Although much of Hawaii’s daily seismic activity occurs within volcanic systems, these widely felt, often coastal, deep earthquakes are not directly related to magmatic processes at all.
In particular, the earthquake of magnitude 6.0 on May 22.nd alarmed the people of the state. This event occurred under the western side of the island of Hawaii, southeast of Captain Cook, at a depth of 14 miles (23 km) below the ocean surface. Two others, unrelated but widely felt, followed a few days later: a magnitude 4.6 on June 2.ndnorthwest of Kyoho, at a depth of 21 miles (24 km); and magnitude -4.7 on June 9.theast of Pepequeo, at a depth of 24 miles (39 km). If these deep earthquakes are not associated with magmatic processes, then 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 land masses on Earth. Hawaii is located not only far from continental landmasses, but also from the tectonic plate boundaries that account for approximately 90% of the world’s seismic activity.
California, Alaska, Russia, Japan, Indonesia and the southwestern Pacific Ocean are regions of high seismic and volcanic activity that make up the Ring of Fire, where tectonic plates slowly collide, move apart and grind against each other. Hawaii is located in the middle of the Pacific Plate and therefore does not produce seismicity associated with 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 is) hot enough to break through the oceanic lithosphere (Pacific Plate), forming undersea volcanoes that become islands. As the Pacific Plate creeps northwest, the hot spot remains stationary, but the volcanic islands move with the plate to form a chain.
Over millions of years, the mantle plume fed magma to shield volcanoes, causing the Hawaiian Island Chain to grow and thicken the lithosphere (the fragile upper layer of the Earth’s mantle). The largest load on the island chain comes from the largest volcanoes: Mauna Kea and Mauna Loa. The lithosphere takes this weight, bending down towards the center of the Earth. About 100 miles off the coast of the Hawaiian Islands, the lithosphere bends upward, forming the so-called Hawaiian Arch.
This cartoon shows how the weight of the volcanoes in the Hawaiian Island Chain can cause the fragile oceanic crust and upper mantle below to buckle, which can lead to deep earthquakes that can be felt throughout the state of Hawaii. USGS chart.
To visualize the weight of islands in the lithosphere, you can imagine a bowling ball lying on a bed. The bowling ball will sink into the blankets and the blankets will form folds radially outward from the bowling ball. These blanket folds are deep faults caused by the bending of plates.
Because the lithosphere is relatively cold and brittle compared to the hotter, more ductile part of the mantle underneath, it can only bend a certain amount before the stress is released as an earthquake. These earthquakes, called flexure events, occur much deeper and less widespread than most earthquakes in Hawaii, which are the result of magma movement and pressure changes or faults within the island itself. These bending events tend to be felt more widely because they occur deeper and the dense lithosphere allows seismic waves to impact a larger area without attenuating as much energy as they travel, which also contributes to potential damage.
Bending events vary in location and depth. They are typically found in coastal or shelf regions and are much deeper than typical seismicity in our volcanic systems. However, not all events in the deep sea are watershed events. For example, the Pahala seismic swarm regularly produces earthquakes greater than magnitude 4 and at depths of more than 18 miles (30 km) off the southeast coast of the island of Hawaii. These are not flexural earthquakes, but are thought to be related to magma transport pathways at hot spots.
HVO, as the authoritative seismic network in the State of Hawaii, analyzes all major local earthquakes. Whether an earthquake is the result of a deep bend or occurs in our volcanic systems, remember to “drop, cover and hold on” when you feel shaking and move away from the coast in case a tsunami occurs.