Hawaii is one of the most seismically active places in the United States, despite sitting far from any major tectonic plate boundary, unlike California or Japan. The state experiences thousands of earthquakes each year, averaging about 100 quakes of magnitude 3 or greater, and a magnitude 5 or greater quake about once a year. This high frequency of seismic activity stems entirely from the islands’ unique volcanic origin and the resulting structural stresses placed on the Pacific Plate. The forces driving these frequent tremors are a direct consequence of the continuous process of island formation and the immense weight of the volcanoes. The mechanisms responsible for Hawaii’s earthquakes fall into two main categories: shallow events tied to active volcanism and deeper events related to the bending of the Earth’s crust.
Hawaii’s Origin: The Hotspot Foundation
The existence of the Hawaiian archipelago is rooted in the Hawaiian Hotspot, a fixed point of high heat rising from deep within the Earth’s mantle. This mantle plume generates magma that pushes through the oceanic crust far from the edges of the Pacific Plate. As the Pacific Plate slowly glides over this relatively stationary magma source, a chain of volcanoes is sequentially formed.
The ongoing movement of the Pacific Plate, traveling generally to the west-northwest, carries the newly formed volcanoes away from the hotspot. The youngest and most volcanically active islands, such as the Island of Hawai‘i, are currently situated directly above the plume. This process has created the vast Hawaiian–Emperor Seamount Chain, a 6,000-kilometer trail of submerged and above-water volcanoes that records the plate’s motion over the last 70 million years. The continuous, localized construction of these immense volcanic mountains on a thin, moving oceanic plate establishes the fundamental condition for the islands’ seismic activity.
Shallow Quakes: The Role of Magma and Flank Movement
The majority of felt earthquakes in Hawaii are shallow, typically occurring at depths less than 15 kilometers, and are directly linked to the active volcanoes, Kīlauea and Mauna Loa. These shallow events are driven by two interconnected processes: the movement of magma and the instability of the volcano flanks. Magmatic pressure causes seismicity as molten rock forces its way through the volcanic plumbing system.
The intrusion of magma into the volcano’s rift zones, which are zones of weakness extending from the summit, creates stress that fractures the surrounding rock. These magma-induced tremors are generally small, often less than magnitude 4, and are used by scientists to track the movement and location of magma beneath the surface. The upward and lateral flow of magma through interconnected, pancake-like chambers called “sills” deep beneath the volcanoes can also trigger swarms of small earthquakes.
A second source of shallow quakes is the gravity-driven sliding of the volcano’s massive flanks, especially Kīlauea’s south flank. The sheer weight of the volcano, combined with the outward push of magmatic intrusions, causes the flank to creep slowly seaward over time. This movement occurs along a sub-horizontal fault, known as a décollement, which lies at the boundary between the volcanic rock and the underlying ancient oceanic crust. When movement along this décollement fault accelerates rapidly, it generates large, shallow earthquakes, such as the magnitude 7.7 Great Kaʻū earthquake in 1868.
Deep Quakes: Crustal Flexure and Stress
A second distinct category of Hawaiian earthquakes are the deeper events, known as flexural earthquakes, which occur at depths typically between 20 and 40 kilometers. These quakes are not caused by the immediate movement of magma but by the Pacific Plate’s structural adjustment to the immense load of the overlying islands. The weight of the volcanic mass causes the rigid lithosphere, which includes the crust and upper mantle, to bend downward, much like a bowling ball placed on a soft mattress.
This downward bending, or flexure, creates significant tensional and compressional stresses within the lithosphere surrounding the islands. The bending stresses are released in the form of deep earthquakes, which can be quite large, sometimes exceeding magnitude 6. These deep quakes often occur beneath the volcanoes or in the surrounding oceanic crust, extending even as far as O‘ahu, hundreds of kilometers from the active volcanoes on the Island of Hawai‘i.
The 2006 magnitude 6.7 Kīholo Bay earthquake is a recent example of a large deep event resulting from this lithospheric bending. These structural adjustments demonstrate that the entire Pacific Plate is under stress from the islands’ weight, leading to deep fracturing in the brittle mantle beneath the crust. The combination of shallow, volcanically-driven faulting and deep, structurally-driven flexure explains the persistent and varied seismic activity across the Hawaiian archipelago.