Volcano Watch: Deep dive into Hawaiʻi’s deep earthquakes

“Volcano Watch” is a weekly column and volcano/earthquake activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

Hawaiʻi Island residents might have felt shaking recently from large, deep earthquakes — including fewer than 24 hours before this column was published when a magnitude-4.5 temblor struck just after 8:15 p.m. July 9 off the west coast of the island.

These deep earthquakes have been recorded since 1912 when Hawaiian Volcano Observatory first started monitoring Hawaiʻi volcanoes.

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

The May 22 magnitude-6.0 earthquake that struck beneath the west side of Hawaiʻi Island, rattling residents statewide — and causing major damage to some homes and other structures — is the most notable recent example of these types of temblors.

It was located just south of Hōnaunau-Nāpōʻopoʻo at a depth of 14 miles below the ocean surface. Two other unrelated — yet widely felt — earthquakes followed a few days later:

• A magnitude-4.6 on June 2 located northwest of Keauhou at a depth of 21 miles below sea level.
• A magnitude-4.7 on June 9 located east of Pepe‘ekeo at a depth of 24 miles below sea level.

If these deep earthquakes are not related to magmatic processes, why do they occur?

The Hawaiian Islands are one of the most unique places in the world — and one of the most geographically isolated landmasses on Earth.

Not only is Hawaiʻi far from continental land masses, it also is 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, creating the Ring of Fire, where tectonic plates slowly collide with, spread apart from and grind against each other.

Hawaiʻi is located in the middle of the Pacific plate and therefore does not experience seismicity related to plate boundary interactions.

Volcanism in Hawaiʻi instead is the result of a mantle plume, a column of hot rock originating near the core-mantle boundary thousands of miles deep in Earth.

Commonly known as a “hot spot,” the mantle plume was — and still is — hot enough to punch through the oceanic lithosphere, also known as the Pacific plate, forming undersea volcanoes that grow into islands.

The hot spot remains stationary as the Pacific plate creeps northwest and those volcanic islands move with it, creating a chain.

That same mantle plume fed magma through millions of years to shield volcanoes in the Hawaiian Islands, causing the 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 its biggest volcanoes, Mauna Kea and Mauna Loa on the Big Island. This weight is accommodated by the lithosphere sagging downward toward the center of Earth.

It only flexes upward about 100 miles offshore in what is known as the Hawaiian Arch.

A way to visualize the weight of the islands on the lithosphere is to picture a bowling ball on a bed. The bowling ball sinks down into the blankets, creating folds radiating outward from the ball.

Since the lithosphere is relatively cool and brittle compared with the hotter, ductile portion of the mantle below, it can only bend so much before the strain is released as an earthquake.

These deep earthquakes — represented by the blanket folds formed by the sinking bowling ball — and deep faulting result from what are called flexure events. They are much deeper and less common than most Hawaiian earthquakes caused by magma movement and pressure changes or faulting within the island itself.

Flexure events are generally more widely felt because they occur deeper, so 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.

The events also are distinguishable by their location and depth, usually in coastal regions or offshore and located much deeper than typical seismic activity within our volcanic systems.

However, not all deep offshore events are flexure events.

The Pāhala seismic swarm for example regularly produces earthquakes larger than magnitude-4 and more than 18 miles deep off the southeast coast of Hawaiʻi Island. These are not flexure earthquakes; instead, they are thought to be related to hot spot magma transport pathways.

Hawaiian Volcano Observatory as the authoritative seismic network in Hawaiʻi 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.

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VOLCANO ACTIVITY UPDATES

KĪLAUEA volcano

VOLCANO ALERT LEVEL: Advisory

Kīlauea has erupted episodically since Dec. 23, 2024, within its summit caldera inside Hawaiʻi Volcanoes National Park.

Episode 50 of summit lava fountaining happened for 7 hours June 27. No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

WHAT’S NEXT: Summit inflation since the end of Episode 50 indicates another lava fountaining episode is possible. Models indicate Episode 51 is most likely to happen between July 11 and 15.

MAUNA LOA volcano

VOLCANO ALERT LEVEL: Normal.

Mauna Loa is not erupting. Read more online in the most recent monthly update about what is happening at Mauna Loa.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

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EARTHQUAKE ACTIVITY

Three earthquakes were reported felt during the past week in the Hawaiian Islands:

• MAGNITUDE-4.5 earthquake at 8:17 p.m. July 9 located 34 miles west-southwest of Captain Cook off the west coast of the Big Island at a depth of 24 miles below sea level.
• MAGNITUDE-3.3 earthquake at 9:16 p.m. July 7 located 8 miles southeast of Pāhala at a depth of 17 miles below sea level.
• MAGNITUDE-3.2 earthquake at 12:26 a.m. July 4 located 8 miles north of Wai‘ōhinu at at a depth of 4 miles below sea level.

Visit the Hawaiian Volcano Observatory website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.