Volcano Watch — Big Island earthquakes

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A few weeks ago, we told you about the more than 67,000 earthquakes we recorded during the past year. Most of those earthquakes were so small that they were not felt and caused no damage.

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A few weeks ago, we told you about the more than 67,000 earthquakes we recorded during the past year. Most of those earthquakes were so small that they were not felt and caused no damage. Of these, 6,772 earthquakes were large enough that we could determine their locations. Only 37 earthquakes were felt on the island, 30 of which had magnitudes greater than 3.0. Of these 30 earthquakes, 22 had magnitudes between 3.0 and 3.5, six had magnitudes between 3.6 and 4.0, and only two had magnitudes greater than 4.0. The largest was a magnitude-5.2 earthquake that occurred on February 1.

These earthquakes do not occur randomly in either space or time. For example, there were no earthquakes with magnitudes larger than 3.0 between May 22 and July 5 or between September 1 and October 8, yet the last week of July had four such events. These 30 events also ranged in depth from less than one mile to as much as 30 miles. Six of the 30 events were shallower than 2.5 miles, 14 were between 3 and 8 miles, and 10 were greater than 15 miles deep.

The deep earthquakes had a wide distribution, with three beneath Mauna Kea Volcano, two offshore south Kona, four offshore the south flank of the island, and one roughly beneath Wood Valley. All of the shallow earthquakes were located beneath Kīlauea Volcano, with two beneath the summit region and the rest within the south flank near the eruption site. Most of the intermediate-depth earthquakes were also located in the south flank of Kīlauea Volcano, although several exceptions occurred, including another one near Wood Valley, one near the south rift zone on Mauna Loa Volcano, and one near Honokaa.

In the past, the Big Island has experienced numerous large damaging or potentially damaging earthquakes. On average, the Big Island experiences one earthquake with magnitude greater than 5.0 each year-and-a-half. Of these, one earthquake about every six years has a magnitude greater than 6.0.

Kalapana was the site of the last earthquakes with a magnitude greater than 6.0, when one of magnitude 6.2 occurred on June 25, 1989. The Kalapana area was also the site of the largest earthquake to occur in this century, when a magnitude-7.2 earthquake occurred on November 29, 1975. A magnitude-6.1 earthquake shook the Kaoiki fault zone between Mauna Loa and Kīlauea Volcanoes on June 27, 1962, and a magnitude 6.7 shook the same area on November 16, 1983. The other large one since 1960, a magnitude-6.2 temblor, took place beneath Honomu on April 26, 1973.

Large, damaging earthquakes have also occurred beneath the Kona coast in 1950, 1951, and 1952, although there has been little activity in that region since. The Kona temblors were probably related to the 1950 eruption of Mauna Loa, so we might expect renewed seismic activity in Kona following a Mauna Loa southwest rift eruption.

Although none of this past year's earthquakes caused but minor damage, they each serve as a reminder that we live in earthquake country and that we must be prepared for the larger, more damaging, earthquakes that occur here with surprising frequency.

Many of these earthquakes are caused by sliding of the flanks of the active volcanoes towards the sea. This movement occurs along a nearly horizontal fault plane that is roughly 5 miles beneath the surface. The flank of the volcano is pushed towards the sea by pressure within the magma system. This slippage of the flanks of Kīlauea and Mauna Loa occurs continuously but is punctuated by larger movements that are associated with the larger earthquakes. Over much longer periods of time, perhaps once every few hundred thousand years, the flanks of these volcanoes fail as giant landslides that dissect the edifices and deposit blocky debris on the sea floor far from the islands.

If you are interested in learning more about these giant landslides that have shaped many of the Hawaiian Islands, Dr. David Clague, Scientist-in-Charge of the U.S. Geological Survey's Hawaiian Volcano Observatory, will present a talk at the Lyman Museum at 7:00 p.m. on Thursday, January 19. The talk will focus on a display of sonar data of the sea floor that reveals many of these catastrophic landslides. Other features of the sea floor around the islands will also be discussed. The Lyman Museum is located at 276 Haili Street in Hilo. Admission is free.