Volcano Watch — It's Sinking in that the Island is Sinking

The net sinking totals more than 30 cm (a foot) per century. Additionally, large earthquakes like those in 1868 and 1975 caused significant down-dropping of portions of the southeast flank of Kīlauea. For example, the 1975 earthquake submerged sections of the coast near Halape and resulted in over 30 cm (a foot) of subsidence as far east as Pohoiki.

The rift zones of active Hawai‘i volcanoes also subside as they are spread apart to accommodate intrusion and eruption of magma. Rift zones are long narrow regions from which eruptions can occur great distances from the summits of Hawaiian volcanoes. Most volcanoes in Hawai`i have two - Kīlauea has a southwest rift zone and a much more active east rift zone.

The ground within rift zones can sink abruptly during magma intrusions and eruptions, sometimes forming a feature known as a "graben" (German for "ditch"). Classic geologic grabens are long, down-dropped areas between two parallel faults.

There are several grabens along Hawaiian volcanic rift zones. Kapoho, which means "the depression," hosts a prominent graben that down-dropped abruptly in 1868, 1924, and 1960" - less than one day before an eruption.

Broader long-term subsidence, up to 15 km (9 mi) wide, also occurs along rift zones with maximum subsidence centered within the rift zone. Because much of the coast southwest of Cape Kumukahi is close to the rift, long-term rift zone subsidence can affect the coastline in this area. Kapoho is where this rift zone meets the sea and probably where subsidence is most evident.

Where the coast is made up of cliffs, small amounts of subsidence are not readily noticed unless the entire cliff becomes submerged. Where the land slopes into the sea, each bit of subsidence results in an inland movement of the sea. With time, familiar locations, roads, and houses can become much closer to the ocean or even submerged beneath it.

The effects of subsidence from all of these processes have been noted all along the coast from Cape Kumukahi to Kaimu. Kapoho school was originally meant to be closer to the coastline, but the site was flooded during the 1868 earthquake-induced subsidence, and the school was relocated inland. The 1924 subsidence (which preceded explosive eruptions at Halema‘uma‘u) allowed seawater into the graben, creating Higashi pond. Subsidence before the 1960 eruption revealed gaping cracks near Kapoho town before lava flows buried the town, the graben, and the pond.

Farther south, there is additional evidence of subsidence in the form of fishpond walls submerged in Kapoho Bay offshore from the Kapoho Beachlots and Vacationland subdivisions. Roads dating back to the monarchy and Hawai`i County have become submerged in the vicinity of Pohoiki (meaning "small depression").

Over the long term, Kapoho has lived up to its name - "depression." - Recent geodetic or deformation monitoring data suggests that the rate of subsidence may have slowed during the last five or six years. This is, however, a temporary slow-down that we should not expect to last. Long-term subsidence is part of the history and future of the lower east rift zone of Kīlauea Volcano.

————————————————————————————————————————————————————————————————

Volcano Activity Update

A prolonged deflation/inflation (DI) event started at Kīlauea's summit on March 5. The deflation caused surface flows on Pulama pali and on the coastal plain to slow down, perhaps stop, during the past week, but not before the flows had reached a few tens of meters (yards) into the National Park. As of this writing (March 11), the volcano had rapidly begun to inflate, and surface activity on the pali and coastal plain will likely increase or resume soon.

At Kīlauea's summit, a spattering and roiling lava surface deep within the collapse pit inset within the floor of Halema‘uma‘u Crater, was barely visible, at times, via Webcam during the past week. The lava surface dropped significantly in response to the deflation phase of the DI event, but has begun to rise slowly back to a higher level since the volcano switched to inflation. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

Three earthquakes beneath Hawai‘i Island were reported felt during the past week - all on Monday, March 8, 2010, at the same location, 32 km (20 miles) northwest of Hilo, and at the same depth, about 32 km (20 miles). A magnitude-4.5 earthquake occurred at 6:29 p.m., H.s.t, a magnitude-3.5 occurred at 6:31 p.m., and a magnitude-1.8 occurred at 6:38 p.m.