Hawaii presents an exception to typical seismic activity, sitting in the middle of the Pacific Plate far from major fault lines. The island chain is a product of a fixed mantle hotspot, a plume of hot rock rising from deep within the Earth. Seismic activity is driven entirely by volcanic processes and the immense weight of the islands themselves.
The Intraplate Seismicity of Hawaii
Hawaii’s seismic activity is intraplate seismicity, occurring within the Pacific Plate rather than at its edge. The Pacific Plate moves slowly northwest, but the underlying hotspot remains stationary. As the plate drifts over this magma source, new volcanoes are continuously built.
This geologic setting links island creation directly to seismic stress. The weight of massive volcanoes, such as Mauna Loa and Kīlauea, imposes a tremendous load on the oceanic crust beneath them. This constant stress is the engine for many earthquakes, concentrated on the Island of Hawaiʻi, the youngest and most volcanically active.
Earthquakes Triggered by Magma Movement
One major source of shaking is tied directly to the movement of molten rock beneath active volcanoes. As magma is injected, accumulates, or withdraws beneath the summit and rift zones, it exerts immense pressure on the surrounding rock. This pressure causes the rock to fracture, generating swarms of small, shallow earthquakes.
These shallow volcanic quakes occur at depths of less than 5 kilometers and are a tool for monitoring active volcanoes like Kīlauea. Scientists use the location and frequency of these tremors to track the pathways and reservoirs of magma as it nears the surface or is intruded into dikes. Inflation and deflation of the summit magma chamber are accompanied by these short-term bursts of seismicity.
Magmatic activity can also trigger deeper earthquake swarms, such as those observed beneath Pāhala. These deeper quakes, occurring around 35 to 42 kilometers, are linked to the intrusion of magma into vast chambers called sills near the base of the crust. As molten rock pulses into these deep sills, it strains the surrounding rock, leading to persistent seismic activity that feeds the shallower volcanic systems above.
Movement of Volcanic Flanks
The most powerful Hawaiian earthquakes result from the structural instability of the volcanic edifices, specifically the seaward movement of their massive flanks. The sheer weight of the shield volcanoes causes their outer slopes to slowly slide toward the ocean. This movement occurs over the basal décollement, a weak, nearly horizontal boundary at a depth of about 8 to 10 kilometers.
The décollement is a massive slip plane formed by layers of ancient marine sediments and hydrothermally altered rock, situated between the base of the volcanic pile and the underlying oceanic crust. Magma injection into the rift zones acts like a wedge, increasing the pressure and pushing the flank block seaward.
While this flank movement is often slow and steady, accumulated stress is sometimes released in a sudden, catastrophic slip. This rapid displacement along the décollement generates large-magnitude earthquakes, such as the 1975 Kalapana earthquake, which registered a magnitude of 7.7. That event caused several meters of horizontal and vertical movement on Kīlauea’s south flank.
Deep Earthquakes Linked to the Hotspot
A third category of Hawaiian seismicity involves non-volcanic earthquakes occurring deep beneath the island structure. These events are located at depths between 20 and 60 kilometers, within the rigid Pacific Plate lithosphere. They are caused by the immense pressure exerted by the weight of the islands on the oceanic plate.
This colossal load causes the lithosphere to flex and bend downward, a process known as isostatic deformation. The bending creates stresses within the plate that, when exceeding the rock’s strength, result in deep-focus earthquakes.
The 2006 Kīholo Bay earthquake (magnitude 6.7 at 29 kilometers depth) is a notable example of a lithospheric bending earthquake. These deep events are widely felt because they occur within the competent, brittle rock of the oceanic plate. They reflect the broad structural adjustments of the Pacific Plate as it accommodates the heavy burden of the Hawaiian volcanoes.