Why do swarms of earthquakes occur around the Ka‘ōiki Pali?

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This question stems from the earthquake swarm that occurred near Nāmakanipaio Campground along the north end of the Ka‘ōiki Pali on February 22-24, 2012.

The answer begins with the magnitude-6.6 Ka‘ōiki earthquake that occurred on November 16, 1983. The earthquake was located 17.5 km (11 mi) west of Halema‘uma‘u beneath the southeast flank of Mauna Loa at a depth of 11 km (7 mi), 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 Ka‘ōiki Pali to the southwest rift zone of Kīlauea volcano.

Within the aftershock sequence, there was a cluster of earthquakes at the north end of the Ka‘ōiki Pali. Between 1960 (when HVO's modern earthquake catalog began) and November 1983, there was an average of about 5 earthquakes per year and no seismic swarms in this area of the Ka‘ōiki Pali. After November 1983, the average at least tripled and included 5 notable seismic swarms.

Swarms in 1990 and 1993 preceded intrusions in Kīlauea'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 3-day-long earthquake swarm that occurred last week. HVO located about 180 earthquakes, the largest being a magnitude-4.3. Most of the quakes were located at depths around 4–5 km (2.5–3 mi) which is much shallower and closer to Kīlauea's summit than the 1983 earthquake.

Ka‘ōiki Pali is one of a series of subparallel faults that extends from the area west of the Nāmakanipaio Campground all the way to Honu‘apo. These faults, which form the Ka‘ōiki-Honu‘apo fault system, were probably created by subsidence of Mauna Loa's southeast flank before Kīlauea 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 Kīlauea's summit and Pāhala.

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 the 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 Kīlauea 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 (Kīlauea) sides of each fault, relative to its northwestern (Mauna Loa) side during subsidence. They are analogous to the Hilina and Hōlei Pali on Kīlauea'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 Ka‘ōiki Pali. Even though strike-slip and normal faulting are evident at the surface, the pattern of ground shaking during the largest earthquakes suggests that they started with another mechanism—deep slippage at the base of Mauna Loa, where it rests on the old ocean floor.

Faults on the south part of the Island of Hawai‘i are subject to forces arising from the movement of magma and forces arising from gravitational breakdown of its volcanoes. The earthquake swarms on the Ka‘ōiki Pali are probably not directly connected with subsurface magma movement but may be linked to changes in pressure within Kīlauea's magma system.

Between Kīlauea's ongoing eruption and Mauna Loa's unrest, most recently in 2004-2006, 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.

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Volcano Activity Update

A lava lake present within the Halema‘uma‘u Overlook vent during the past week resulted in night-time glow that was visible from the Jaggar Museum overlook. The lake, which is normally about 90–115 m (295–377 ft) below the floor of Halema‘uma‘u 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 Kīlauea's east rift zone, surface lava flows were advancing slowly down the pali over the past week. As of Thursday, March 1, flows were active in the middle part of Royal Gardens subdivision, reaching down to an elevation of about 180 m (600 ft). 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 Hawai‘i Island were reported felt this past week. Between 9 p.m. Thursday, February 23, and 9:30 am Friday, February 24, HST, eight magnitude-1.6 to -4.3 earthquakes were located 4–5 km (2.5–4.5 mi) northwest of Kīlauea summit at a depth of 4–7 km (2.5–3 mi). The two largest were a magnitude-4.1 earthquake at 9:02 p.m. on Thursday and magnitude-4.3 at 3:52 a.m. on Friday. A magnitude-2.3 earthquake occurred at 10:37 p.m. on Saturday, February 25, and was located 5 km (3 mi) southeast of Kapoho at a depth of 0.3 km (0.2 mi).