughly 7 to 9 kilometers, about 4 to 6 miles, underground at the boundary between Kilauea’s volcanic pile and the older oceanic crust it rests on. It is the surface along which the entire flank, not just the coastline, glides seaward as magma pressure inside the volcano’s East Rift Zone pushes outward.

A detachment fault carries the whole flank seaward

Unlike a typical earthquake fault that ruptures once and stays quiet, the Kilauea décollement moves almost continuously, and the rate of that movement tracks what the volcano’s magma system is doing above it. During the 1983-to-2018 Puu Oo-Kupaianaha eruption, one of the longest-running eruptions Kilauea has produced in the modern instrumental record, the flank’s average creep held near 8 centimeters a year. Conway describes the flank as capable of moving faster or slower depending on what stage of activity the volcano is in, with rates that stay close to steady during long eruptions but jump abruptly during major intrusions of magma into the rift zone or during slow-slip earthquakes, a distinct category of quiet, weeks-long fault movement that releases roughly as much energy as a moderate earthquake without producing the shaking of one.

Kilauea’s south flank has generated five or more magnitude-4.0-or-greater earthquakes in a single year before, each one a small, sudden release of pressure as the detachment fault lurches forward over the ocean crust rather than a sign its overall creep rate has changed. Those slow-slip events, the subject of a separate Volcano Watch dispatch from the observatory, tend to recur on a roughly annual cycle tied to how the volcano’s magma system is behaving that year.

The 2018 earthquake moved the coast half a meter in seconds

The steady creep gave way to something far more abrupt on May 4, 2018, when a magnitude-6.9 earthquake struck as Kilauea’s lower East Rift Zone erupted. GPS stations recorded up to roughly 0.5 meters, about 1.5 feet, of sudden seaward motion at the surface of the south flank, a jump many times larger than an ordinary year’s worth of creep compressed into moments. Conway attributes that surge to the same magmatic intrusion driving the lower rift zone eruption, which piled additional pressure onto the flank on top of the detachment fault’s normal seaward pull.

The middle section of the East Rift Zone contracted sharply during the 2018 event, then began expanding again as magma refilled that part of the system, a push-and-pull Conway says has played out before, including during a 2007 intrusion known as the “Father’s Day” event that produced its own brief contraction of the rift zone.

The flank kept adjusting for two and a half more years

The south flank did not settle immediately once the 2018 eruption ended. Conway’s monitoring data show roughly an additional 10 centimeters, about 4 inches, of adjustment along the flank’s surface accumulated over the following two and a half years, a slow release of the stress the earthquake and eruption had built up. She describes that continued drift as expected and not a sign of any new hazard, distinct from the abrupt jump the earthquake itself produced.

The two-stage pattern, a sharp jump followed by a long, slow tail of adjustment, is what Hawaiian Volcano Observatory watches for after any major rift-zone intrusion, since the tail can keep releasing stress for years after the dramatic earthquake itself has faded from the news. Along the same stretch of coastline, the Holei Pali escarpment rises roughly 400 meters, about 1,300 feet, above the road that cuts through Hawaii Volcanoes National Park, a visible marker that the Hilina fault system Conway studies has been reshaping this part of the island long before 2018.

GPS stations separate the baseline creep from earthquake jumps

Hawaiian Volcano Observatory’s dense GPS network is what lets scientists tell the difference between the flank’s steady baseline creep, its earthquake-driven jumps and its slower post-quake adjustments, rather than treating every uptick in motion as a fresh hazard. Conway says the relationship between Kilauea’s tectonics and its volcanism remains an active area of study, with the flank’s slide continuing to unfold in ways the observatory’s monitoring network is built to catch as it happens rather than after the fact.

That distinction matters because a headline number like 8 centimeters a year sounds almost imperceptible until it is set against the roughly half-meter jump the 2018 earthquake produced in a matter of seconds. The gap between those two figures is the entire reason Hawaiian Volcano Observatory keeps GPS receivers bolted to the flank year-round instead of relying on occasional surveys, since only continuous monitoring can catch the moment ordinary creep turns into something faster.

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This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.