Volcano Watch — Have you ever experienced that sinking feeling?

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There is abundant evidence that the island of Hawaii is sinking, and that different parts of the island sink, or subside, at different rates and for different reasons. At any location, the net subsidence is the sum of these different types of subsidence and any change in global sea level.
 

There is abundant evidence that the island of Hawaii is sinking, and that different parts of the island sink, or subside, at different rates and for different reasons. At any location, the net subsidence is the sum of these different types of subsidence and any change in global sea level.

The first type of subsidence is widespread and affects the entire island.In the short term, say the last 50 years, there are detailed tide gauge data that show that Hilo has sunk, relative to Honolulu, at a rate of 2.3 millimeters per year, or roughly 4.5 inches in 50 years. At the same time, global sea level has risen about 1.8 millimeters per year, so that Hilo has actually sunk about 8 inches relative to sea level in that 50-year period.

Additional long-term data derived from the Hilo scientific drill hole, described in a column in early September, suggest a slightly slower subsidence rate of 2.2 millimeters per year averaged over the last 39,000 years. This long-term rate is highly dependent on changes in absolute sea level causes by adding or subtracting ice to the polar ice caps. In the last 50 years, the change in global sea level was small, compared with the large changes that accompany formation of icecaps during glacial periods and melting of ice during interglacial periods.

A third line of evidence that the island is sinking is the presence of a series of drowned coral reefs around much of the northern half of the island. These reefs mark the location of the shoreline at different times in the past. Offshore Kohala Volcano is a sequence of six coral reefs, with the deepest, and oldest, at a depth of about 1,335 meters (4,380 feet). This reef has been sampled, and the recovered coral fragments have been radiometrically dated at about 465,000 years old. The shallowest, and youngest, reef is about 15,000 years old. The inferred subsidence rate for the island over the last 500,000 years is about 2.6 millimeters per year. This rate is slightly greater than that inferred for the more recentpast, although the estimates are not statistically different.

The formation of these reefs is a fascinating story of its own. Each of these reefs drowned when global sea level rose while the island was slowly sinking. The reefs could not grow fast enough to keep up with the rapid change in relative sea level. Global sea level rises when the polar ice caps melt, or during warm interglacial periods. When global sea level is falling due to addition of ice to the polar ice caps, the island is likewise sinking, so relative sea level is stable and the reefs flourish. The result of all this is counter-intuitive: the reefs around Hawaii flourish during cold periods and die during warm periods.

An important question to ask is "why is Hawaii slowly sinking?" The answer lies in the great weight of the islands that slowly bends the underlying lithosphere, the outer, nearly rigid, 50-mile thick layer of the Earth. As the volcanoes grow, their weight is greater than the lithosphere can support. The result is that the lithosphere flexes downward under the increasing weight of the growing island. The downward flexing is a response to increases in the weight of the island, and is most rapid while the island is rapidly growing. The older Hawaiian Islands have already completed their period of rapid growth and rapid subsidence. Their much slower subsidence rates result in much smaller changes in relative sea level that allow for growth of coral reefs and formation of white sand beaches.

There is a second type of subsidence associated with large earthquakes on Hawaii. The last earthquake that produced coastal subsidence was the magnitude 7.2 event in 1975. The epicenter was located near Kalapana, and the coastal subsidence associated with the earthquake was limited to the south flank ofKīlauea Volcano. However, the amount of subsidence along the coast was truly impressive. The greatest subsidence occurred near Halape, where the coast sank about 3.0 to 3.5 meters (10-11.5 feet). The amount of subsidence decreased to the east and was about 1 meter (3 feet 4 inches) at Kaena Point, 0.5 meters at Kalapana, and 0.25 meters at Kapoho. To the west, subsidence was limited to the south flank of Kīlauea and so Punaluu did not subside. Along the south flank of Kīlauea, this type of subsidence represents the most important type as a single earthquake-related subsidence event can be equivalent to up to 1,500-years worth of the slow subsidence discussed above.

There are several other areas of the island that may be subject to this type of earthquake-related catastrophicsubsidence. They are sites of intermediate-depth earthquakes related to seaward slippage of large landslide structures and are located between Palima Point and Naalehu, and between Kealakekua Bay and a few miles south of Hookena. Although these areas are not as active as the south flank of Kīlauea Volcano, they have the potential for large earthquakes and related catastrophic subsidence.

There is yet another type of vertical motion that occurs along the active south flank of Kīlauea Volcano. This motion occurs between major earthquakes and can be in the opposite sense, that is, the coast may be uplifted. This type of motion is more poorly constrained, but thecentral south flank of Kīlauea at Apua Point has uplifted at rates of perhaps a few centimeters per year following the 1975 Kalapana earthquake. Farther east, the amount of uplift is smaller.

A final type of subsidence is related to extension across the lower east rift zone near Kapoho. There, the coast continued to subside following the 1975 earthquake. For the last 10 years, this subsidence appears to be occurring at rates of a few centimeters per year as the rift slowly widens. At times in the past, particularly in 1924 and in 1960, sections of the coast near Kapoho have subsided dramatically as magma was intruded into the lower east rift zone. Such subsidence events were nearly as large as those associated with the 1975 earthquake.