Categorized | Earthquake, Featured, Sci-Tech, Volcano

Volcano Watch: Why do swarms of earthquakes occur around the Ka‘oiki Pali?


View Earthquake Swarm (2/22/12) in a larger map

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

Seismology and Seismometry. Ka‘oiki aftershocks recorded on a rotating drum seismograph at Desert station located within a few miles of the epicentral area of the Hawaii earthquake of November 16, 1983. USGS Photo

Seismology and Seismometry. Ka‘oiki aftershocks recorded on a rotating drum seismograph at Desert station located within a few miles of the epicentral area of the Hawaii earthquake of November 16, 1983. USGS Photo

This questions stems from the earthquake swarm that occurred near Namakanaipaio Campground along the north end of the Ka`oiki Pali on February 22-24, 2012.

The answer begins with the magnitude-6.6 Ka`oiki 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`oiki 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 Ka`oiki 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`oiki Pali. After November 1983, the average at least tripled and included 5 notable seismic swarms.

Swarms in 1990 and 1993 preceded intrusions in Kiauea’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.

Earthquakes in the Volcano area for the past two weeks. Map generated at 9:06 a.m. Friday, Feb 24, 2012

Earthquakes in the Volcano area for the past two weeks. Map generated at 9:06 a.m. Friday, Feb 24, 2012

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 Kilauea’s summit than the 1983 earthquake.

Ka`oiki 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 Ka`oiki-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 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.

Seismographs at the Thomas A. Jagger Museum at Hawaii Volcanoes National Park. Photo by Baron Sekiya | Hawaii 24/7

Seismographs at the Thomas A. Jagger Museum at Hawaii Volcanoes National Park. Photo by Baron Sekiya | Hawaii 24/7

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 Ka`oiki 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`oiki 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 2004-2006, 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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