April 18, 1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Earthquake Risk Can Be Reduced

Regular readers of this column should not have been surprised last week by the news article stating that the earthquake risk on the Big Island is as high as that of California. Although we are located in the middle of the Pacific plate and not at the contact between two plates, we experience large earthquakes resulting from stresses produced here by our dynamic volcanoes and by the gravitational load of the island on the Earth's crust.

The earthquake hazard on the island will always exist, but through a continuing program of public awareness and preparedness, we can mitigate the effects of a large temblor. Staff members of the U.S. Geological Survey's Hawaiian Volcano Observatory (HVO) regularly speak to school, community, and civic groups about the seismicity of the area. We have devoted many of these columns outlining the primary dangers of a great earthquake - structural failure and tsunami.

The only practical way to prevent injuries or death from a locally generated tsunami is to immediately rush to higher ground when a large earthquake is felt while at the seashore. If the populace can develop this as an automatic response, the danger will be diminished.

Campers at Halape were inundated by the ocean within 30 seconds after the ground stopped shaking from the 7.2 magnitude earthquake in 1975. The tsunami consisted of five waves, and the largest had a height of 14.8 m (48 ft). From their origin near Kalapana, waves reached Hilo in 20 minutes and Kailua-Kona in 27 minutes.

The failure of structures by severe shaking can be decreased by building new structures and retrofitting older structures to withstand the strongest motion to be expected in a large earthquake. Accurately predetermining the maximum acceleration is not possible, and authorities can only depend upon the past seismic record.

A major objective of the Hawaii State Earthquake Advisory Board (HSEAB) is to evaluate seismic zonation and building codes throughout the state and to make recommendations to the Hawaii State Civil Defense (HSCD). HSEAB is composed of seismologists, geologists, engineers, architects, planners, and emergency managers - including HVO staff members. With seismic data from the U.S. Geological Survey through HVO, HSEAB advised HSCD to apply to the International Conference of Building Officials (ICBO) for a change in the zonation of Hawaii County from Seismic Zone 3 to 4. This is the highest zone.

ICBO is the organization that administers and publishes revised editions of the Uniform Building Code every three years. The application by HSCD was adopted by ICBO, and the 1997 edition of their Uniform Building Code will reflect Hawaii County in Seismic Zone 4. The County of Hawaii will officially implement Seismic Zone 4 construction requirements after the 1997 edition of the Uniform Building Code is passed and signed as a County ordinance.

Heading for high ground if at the seashore or being in a well-constructed building may save your life during the next big earthquake.

Eruption Update: April 18

Eruptive activity within Pu`u `O`o crater continues with the lava pond about 46 meters (150 ft.) below the lowest section of the rim. The two large surface flows reported last week were stagnant, but a new flow emanating from the 2,500-ft elevation has traveled 2.5 km (1.5 mi) to the southeast. The distal end of this flow is about 1.5 km (1 mi) from the top of Pulama Pali. Overflows from the collapse pits at the base of Pu`u `O`o mantle the western and southwestern flank of the episode 51 shield. Occasional fountaining up to 12 m (40 ft) can be seen in the 51 vent area.

Recent Earthquakes

A magnitude 3.8 earthquake located 0.9 km (0.54 mi.) beneath Waldron Ledge was felt by residents and visitors of Hawaii Volcanoes National Park and the Volcano community at 2:55 p.m. on Wednesday, April 16. The earthquake triggered a small slide from the caldera wall beneath Waldron Ledge.