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June 18, 1998

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

Why do we have so many earthquakes?

This week marks the ninth anniversary since the last large (greater than 6 magnitude) earthquake on the island of Hawaii. At 5:27 p.m. on Saturday, June 25, 1989, a magnitude 6.1 earthquake shook the Hawaiian Islands. The epicenter of the earthquake was 11 km (6.6 mi) west of Kalapana at a depth of 9 km (5.4 mi).

The earthquake, which caused damage totaling over a million dollars on the island of Hawaii, was felt as far away as O`ahu. Five homes were completely destroyed in the Puna District, and 10 others sustained major structural damage. Minor damage, including cracked walls, ceilings, and concrete pads, together with shifted foundations, was reported for a hundred houses.

A small tsunami was generated by the earthquake. Maximum wave heights of 0.57 m (1.9 ft), 0.21 m (0.7 ft), and 0.14 m (0.46 ft) were recorded at Honuapo, Kapoho, and Hilo, respectively. Ground cracks were observed in Kalapana, and minor landslides occurred in Puna and along the Hamakua coast.

The epicenter of the earthquake was in the south flank region of Kilauea Volcano. This block of land is the most active seismic area in the United States. In the past 45 years, four large earthquakes occurred in this area. The largest was the magnitude-7.2 earthquake of November 29, 1975.

Why is the south flank of Kilauea so active seismically? Two forces, magmatic pressure and gravity, combine to stress this block, and a layer of ocean sediments at the base of the block allows easy slippage.

The sources of magmatic pressure are the rift zones of Kilauea Volcano which outline the northern boundary of the south flank. These rift zones are comprised of numerous dikes or sheet-like intrusions that may be several meters thick and several kilometers long. Whenever a new dike is wedged into the rift zone, the volcano adjusts to accommodate the added mass. Because Mauna Loa acts as a buttress to the north flank of Kilauea, only the unbuttressed seaward (south) flank of the volcano can move to make room for any new material.

The rift zones are also the sites of eruptions which continually build higher, linear edifices. As the height of the rift zone increases, gravitational forces act to lower it, and these forces are translated to the sides of the ridge. Again, the stresses are relieved by the mobile unbuttressed flank.

If the south flank block did not have a weak basal layer on which to slip, intrusions into the rift zones would stop as would eruptions, and there would be no large earthquakes. However, the south flank block sits on an old ocean sediment layer. Faulting within this layer allows the overriding block to slip seaward to relieve the magmatic and gravitational stresses acting on it.

Global Positioning System (GPS) measurements by the U.S. Geological Survey's Hawaiian Volcano Observatory show that the south flank is presently moving seaward about 0.1 m (4 in) annually. Geodetic measurements spanning the November 1975 earthquake indicated that some coastal areas moved nearly 10 meters (33 ft) seaward.

As long as magma continues to enter Kilauea Volcano, the south flank will continue to move seaward, and we will continue to have large earthquakes from that region.

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano from the Pu`u `O`o vent continued unabated during the past week. The lava flows through a network of tubes to the seacoast and enters the ocean at two locations-Waha`ula and Kamokuna. The public is again reminded that these two areas are extremely dangerous. The National Park Service has restricted access to them because of frequent explosions that accompany collapses of the growing lava bench.

There were no earthquakes reported felt in the past week.

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Updated: 25 June 1998