The Hawaiian Islands are one of the most seismically active regions in the United States, experiencing thousands of earthquakes each year. This high frequency of seismic activity is not driven by the tectonic plate boundaries common to places like California or Alaska. Instead, the persistent shaking is directly linked to the volcanic processes that created and continue to build the islands. Earthquakes in Hawaii result from the constant movement of magma and the immense, shifting mass of the volcanoes themselves. The unique geological setting means that while many earthquakes are small, the islands are also subject to occasional, very large events that pose distinct hazards.
Unique Geological Drivers of Hawaiian Seismicity
Hawaiian earthquakes are largely non-tectonic, meaning they do not result from the grinding of two major continental plates. The primary cause of seismicity is the presence of an active volcanic hotspot beneath the Pacific Plate. The pressure exerted by molten rock moving beneath the surface is a constant source of stress on the surrounding crust.
The movement of magma within the active volcanoes, such as Kīlauea and Mauna Loa, is one main mechanism for earthquakes. As magma accumulates in shallow reservoirs or pushes through subterranean plumbing systems and rift zones, it fractures the rock layers above and around it, creating swarms of small to moderate earthquakes. These volcanic quakes are essential for scientists to monitor, as they often signal changes in eruption potential.
A second, more potent mechanism involves gravitational slumping, where the massive weight of the volcanic edifice causes the flanks of the islands to slide seaward. This slow, continuous movement occurs along a nearly horizontal detachment fault, or décollement, that separates the overlying volcanic rock from the ancient oceanic crust below. When friction along this fault is overcome, the flank can lurch rapidly, causing large, shallow earthquakes, such as the magnitude 7.9 event in 1868.
Seismicity Patterns and Frequency Across the Islands
The vast majority of seismic activity in the Hawaiian archipelago is concentrated on the Island of Hawaiʻi, often called the Big Island, particularly around the active volcanoes of Kīlauea and Mauna Loa. This is the youngest and most volcanically active island, which explains the high frequency of quakes in this area. Most of the recorded thousands of quakes each year are minor, with approximately 90% registering below magnitude 3.0, making them undetectable without instruments.
The islands experience an average of about 100 quakes of magnitude 3.0 or greater annually, with around one event of magnitude 5.0 or larger occurring each year. Major quakes, such as those of magnitude 6.0 or greater, happen less frequently, with an average of one every ten years. These larger events occur at shallow depths, between 5 and 20 kilometers, along the décollement fault beneath the seaward flanks of the active volcanoes.
Earthquakes also occur at much deeper mantle depths, sometimes more than 40 kilometers below the surface. These deep quakes are thought to be caused by the bending of the Earth’s lithosphere under the immense weight of the entire island chain. While these deep events can be felt across the islands, they are less frequent and less damaging than the shallow, flank-related earthquakes.
The Interplay Between Earthquakes, Eruptions, and Tsunami Risk
The unique geological setting means that earthquakes and volcanic eruptions are closely connected phenomena in Hawaii. Large flank earthquakes can sometimes trigger volcanic eruptions by relieving pressure on the magma chamber or by opening new fissures in the volcano’s structure. For instance, the 2018 magnitude 6.9 earthquake was immediately followed by a significant volcanic sequence at Kīlauea.
Conversely, periods of intense magma movement and pressure buildup within a volcano can induce earthquake swarms as the molten rock forces its way through the crust. This interdependency means seismic activity can be a precursor to an eruption, and an eruption can then cause further seismic events. Monitoring the patterns of these quakes helps scientists forecast potential volcanic hazards.
The most significant tsunami hazard in Hawaii comes from locally generated waves, which are a direct consequence of large, shallow, flank-related earthquakes. When the seaward flank of a volcano rapidly slips in a major earthquake, it can cause a massive submarine landslide or slump. This sudden displacement of a large volume of water generates a tsunami that travels at high speed, giving coastal communities very little warning time.
A locally generated tsunami can reach the nearest shorelines in a matter of minutes, sometimes as quickly as five to forty minutes. The rapid arrival time and potential for destructive wave heights make these localized events a major concern for public safety. This hazard is distinct from distant tsunamis, which originate from earthquakes around the Pacific Ring of Fire and allow for hours of warning.