Though most of Hawaii’s day-to-day seismic activity occurs within volcanic systems, these widely felt, often offshore, deep earthquakes are not directly related to magmatic processes at all. 

Most notably, the magnitude-6.0 earthquake on May 22^nd rattled residents statewide. This event occurred beneath the west side of the Island of Hawaiʻi, southeast of Captain Cook at a depth of 14 miles (23 km) below the ocean surface. Two others that were unrelated, yet widely felt, followed a few days later: a magnitude-4.6 on June 2^nd, northwest of Keauhou at a depth of 21 miles (24 km); and a magnitude-4.7 on June 9^th, east of Pepeʻekeo at a depth of 24 miles (39 km). If these deep earthquakes are not related to magmatic processes, why do they occur?

The Hawaiian Island Chain is one of the most unique places in the world and one of the most geographically isolated landmasses on Earth. Not only is Hawaii far from continental land masses, but it is also far from tectonic plate boundaries, which are responsible for roughly 90% of the world’s seismic activity. 

California, Alaska, Russia, Japan, Indonesia, and the Southwest Pacific are regions of high seismic and volcanic activity that make up the Ring of Fire, where tectonic plates slowly collide with, spread apart from, and grind against each other. Hawaii is located in the middle of the Pacific plate and therefore does not produce seismicity related to plate-boundary interactions. 

Volcanism in Hawaii is the result of a mantle plume, a column of hot rock originating near the core-mantle boundary, thousands of miles deep in the Earth. Commonly known as a “hot spot,” the mantle plume was (and still is) hot enough to punch through the oceanic lithosphere (the Pacific plate), forming undersea volcanoes that grow into islands. As the Pacific plate creeps northwest, the hot spot remains stationary and those volcanic islands move with the plate, creating a chain. 

Over millions of years, the mantle plume has fed magma to shield volcanoes, causing the Hawaiian Island Chain to grow and become heavier atop the lithosphere (the brittle upper layer of the Earth’s mantle). The load of the island chain is heaviest beneath the biggest volcanoes: Mauna Kea and Mauna Loa. The lithosphere accommodates this weight by sagging downward toward the center of the Earth. About 100 miles offshore of Hawaii, the lithosphere flexes upward in what is known as the Hawaiian Arch. 

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A way to visualize the weight of the islands on the lithosphere is to picture a bowling ball on a bed. The bowling ball will sink down into the blankets, and the blankets create folds radially outward from the bowling ball. These blanket folds represent the deep faulting resulting from plate flexure. 

Since the lithosphere is relatively cool and brittle compared to the hotter, ductile portion of the mantle below it, it can only bend so much before the strain is released as an earthquake. These earthquakes, called flexure events, are much deeper and less common than most Hawaiian earthquakes, which are the result of magma movement and pressure changes or faulting within the island itself. These flexure events are generally more widely felt because they occur deeper and the dense lithosphere allows the seismic waves to impact a larger footprint without attenuating as much energy as they propagate, which also contributes to the potential for damage.

Flexure events are distinguishable by their location and depth. They are usually in coastal regions or offshore and much deeper than typical seismicity within our volcanic systems. However, not all deep offshore events are flexure events. For example, the Pāhala seismic swarm regularly produces earthquakes larger than magnitude-4 and more than 18 miles (30 km) deep off the southeast coast of the Island of Hawaiʻi. These are not flexure earthquakes but thought to be related to hot spot magma transport pathways. 

As the authoritative seismic network in the State of Hawaii, HVO is tasked with reviewing all large local earthquakes. Regardless of whether an earthquake is a deep flexure event or occurring within our volcanic systems, remember to “drop, cover, and hold-on” when you feel shaking, and move away from the coast in case a tsunami is generated.