The earthquake on Hawai`i island last week was caused not by slipping tectonic plates, but by the weight of the island bending the underlying lithosphere.
Fact is, the Hawaiian islands are slowly sinking under their own weight, even as sea levels rise.
Turns out that each is causing roughly half of the islands' shoreline loss (see Hilo estimate below).
As our volcanoes grow, their weight is greater than the lithosphere (the outer, nearly rigid, 50-mile thick layer of the Earth) can support.
The result is that the lithosphere flexes downward under the increasing weight of the growing island.
No wonder we're losing our island beaches at such a high rate.
According to UH scientists, "As much as 25 percent of sandy beach land on O'ahu and Maui has been lost in the past 50 years," based on comparison of old and new aerial photos and maps of waterfront property, and beach loss "is continuing in many places across the state at a rate of between 6 inches and 12 inches a year" (via Mike Leidemann story in Honolulu Advertiser).
Of course, much of our beach loss results from short-sighted "shoreline hardening" projects, as Leidemann's story makes clear.
Yet, the State is well-advised to plan for island shorelines moving progressively inland.
For example, 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, according to a USGS study. 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.
As this subsidence continues, we know that:
"Hawai`i island will change shape in the future as Kilauea extends its southern and eastern shorelines and as Loihi grows to sea level and eventually coalesces with the island. At the same time, however, continued subsidence will reduce the size of Kohala, Mauna Kea, Hualalai, and Mauna Loa Volcanoes. At the present rate of subsidence of about 1/8 inch per year, Kohala will become a separate island about 350,000 years from now. Between 600,000 and 850,000 years from now, Hualalai and Mauna Kea will become separate islands; exactly when that time will come cannot be estimated without knowing when Mauna Loa will stop erupting."
Either way, it is sobering to note that sea level rise is only half of our longterm challenge.