What’s causing the swarm near Kaoiki Pali?

SPECIAL TO WEST HAWAII TODAY

Why do swarms of earthquakes occur around the Kaoiki Pali?

This questions stems from the earthquake swarm that occurred near Namakanaipaio Campground along the north end of the Kaoiki Pali on Feb. 22 to 24.

The answer begins with the magnitude-6.6 Kaoiki earthquake that occurred on Nov. 16, 1983. The earthquake was located 11 miles west of Halemaumau beneath the southeast flank of Mauna Loa at a depth of 7 miles, and it caused more than $6 million in damage. The earthquake was followed by several thousand aftershocks over a large area extending southeast beyond the Kaoiki Pali to the southwest rift zone of Kilauea volcano.

Within the aftershock sequence, there was a cluster of earthquakes at the north end of the Kaoiki Pali. Between 1960 (when Hawaiian Volcano Obseratory’s modern earthquake catalog began) and November 1983, there was an average of about five earthquakes per year and no seismic swarms in this area of the Kaoiki Pali. After November 1983, the average at least tripled and included five notable seismic swarms.

Swarms in 1990 and 1993 preceded intrusions in Kilauea’s upper east rift zone by a few months. Swarms in 1997 and 2006 did not immediately precede any change in activity. Several of these swarms, but not all, included a magnitude-4 or greater earthquake.

This brings us to the three-day-long earthquake swarm that occurred last week. HVO located about 180 earthquakes, the largest a magnitude-4.3. Most of the quakes were located at depths around 2.5 to 3 miles, which is much shallower and closer to Kilauea’s summit than the 1983 earthquake.

Kaoiki Pali is one of a series of subparallel faults that extends from the area west of the Namakanipaio Campground all the way to Honuapo. These faults, which form the Kaoiki-Honuapo fault system, were probably created by subsidence of Mauna Loa’s southeast flank before Kilauea was built against (or on) it. You can see the surface expression of these faults in the stepped terrain just mauka of Highway 11 between Kilauea’s summit and Pahala.

But the fact that the steps are draped by an unbroken 9,000-year-old lava flow shows that, if these faults are still active at depth, their movements are too minor to result in a surface rupture despite the earthquakes centered there. This is reaffirmed in 2012 by the lack of any shift of the ground surface in response to recent earthquakes.

The entire southeast flank of Mauna Loa frequently hosts earthquakes in response to being squeezed and stretched between two very active volcanoes. HVO records show four damaging earthquakes in this area between 1941 and 1983, but none since. Earthquakes in 1974 and 1983 are believed to have enabled Mauna Loa’s eruptions in 1975 and 1984.

The faults and cracks in the area between Mauna Loa and Kilauea tell multiple stories about the sense of movement. First, the fault system itself is made of normal faults, suggesting a downward motion of the southeastern (Kilauea) sides of each fault, relative to its northwestern (Mauna Loa) side during subsidence. They are analogous to the Hilina and Holei Pali on Kilauea’s south flank. Farther up Mauna Loa, near the epicenters of the 1974 and 1983 earthquakes, the ground surface is cracked in a strike-slip manner, suggesting differential sideways movement of the surface parallel to the Kaoiki Pali. Even though strike-slip and normal faulting are evident at the surface, the pattern of ground shaking during the largest earthquakes suggests they started with another mechanism — deep slippage at the base of Mauna Loa, where it rests on the ocean floor.

Faults on the south part of the Hawaii Island are subject to forces arising from the movement of magma and forces arising from gravitational breakdown of its volcanoes. The earthquake swarms on the Kaoiki Pali are probably not directly connected with subsurface magma movement but may be linked to changes in pressure within Kilauea’s magma system.

Between Kilauea’s ongoing eruption and Mauna Loa’s unrest, most recently in 2004-06, many forces have been at play in this intermediate region. As forces in the crust change in response to magma intrusion or gravitational subsidence, we can probably expect more of these earthquake swarms.

Kilauea activity update

A lava lake present within the Halemaumau Overlook vent during the past week resulted in nighttime glow visible from the Jaggar Museum overlook. The lake, which is normally about 295 to 377 feet below the floor of Halemaumau Crater and visible by HVO’s webcam, rose and fell slightly during the week in response to a series of large deflation-inflation cycles.

On Kilauea’s east rift zone, surface lava flows were advancing slowly down the pali over the past week. As of Thursday, flows were active in the middle part of Royal Gardens subdivision, reaching down to an elevation of about 600 feet. These flows are following the general path of earlier Peace Day flows, which may bring them close to the last few standing structures in Royal Gardens. There are no active flows on the coastal plain, and there is no active ocean entry.

Nine earthquakes beneath Hawaii Island were reported felt in the past two weeks. Between 9 p.m. Feb. 23 and 9:30 a.m. Feb. 24, eight magnitude-1.6 to -4.3 earthquakes were located 2.5 to 4.5 miles northwest of Kilauea summit at a depth of 2.5 to 3 miles. The two largest were a magnitude-4.1 earthquake at 9:02 p.m. Feb. 23 and magnitude-4.3 at 3:52 a.m. Feb. 24. A magnitude-2.3 earthquake occurred at 10:37 p.m. Feb. 25 and was located 3 miles southeast of Kapoho at a depth of 0.2 mile.

Visit the HVO web site, hvo.wr.usgs.gov, for detailed Kilauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes and more; call 967-8862 for a Kilauea summary; email questions to askHVO@usgs.gov.

Volcano Watch is a weekly article and activity update written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.