The Hawaiian Islands are known for active volcanoes, but they also have one of the highest earthquake hazards in the United States. This high risk is surprising because Hawaii sits firmly in the middle of the Pacific Plate, far from the major tectonic plate boundaries where powerful earthquakes typically occur. The constant internal geological activity that builds the islands also tears them apart. This internal stress generates thousands of earthquakes each year, ranging from tiny tremors to rare but highly destructive magnitude 7.0 events. These seismic events are not caused by the collision or sliding of plates but are instead a direct consequence of magma movement and the sheer weight of the massive volcanic edifices.
Hawaii’s Unique Hotspot Origin
The Hawaiian Islands owe their existence to a mantle plume, or hotspot, fixed deep within the Earth. This plume is a persistent upwelling of hotter, buoyant material that supplies magma to the crust above it.
The Pacific Plate, which carries the islands, moves slowly northwestward over this fixed plume. As the plate moves, volcanoes are successively built up, carried away from the magma source, and then become extinct, creating the long Hawaiian-Emperor seamount chain. This process contrasts sharply with tectonic earthquakes common at plate boundaries, such as those in California or Japan. Hawaiian seismicity is mid-plate volcanism, driven by the forces of construction and gravity rather than plate tectonics.
Shallow Quakes Driven by Magma Movement
The most common type of earthquake in Hawaii is shallow and directly related to the movement of molten rock beneath the active volcanoes, Kīlauea and Mauna Loa. These quakes originate at depths less than 10 miles (16 kilometers) below the surface. They occur when magma pushes through subterranean pathways, forcing the surrounding brittle rock to fracture.
Magma moving laterally into a volcano’s rift zone, or vertically toward a summit reservoir, creates immense pressure that stresses the crust. This process is tracked by scientists as the number of small quakes, often less than magnitude 4, increases in a localized swarm. Deep, pancake-shaped chambers called sills, located approximately 22 to 26 miles (35 to 42 kilometers) underground, channel magma toward the shallower reservoirs of Kīlauea and Mauna Loa. The intrusion of magma into this network causes a constant cascade of small, localized earthquakes.
Pressure changes within the magma storage regions, such as the inflation and deflation of a reservoir, also stress the overlying and surrounding rock. These shallow events are often precursors or companions to eruptions, providing a real-time signal of the dynamic changes occurring in the volcanic plumbing system. While individually small, the sheer frequency of these quakes—thousands per year—makes them a constant feature of the local seismic environment.
Deep Quakes from Gravitational Flank Instability
The largest and most destructive Hawaiian earthquakes are caused not by magma directly, but by the massive weight of the volcanic structures leading to gravitational instability. Immense volcanoes, such as Kīlauea and Mauna Loa, are built on a layer of weak, ancient oceanic sediment that covers the original seafloor. The sheer mass of the volcanic edifice causes its entire flank to spread and slowly slide seaward.
This sliding occurs along a low-angle, sub-horizontal feature known as a décollement, or detachment fault, located about 5 to 6 miles (8 to 10 kilometers) beneath the volcano. The décollement acts as a zone of weakness separating the younger, overlying volcanic rock from the older oceanic crust below. Over decades, stress builds up as the flank creeps outward, driven by gravity and internal pressure from magma intrusions.
When this accumulated stress overcomes the friction along the décollement, the flank can suddenly slip, resulting in a major earthquake. The magnitude 7.7 Kalapana earthquake in 1975, which caused significant damage and a destructive local tsunami, resulted from this rapid flank movement. These powerful events are structural adjustments to the island mass, with large displacement along the décollement fault being the primary mechanism for Hawaii’s highest-magnitude seismicity.
Assessing Seismic Hazards
The combination of shallow, magma-driven quakes and deep, flank-instability quakes creates a complex and persistent seismic hazard profile. Hawaiian earthquakes are characterized by a high number of small, daily tremors and a much rarer occurrence of large, damaging events. Historically, the state averages about one magnitude 6.0 or greater earthquake every 10 years, and two magnitude 7.0 or greater quakes every 100 years.
The risk to public safety involves not only ground shaking from large events but also the secondary hazards they trigger. The rapid seaward movement of a volcano flank during a large décollement earthquake can displace enormous volumes of ocean water, generating highly destructive, localized tsunamis. This occurred during the 1975 Kalapana event, causing loss of life and significant coastal damage.
Intense ground shaking on the steep slopes of the volcanoes can also cause landslides and rockfalls. The unstable nature of the volcanic slopes, especially in areas with fault systems like the Hilina Fault System on Kīlauea’s south flank, means a large earthquake can easily destabilize the land. These combined direct and indirect hazards contribute to Hawaii’s high risk for seismic activity.