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July 16, 1998

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


Are We Breaking Away - The Great Crack

In a recent national television program on tsunami, attention was focused on the Great Crack in the southwest rift zone of Kilauea. The size of the crack was presented as evidence that the south flank was breaking away from the island. The gaping fissure is impressive for its continuous length of 13 km (8 mi), width up to 15 m (50 ft), and depth of 20 m (66 ft). This feature, however, is the result of crustal dilation from magmatic intrusions into the rift zone and not from the seaward movement of the south flank. There is no evidence that the Great Crack is getting bigger at this time or that the island is tearing apart along this seam.

The Great Crack is one of a series of cracks, eruptive fissures, and cones that outline the southwest rift zone of Kilauea Volcano. It is located toward the far end of the rift zone but does not extend to the seacoast. Exactly when the crack formed is unknown, and although it is continuous for a great length, all of it may not have been formed at the same time. There is some evidence that parts of it may have been formed above a large lava tube.

Where the crack is narrow enough that opposing walls can be compared, matching features fit together like pieces of a jigsaw puzzle. This suggests that a simple widening caused the crack. Opposite walls also have no vertical offset, so south flank subsidence did not influence the formation of the crack.

In 1823 lava welled out of the lower 10 km (6 mi) of the Great Crack. The ensuing Keaiwa eruption is noteworthy for its very fluid lava that reportedly flowed rapidly down to the sea. Some scientists believe that the fluidity of the lava resulted in a veneer left high on older structures - the "Lava Plastered Cones" - as the lava swept its way to the coast.

The present intact nature of the delicate 1823 flow indicates that there was no growth of the Great Crack during the largest historic earthquake (magnitude 7.9) in 1868. Measurements spanning the magnitude-7.2 south flank earthquake in 1975 show no change across the area. Thus, the two largest earthquakes in historic time have not affected the Great Crack.

Similar, though shorter, wide cracks occur on the east rift zone. One, between Napau Crater and Pu`u `O`o, has existed for many decades, as did another east of Pu`u `O`o, now filled in with new lava flows. Thus, the Great Crack is not unique; it is simply longer than the others.

The east and southwest rift zones together with the Koa`e fault system form the north and west boundaries of the south flank block of Kilauea. Bathymetric studies around the Hawaiian Islands identify numerous giant submarine landslides from the islands. The south flank block is identified as a possible future landslide that could generate a large tsunami, but these phenomena occur so infrequently (perhaps once every few tens of thousands of years) that we should not lose any sleep over it ever happening. We will have ample warning before it goes.

What we should concern ourselves with are the large (greater than magnitude 6) earthquakes that frequently originate in the south flank. We must build structures that can withstand the shaking caused by these earthquakes and ensure that furnishings are also well secured. We must become aware that a large earthquake can generate a tsunami, as in 1868 and in 1975, and if we are near the seacoast when the ground shakes, we should move inland to higher ground as quickly as possible.

The total breakaway of the south flank block of Kilauea mentioned in the television program is not taking place at this time. The U.S. Geological Survey's Hawaiian Volcano Observatory is constantly monitoring this area and will immediately inform the Hawaii County Civil Defense agency if anything unusual occurs.

Eruption and Earthquake Update

There were no significant changes in the eruptive activity of Kilauea Volcano at the Pu`u `O`o vent. Lava continues to flow through a network of tubes from the vent to the coast and enters the ocean only at Kamokuna. The public is again reminded that this area is extremely dangerous because of frequent explosions that accompany collapses of the growing lava bench. Lava stopped entering the ocean at Waha`ula during the past week.

One earthquake was reported felt during the past week. Residents of Hilo, Puna and Volcano were awakened by a magnitude 4.1 earthquake at 4:50 in the morning on Tuesday, July 14. The earthquake originated 5 km (3 mi) southeast of Makaopuhi Crater at a depth of 7.5 km (4.5 mi).

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Updated: 31 July 1998