Volcano Watch — Large earthquakes and inflated or inflating volcanoes

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Friday, November 29, marked the 27th anniversary of the second largest earthquake to occur in Hawai`i. The magnitude-7.2 earthquake originated beneath the south flank of Kilauea Volcano, and that side of the volcano moved 8 m (26 ft) seaward and 3.5 m (11.5 ft) down. What caused the earthquake?

When magma from a deep mantle source enters a volcano, some of it erupts, but most of it accumulates within the volcano. To accommodate the accumulated magma, the flanks of the volcano are pushed out. In the case of Kilauea Volcano, its north flank is buttressed by Mauna Loa, the world's largest volcano, so only the unbuttressed south flank moves seaward to accommodate the increased volume of magma.

The seaward movement of Kilauea's south flank can be slow, about 8 to 10 cm (3 to 4 inches) per year, as we have detected with our continuous geodetic measurements, or it can be rapid, as during the 1975 earthquake.

The causal relationship of magmatic stresses can be inferred from other large flank earthquakes. The largest earthquake in Hawai`i, the April 2, 1868, Mauna Loa south flank temblor estimated to have a magnitude of 7.9, occurred as a dike was forcing its way from the summit down the southwest rift zone of Mauna Loa. This was preceded a few days earlier on March 28 by another Mauna Loa south flank earthquake with an estimated 7.0-magnitude.

On October 5, 1929, an earthquake with a magnitude of 6.5 occurred beneath Hualalai Volcano. It was the largest of a swarm of earthquakes accompanying an intrusion of magma within Hualalai.

A magnitude-6.0 Mauna Loa south flank earthquake beneath the Ka`oiki fault zone was the harbinger that allowed scientists at HVO to forecast the 1942 Mauna Loa northeast rift zone eruption. The pattern of a large or moderately large earthquake before a Mauna Loa eruption was noticed for the last three Mauna Loa eruptions. The time period between the earthquake and the ensuing eruption differed for the three eruptions.

The June 1, 1950, southwest rift eruption of Mauna Loa was preceded by a magnitude-6.2 earthquake on May 29. The exact location of the earthquake is uncertain, but it is thought to be from the upper southwest rift zone. Two large earthquakes occurred on the southwest flank of Mauna Loa following the 1950 eruption.

On August 21, 1951, a destructive magnitude-6.9 earthquake near Kealakekua relieved the accumulated strain caused by the magmatic intrusion of the 1950 dike into the southwest rift zone of Mauna Loa. A magnitude-6.0 aftershock on May 23, 1952, moved the flank farther seaward.

A magnitude-5.5 Ka`oiki earthquake on November 30, 1974, and a magnitude-4.7 summit earthquake on December 15 are associated with the increase in seismic activity before the July 5, 1975, summit eruption of Mauna Loa. Intense earthquake activity continued for weeks after the day-long eruption ended as a dike intruded the northeast rift zone.

Another large Ka`oiki earthquake occurred on November 16, 1983, and heralded the inflated state of Mauna Loa Volcano. The magnitude-6.6 earthquake was followed on March 25, 1984, by a summit and northeast rift zone eruption that lasted 23 days.

Back on Kilauea Volcano, the south flank again rapidly shifted seaward on June 25, 1989, when a magnitude-6.1 earthquake struck. This probably was the result of the volcano trying to accommodate the intrusions of magma into the east rift zone.

Not all large Hawaiian earthquakes are directly related to magmatic stresses. A family of deep earthquakes is caused by the sheer weight of the volcanoes. The lithosphere is depressed, flexed, and occasionally fractured. The magnitude-6.2 Honomu earthquake on April 26, 1973, and the magnitude-6 earthquake north of Maui on January 22, 1938, are examples of this type of earthquake.

So, the next time you feel an earthquake, it probably will be because the volcano is over-stressed and seeking relief. The alternative is to erupt.

Volcano Activity Update

Eruptive activity of Kilauea Volcano continued unabated at the Pu`u `O`o vent during the past week. Lava flows through a tube system from the vent to the sea.. Lava continues to enter the ocean at the Wilipe`a and West Highcastle lava deltas. Only the Lae`apuki entry of the east arm of the Mother's Day flow is active, but numerous surface breakouts are observed in the coastal flats from the inflating flow. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying sudden collapses of the new land. The steam clouds are highly acidic and laced with glass particles. The National Park Service has erected a rope barricade to delineate the edge of the restricted area. Do not venture beyond this rope boundary and onto the lava deltas and benches.

One earthquake was felt in the week ending on November 27. A resident of Mt. View was shaken at 12:07 p.m. on November 23. The magnitude-2.3 temblor was located 5 km (3 mi) west of Kea`au at a depth of 13.6 km (8.2 mi).

Mauna Loa is not erupting. The summit region continues to inflate, but no earthquakes were located in the area for the last seven days.