Moving magma, eruptions, and ground shaking from strong earthquakes produce an abundance of ground fractures and subsidence features (downward movement of ground) that profoundly affect the landscape, human activity, and infrastructure. Ground fractures and subsidence most commonly occur at the summits or rift zones of active Hawaiian volcanoes during magma intrusions and eruptions. They also occur over broad areas on the flanks of Mauna Loa and Kīlauea during large earthquakes. Additionally, the slow but continuous subsidence of the Island of Hawai‘i and the threat of sea-level rise also present a long-term hazard to shoreline facilities.
At Hawaiian volcanoes, magma moves toward the surface in long, narrow bladelike bodies (called dikes) typically less than a few meters (yards) wide. Dikes commonly are 5 to 10 km (3 to 6 mi) long and 2 to 3 km (1.2 to 1.9 mi) tall. During an intrusion, rising magma deforms the ground surface, with the most striking examples being large fractures and a broad, low area between parallel fractures, called a graben. During an eruption and intrusion at Kīlauea Volcano's East Rift Zone in December 1965, hundreds of cracks opened across a broad area about 13 km (8 mi) long and 1.6 km (1 mi) wide. Some cracks opened at least 3 m (10 ft) and moved up or down at least 1 m (3 ft); the fractures seriously damaged the Chain of Craters and Hilina Pali roads in Hawai`i Volcanoes National Park.
Earthquakes, especially those of magnitude-6 and above, produce strong ground shaking over a large area, so they often cause ground fractures and subsidence over much broader areas than moving magma. Fractures typically occur on, or around, a landslide or slump block that is shaken loose by an earthquake. They also form next to unstable crater walls as loose soil and rocks adjust to the shaking.
Vertical movement along shallow below-ground faults can also result in fractures or offsets of the ground surface. Large earthquakes beneath the flanks of Mauna Loa and Kīlauea that occur along the boundary between the ocean crust and Island of Hawai‘i have caused sizeable areas to move seaward and subside by several meters (yards). For example, the magnitude-7.7 earthquake on November 29, 1975, resulted in Kīlauea's south flank moving seaward 4 to 8 m (13 to 26 ft) and subsiding as much as 3.5 m (11.5 ft). Over thousands of years, repeated horizontal and vertical displacements in this area have created the stepped topography of the volcano's southern coastline, each step as high as tens to several-hundred meters (yards).
Ground settling may also occur at or around an active vent if magma beneath the vent drains away and removes the underlying support for the ground surface. The lack of ground support can lead to the formation of small collapse craters (called pit craters) tens of meters (yards) across. When large volumes of magma are withdrawn from the summit reservoirs beneath Kīlauea and Mauna Loa volcanoes, calderas several kilometers (miles) across and hundreds of meters (yards) deep may form.
The Island of Hawai‘i is sinking at a rate of a few millimeters (fractions of an inch) per year. The enormous load (weight) of its volcanoes have caused the underlying ocean crust and lithosphere to sag over a large area.
Based on tide-gauge measurements between 1947 and 1983 at Hilo, Hawai‘i, the rate of sinking was 2.4 mm (0.09 inch) per year, equivalent to 2.4 cm (0.9 inch) per decade. The average worldwide rise of sea level since 1992 is about 3 cm (1.2 inches) per decade. Add these together and Hawai‘i is submerging beneath sea level by as much as 5-6 cm (2 to 2.4 inches) per decade. This is equivalent to 50 to 60 cm (1.6 to 2 ft) per century! Submergence from the slow, continuous subsidence and seal-level rise will eventually endanger shoreline infrastructure and development. Low-lying coastal areas on the Island of Hawai‘i will become increasingly threatened by damage from storm waves and tsunamis, and eventually by inundation.