June 28, 1996

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Deep Earthquakes

Each year, the Hawaiian Volcano Observatory records thousands of earthquakes with its island-wide network of seismographs. Often, many of these earthquakes are directly related to volcanic activity and indicate movement of magma beneath the summits or rift zones of the volcanoes. Other earthquakes are less obviously related to volcanic activity, as they occur deeper within the volcanoes or beneath the flanks of the volcanoes, away from the shallow activity related to visible lava. However, all earthquakes in Hawaii are ultimately due to the presence of the volcanoes.

While fewer earthquakes are routinely recorded from Mauna Kea than are earthquakes beneath Kilauea or Mauna Loa, they are clearly recognizable numbers. They are typically deeper than Kilauea earthquakes, extending roughly between 10 and 60 km beneath the Earth's surface. Because of their deeper origin, the seismic waves are less attenuated, or dampened, on their way to the Earth's surface. In general, these earthquakes are more widely felt than shallower earthquakes of equal magnitude.

Since January, we have recorded over 100 earthquakes occurring within a 25-km radius of Mauna Kea's summit. Their depths ranged from 5 to 48 km beneath the summit, and the largest of these earthquakes was M 4.4, recorded on January 21. The most recent Mauna Kea earthquake was M 2.3, recorded on Friday morning, June 28.

As with earthquakes that we record from elsewhere beneath the island, we use the earthquake hypocentral location and other source properties to develop our ideas of how the volcanoes and the islands have evolved. While we believe that many of the deeper earthquakes beneath Kilauea and Mauna Loa partly signal the upward movement of magma from the hot spot, we view the earthquakes beneath Mauna Kea differently.

According to the theory of plate tectonics, the Hawaiian Islands are formed as the lithospheric Pacific Plate moves to the northwest over a relatively stationary "hot spot" in the Earth's mantle. The islands are built atop the ancient oceanic crust as lava is erupted onto the Earth's surface. As the volcano grows, more weight is loaded onto the underlying lithosphere.

While a volcano is highly active, there are a number of processes that result in forces that lead to earthquakes. As a volcano becomes dormant, it evolves more gradually. In Hawaii, as the island moves away from the hotspot, the underlying lithosphere becomes cooler and more brittle, and also more capable of sustaining earthquakes. As this lithospheric evolution takes place, there are occasional earthquakes to release energy accumulated within the plate.

Because of the relatively random patterns of occurrence of the Mauna Kea earthquakes that we observe, we do not interpret them to be tied to future eruptions. Rather, they indicate continuing lithospheric adjustments beneath the island. Changes in the observed patterns, from our continuing seismic monitoring, might warrant a reevaluation of this view.

Kilauea Eruption Update

The eruption from flank vents on the western side of Pu'u 'O'o continues unabated. Lava enters the ocean at four points along a one-mile stretch between Lae'apuki and Kamoamoa. A large block of the Kamoamoa/Lae'apuki bench slid into the ocean at about 11:00 PM on June 22. This triggered a series of hydrovolcanic explosions that were so violent, our Kilauea seismic network was able to record them for nearly two hours.

Two earthquakes were felt since our last article. Residents of Waikii, Ahualoa, and Waimea were shaken by a magnitude 3.0 temblor on Sunday, June 9 at 11:37 PM. The earthquake was located 2 miles west of the summit of Mauna Kea at a depth of 16 miles. A magnitude 4.4 earthquake occurred 11 minutes after midnight on Friday morning, June 14, and it was felt throughout the island of Hawaii except the Kohala district. The epicenter of the earthquake was 32 miles southeast of Pahala and at a depth of 27 miles beneath Loihi Volcano. There were no reports of damage from either quake.