Earthquakes occur frequently in Hawaii, establishing the islands as one of the most seismically active regions in the United States. Thousands of tremors are recorded across the state each year, though most are too small to be felt. The unique geological setting, far from any tectonic plate boundary, results in a distinct pattern of seismic activity. This activity is directly linked to the forces that created and continue to shape the volcanic archipelago.
The Dual Causes of Hawaiian Seismicity
Hawaiian earthquakes are not the result of continental plates grinding past each other, but rather the consequence of two primary, localized forces: volcanic pressure and structural adjustments to the Earth’s crust. These mechanisms are distinct from the large-scale plate boundary earthquakes seen elsewhere. The first and most common type is directly caused by the movement of magma beneath the active volcanoes, primarily Kīlauea and Mauna Loa.
As molten rock moves through underground channels and accumulates in shallow reservoirs, it exerts pressure on the surrounding rock. This pressure causes the rock to fracture, creating numerous small, shallow earthquakes that often occur in swarms. These events provide scientists with valuable data, acting as a real-time tracking system for magma migration. Volcanic earthquakes are typically magnitude 5.0 or smaller, but their high frequency defines seismicity near the active rift zones.
The second primary cause involves large-scale structural forces acting on the Pacific Plate. The weight of the Hawaiian Islands, particularly the Big Island, presses down on the underlying oceanic lithosphere, causing it to bend or flex. This downward bending creates stress fractures in the crust and upper mantle, resulting in flexural earthquakes.
These flexural events tend to be deeper than volcanic quakes, often occurring 15 to 60 kilometers beneath the surface. A related and often more damaging structural event is the flank earthquake, which occurs along a shallow detachment fault beneath the volcano. This fault allows the massive flanks of the volcanoes to slide seaward due to gravity and magma pressure, sometimes resulting in large, destructive earthquakes, such as the magnitude 7.7 event in 1975.
Geographic Concentration and Frequency
The majority of seismic activity is concentrated on and around the Island of Hawaiʻi (the Big Island). This concentration is a direct consequence of the island’s young age and its two active shield volcanoes, Kīlauea and Mauna Loa. The southern districts, where the most active volcanic systems are located, experience the highest frequency of tremors.
Volcanic earthquakes are characterized by high frequency and shallow depth, usually less than 5 kilometers below the summit and rift zones. These events often manifest as intense earthquake swarms, which are periods of elevated seismic activity without a clear mainshock. In contrast, deeper flexural earthquakes are less frequent but can occur anywhere beneath the island chain, sometimes causing shaking to be felt across multiple islands, even as far as Oʻahu.
The state experiences an average of 400 to 500 earthquakes of magnitude 3.0 or greater each year, with most being small and unnoticed. More significant events of magnitude 5.0 or higher occur a few times per year. Historical records show that a magnitude 6.0 or greater earthquake occurs roughly once every decade, and a magnitude 7.0 or greater event happens about twice every century, demonstrating the potential for significant, damaging shaking.
Associated Hazards and Monitoring Systems
Hawaiian earthquakes pose several distinct hazards, one of the most serious being the potential for a locally generated tsunami. Large earthquakes that occur offshore or along the shallow, seaward-sliding flanks of the volcanoes can rapidly displace massive volumes of water. For example, the magnitude 7.7 Kalapana earthquake in 1975 generated a destructive local tsunami that resulted in fatalities and significant damage.
Large earthquakes can also impact volcanic processes. Ground shaking can destabilize the rift zones and slopes of the volcanoes, potentially triggering new eruptions or accelerating the movement of magma toward the surface. The stress changes caused by major seismic events can unlock pathways for magma, influencing the timing and location of a volcanic outbreak.
To track and mitigate these hazards, the region relies on a sophisticated network of instruments and agencies. The USGS Hawaiian Volcano Observatory (HVO) monitors all seismic activity across the state, utilizing a dense network of seismometers primarily focused on the active volcanoes of the Big Island. HVO’s data helps scientists track magma movement and assess the risk of both volcanic and seismic hazards in real-time.
The HVO’s seismic network is integrated into the Hawaii Integrated Seismic Network (HISN), which also includes instruments operated by the Pacific Tsunami Warning Center (PTWC). The PTWC uses this real-time seismic data to rapidly detect large offshore and flank earthquakes. Their performance target is issuing initial local tsunami warnings within 90 seconds of an event’s origin time. This comprehensive monitoring infrastructure provides timely alerts to protect the public from earthquake shaking and subsequent tsunamis.