An hour before the main shock, a magnitude 5.4 foreshock rolled across the Big Island and was felt as far away as Oahu, more than 200 miles to the northwest. It was a warning shot. At 12:33 p.m. on May 4, 2018, a magnitude 6.9 earthquake -- the largest to hit Hawaii since 1975 -- struck near the south flank of Kilauea. The quake was not a separate event from the volcanic chaos already unfolding across lower Puna; it was part of the same system, a consequence of magma forcing its way through Kilauea's East Rift Zone and, in turn, a trigger for what came next.
Kilauea had been restless since late April 2018, with new lava outbreaks signaling a major shift in the volcano's plumbing. The earthquake's epicenter lay on Kilauea's southeastern flank, where the island's mass has been gradually collapsing seaward for millennia. Analysis of seismic waveforms revealed that the rupture occurred on a thrust fault dipping at roughly 7 degrees -- at the interface where Hawaiian volcanic rock sits atop older oceanic sediments. This shallow, nearly horizontal fault plane made the earthquake similar in character to the devastating 1975 Kalapana earthquake, which killed two people and injured 28 at the same location. The slow rupture speed confirmed that the break propagated along a weak zone, not through solid basalt.
The earthquake's most significant physical consequence was invisible from shore. The Hilina Slump, the massive block of Kilauea's south flank that has been creeping seaward at about 10 centimeters per year, lurched roughly two feet toward the ocean in a single moment. This was not the first time. The same structure had moved during earthquakes in 1868 and 1975, and it moves aseismically between them. But each sudden displacement carries risk: the slump has generated tsunamis before, and the 2018 earthquake produced a small one -- 40 centimeters at Kapoho, 20 centimeters at Hilo, and 15 centimeters at Honuapo. The Pacific Tsunami Warning Center issued a local advisory, but the waves remained well below dangerous levels.
The 2018 earthquake did not occur in isolation. It was tied to the movement of magma through Kilauea's East Rift Zone, which had begun producing new lava outbreaks in lower Puna just days earlier. The relationship between seismic and volcanic activity at Kilauea is well documented: the 1868 and 1975 earthquakes both preceded or accompanied major eruptive episodes. In 2018, the earthquake preceded further volcanic activity that would continue for months, ultimately destroying over 700 homes in the Leilani Estates and Kapoho neighborhoods. The earthquake was a symptom of the same deep process -- magma injection destabilizing the south flank -- and simultaneously a catalyst, relieving pressure in ways that opened new pathways for lava.
Hawaii Island sits far from any tectonic plate boundary, but its earthquakes are no less powerful for that. The island's active volcanoes generate constant minor seismicity as magma moves beneath them, punctuated by larger tectonic events caused by the gradual collapse of volcanic flanks under their own weight. The south flank of Kilauea is the most active zone, sliding seaward across a detachment surface at the top of the oceanic crust roughly 8 to 10 kilometers deep. The 2018 earthquake was a Mercalli VIII event -- classified as Severe -- that cracked roads, triggered landslides, and damaged buildings. But compared to the potential energy stored in the Hilina Slump, it was modest. The question for geologists is not whether Kilauea's flank will move again, but when, and how far.
Epicenter at 19.37°N, 155.03°W, on Kilauea's south flank near the coast. The Hilina Pali cliff system is visible as parallel scarps running northeast. Chain of Craters Road descends from the summit caldera to the coast. Nearest airport: PHTO (Hilo International Airport) to the northeast. The 2018 lava flows from the lower East Rift Zone are visible as dark expanses in the Leilani Estates / Kapoho area to the east. Best viewed from 5,000-10,000 ft altitude.