Volcano Watch — Earthquake reflects instability of Kīlauea's south flank

The earthquake that woke many Big Island residents at 5:47 a.m. on Monday, June 30, was the biggest on the island since February 1, 1994, and the largest on Kīlauea's south flank since 1989. Its magnitude (M) was about 5.3 (the exact value is still being refined), big enough to cause extensive non-structural damage in the South Hilo and Puna districts. The quake's location, about 10 km (6 miles) west of Kaimu, is near that of the M 7.2 earthquake of 1975, the M 6.5 and 6.0 quakes of 1954, and the M 6.1 shock of 1989. Its depth, about 10 km (6 miles), is typical of other large earthquakes beneath the south flank of Kīlauea but is shallower than for many quakes on the Big Island. The relatively shallow depth helps account for the notable damage in Hilo.

Taking place only 13 minutes before the transfer of Hong Kong to China, the earthquake could be taken as a Big Island contribution to the festivities. Be that as it may, what caused the earthquake? Earthquakes occur when the earth's crust is stressed to the breaking point. The stress can be relieved, and stability momentarily achieved, only by breaking rocks in the crust. Earthquakes are the signals of breaking rocks.

Several factors contribute to the buildup of stress and the overall instability of the south flank of Kīlauea. Gravity acts on the volcano, forcing it to move away from the rest of the island. The weight of the island on the old sea floor causes internal movements and readjustments. Intrusion of magma into the rift zones pushes the flank outward. Hydrothermal alteration of rocks within and south of the rift zones weakens the edifice and makes it prone to failure. Slow southward creep of hot, dense intrusive rock bulldozes the south flank outward.

All of these factors, and perhaps others, combine to make the south flank of Kīlauea one of the world's least stable and most mobile areas. Since early in this century, geodetic surveys show that parts of the coastline have moved seaward more than 10 m (30 ft). Contemporary measurements indicate rates for the past several years of as much as 10 cm (4 inches) per year. Currently the south flank is also going up, although far more slowly than it is moving outward.

In an ideal situation, with no resistance to movement and no friction, all of this motion could occur without earthquakes. Such a situation is found only in textbooks, however. In the real world, there is much resistance, and hence much seismicity, as rocks have to break in order to move. Kīlauea's south flank is very active seismically because it is moving a lot.

Monday's earthquake reflects breakage of part of the flank to accommodate the inexorable movement caused by the factors listed above. Geodetic surveys currently underway should provide tangible evidence of this movement. In the past, larger earthquakes have accompanied huge ground movements. The largest in this century was on November 29, 1975, when the coastline moved out as much as 6 m (20 ft) and down as much as 3.5 m (11 ft). It was this movement that disturbed the neighboring sea floor and caused the tsunami that killed two campers at Halape.

Last Monday's shake is a reminder of what is yet to come. Consider now if your glassware is safely stored, if that framed picture hanging over your pillow really needs to be there, and if your water heater and propane tanks are securely strapped to a solid surface. When the next M 7.2 (or larger) earthquake comes, shaking will be much stronger and the damage (including structural damage to buildings) far more serious than during the tickler of June 30.

Volcano Activity Update

Kīlauea's east rift zone eruptive activity continued during the past week with lava production from three vents. The spatter cone within Pu`u `O`o Crater produced flows which resurfaced and raised the crater floor. Intermittent fountains from the west flank vent have sent flows to the west and south for short distances. Other small channeled lava flows originate from a perched lava pond on the south side of Pu`u `O`o and extend for less than 1.5 km to the south.

A magnitude 3.5 aftershock of the M 5.3 earthquake was felt by Hilo and Puna residents at 6:03 a.m. on Monday morning.