# Volcano Watch — The Big Island is sinking under weight of volcano

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A magnitude 3.8 earthquake occurred about 15 miles beneath Mauna Kea Volcano at 1:48 a.m. on January 21. Similar deep earthquakes occur persistently beneath the Big Island but are not restricted to any particular location.

The Big Island is sinking under weight of volcano

(Public domain.)

A magnitude 3.8 earthquake occurred about 15 miles beneath Mauna Kea Volcano at 1:48 a.m. on January 21. Similar deep earthquakes occur persistently beneath the Big Island but are not restricted to any particular location. The largest such earthquake recorded in the last 20 years was a 1973 quake, with magnitude 6.3, located about 15 miles beneath Honomu.

These earthquakes are not caused by movement of magma, either directly or indirectly. Instead, they are caused by bending of the ocean lithosphere beneath Hawaii. The lithosphere is the nearly rigid outer layer of the Earth and is roughly 50 miles thick beneath Hawaii. As the volcanoes grow, their weight is greater than the lithosphere can support. The result is that the lithosphere flexes downward under the increasing weight of the volcanoes. As the lithosphere bends, earthquakes are produced. Most of these earthquakes occur near the top of the lithosphere, which is more brittle than the deeper parts because it is cooler.

One result of this downward bending is that the Islands sink or subside. This effect can be seen by changes in sea level recorded at tide gauges around the Islands. Hilo, for example, is sinking at a rate of nearly one tenth of an inch each year. This doesn't sound like much, but over 100 years, it represents 10 inches of subsidence.

Additional evidence for the geologically rapid sinking of the Islands is found offshore. The figure shows a series of drowned coral reefs that are located off the west coast of Kohala Volcano. These reefs mark successive shorelines of the Big Island, with the deepest reef being the oldest (about 465,000 years old) and the shallowest being the youngest (about 15,000 years old). Their ages were determined by dating corals recovered from each reef.

The reefs drowned when the sea level rose as the island was sinking, and the reefs could not grow fast enough to keep up. The sea level rises when the polar ice caps melt during warm-climate periods. Each new reef formed when sea level was dropping, thereby keeping the sinking shoreline stable, relative to sea level, and allowing the corals time to grow. Sea level falls as the polar ice caps grow during cold-climate periods.

The deepest reef is now located 4,380 feet below sea level, thereby demonstrating that the Big Island has sunk at least this much, and is still sinking, at a rate of nearly one tenth of an inch per year. Similar drowned reefs located off the eastern coast of Haleakalā Volcano are at depths as great as 6,900 feet. These reefs are also still sinking, but much more slowly than those offshore from Kohala Volcano.

Such subsidence is the major reason that only the youngest of the Hawaiian volcanoes form high islands. The older volcanoes have subsided so much that they are no longer above sea level. These older, submerged volcanoes comprise the Leeward Hawaiian Islands, extending westward to Midway Island, and additional, even older volcanoes named the Emperor Seamounts.

### Volcano Activity Update

The eruption on the East Rift Zone of Kīlauea Volcano continues with low-volume, steady effusion of lava from the breakout at about the 1,900-foot level of the tube downslope from the Kupaianaha vent. This is the same area that has had active flows for the last month. The volume of lava continues to decline slowly. The lava pond within Puu O`o is active and produces bright glow at night. It is likely that the Kupaianaha vent will stop erupting in the near future, and episodic fissure eruptions will commence in the vicinity of the episode 49 [vents or, possibly, along the upper East Rift Zone near Pauahi Crater].

*[bracketed text not published in Hawaii Tribune-Herald]