Sea and land level changes in Hawai'i

Abstract

An overall rise in sea level has been recorded by all Hawaii tide gauges over the last century; however, the rates vary considerably between islands. In particular, the sea level rise rate is higher at Hilo (3.3 mm/yr) on the island of Hawaii at the southeast end of the island chain, than at Honolulu (1.4 mm/yr) on the island of Oahu about 335 Ian to the northwest. This difference has been attributed to island subsidence associated with active volcanism at the southeast end of the Hawaiian ridge. Continuous GPS measurements collected over the past 5 years are used to examine the relative vertical movements of the main Hawaiian Islands in an attempt to reconcile the observed difference in sea level rates. The rates of vertical crustal motion are estimated in a reference frame realized using a network of 30 GPS stations spread across the Pacific region. Although absolute vertical motion rates are not yet obtainable, the differential rates of vertical crustal motion are determined to within approximately ± 0.5 mm/yr at the 95% confidence level. The geodetic measurements indicate that vertical velocities within the main Hawaiian Islands are similar. Hilo is subsiding relative to Honolulu, but the difference in rates is only 0.5 mm/yr, considerably less than the 1.9 mm/yr difference suggested by tide gauge observations. Historical hydrographic data suggest that steric sea level trends since 1945 vary considerably along the Hawaiian Ridge, with rates increasing from the northwest to the southeast. It is suggested that the difference in Hilo-Honolulu sea level rise rate is due in part to upper ocean thermal variations. The notion that oceanographic effects influence differential rates of sea level rise at Hawaii challenges previous interpretations based solely on variable crustal motion. The absolute rate of vertical crustal motion at Honolulu has been constrained by coral-age data to be less than 0.1 mm/yr since the last interglacial. A reanalysis of the age-depth relationship obtained from submerged Holocene corals recovered from the Hilo drill hole is consistent with an absolute rate of crustal subsidence anywhere between - 2.7 and 0 mm/yr. Given this wide range of velocities, the differential Holocene subsidence rates from coral data do not conflict with our reinterpretation of the tide gauge data.