Report on the State of Physical Infrastructure in Hawaii

Unlike many other areas where a shift in tectonic plates is the sole causeof an earthquake, 95 percent of the earthquakes in Hawaii are linked tovolcanic activity. These earthquakes can occur before or during eruptions,or as magma (molten rock) travels underground. The earthquakesdirectly associated with the movement of magma are concentrated on thesouthern part of the island of Hawaii and along its Kona coast beneathtwo of the island’s active volcanoes, Kilauea and Mauna Loa. Kilauea isone of the world’s most active volcanoes and most of the earthquakes areassociated with incremental movement of the volcano’s flank towardsthe sea. A few of the island’s earthquakes are less directly related tovolcanism; these earthquakes originate in zones of structural weakness atthe base of the volcanoes or deep within the earth beneath the islands.Since 1952 there have been 34 eruptions, and eruptive activity hasbeen continuous along Kilauea’s east rift zone since January 1983. Veryshallow earthquakes frequently precede or accompany an eruption.Hundreds of such earthquakes make up swarms that commonly occurover a period of several hours or days before an eruption as magma forcesits way into a new area. These earthquakes are seldom large enoughto cause widespread damage, but they may produce extensive groundfracturing close to the potential eruption site. Once an eruption begins,the earthquakes usually diminish.The most recent and significant earthquake in the state occurred off thenorthwest coast of Kiholo, Hawaii in October 2006. Categorized as a 6.7magnitude event, the costs for repair and restoration of critical infrastructurefollowing the Kiholo Bay Earthquake are $100 million. Estimateddamage to infrastructure for the same event is more than $120 million.Engineers, seismologists, architects, and planners have carefully evaluatedseismic hazards related to building construction. They have devised asystem of classifying seismic hazards on the basis of the expected strengthof ground shaking and the probability of the shaking actually occurringwithin a specified time. The results are included in the Uniform BuildingCode (UBC) seismic provisions.The UBC seismic provisions contain six seismic zones, ranging from 0(no chance of severe ground shaking) to 4 (10 percent chance of severeshaking in a 50-year interval). The shaking is quantified in terms ofg-force, the earth’s gravitational acceleration. The diagrams on thefollowing page depict seismic zonation.14