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June 17, 1999

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


Felt Earthquakes In June

Two magnitude-4.4 earthquakes rocked the Big Island this month. The first occurred on June 3 and was located 13 km (7.8 mi) east of Waimea at a depth of 38.3 km (23 mi). The second was a week later on June 10 and was located 21.3 km (12.8 mi) southeast of Punalu`u at a depth of 52 km (31.2 mi).

Although the epicenters of the earthquakes were 125 km (75 mi) apart, the two earthquakes were related. Both earthquakes belong to a family of deep earthquakes that result from the weight of the islands depressing the lithosphere. Normally, the explanation for these earthquakes is given as "crustal adjustment" because the public is familiar with the Earth's crust. The lithosphere, however, is composed of the crustal layer and about 75 km (45 mi) of the outer mantle. It is the thickness of the Pacific plate on which the Hawaiian Islands ride to the northwest.

As more weight is added to the island by eruptions, the plate usually flexes or bends in response to the increased load. At times, the load exceeds the strength of the plate or is added too quickly for the plate to adjust. This results in brittle failure of the plate and an earthquake. The rate of loading decreases as the volcano becomes less active, and these deep earthquakes become less frequent around the older islands.

However, the plate's adjustment to the increased load is slow and continues after the island has passed the hot spot. The continued adjustments make deep earthquakes like these possible away from the Big Island. In fact, the largest earthquake from this deep source in modern times was a magnitude-6.7 that occurred north of Maui on January 22, 1938. Deep earthquakes have been located off of Lana`i, Moloka`i, Kaho`olawe, and Maui. The most recent destructive earthquake from this family was the magnitude-6.2 Honomu earthquake on April 26, 1973.

The flexure or bending of the lithosphere has another noticeable result. As the plate bends, the islands subside. Evidence for the subsidence of the islands come from many sources. Tide gage records indicate that the Big Island is sinking more than 2 mm (0.1 inch) per year. Dead coral reefs are found more than 1300 m (4400 ft) deep off the Kohala coast. When alive, these reefs were near the surface of the sea. Core recovered from the Hawai`i Scientific Drilling Project hole in Hilo indicates that the drill site at the Hilo airport has subsided 1,090.6 meters (3,578 ft). The sinking of the islands, together with the rising sea level due to melting of glacial ice, can make some of our favorite beach hangouts or fishing holes wet and inaccessible in the course of our lifetime.

Over a longer time span, more dramatic changes occur. The islands of Maui, Kaho`olawe, Lana`i, and Moloka`i are thought to have been one island when they were at the latitude of Hawaii island. With continued subsidence, the island of Maui will become two islands when the lowlands between Haleakala Volcano and West Maui Volcano are flooded by the sea. What was once one large island, Maui Nui, will become five small islands. The same is true for this island, which may become five islands, then three small islands (parts of Mauna Loa, Mauna Kea, and Hualalai).

Eruption Update

A pause in eruptive activity commenced early Monday morning shortly after midnight. The start of the repose was detected by tilt instruments, which recorded a change in tilt at the summit of Kilauea Volcano. Slow inflation of the summit region indicated that magma was accumulating in the summit storage chamber and not being erupted from the Pu`u `O`o vent. Lava stopped flowing into the sea near Kamokuna at about 6:00 p.m. on Monday, as the tubes drained. Eruptive activity has not resumed as this article is being written on Thursday, June 17. These pauses in eruptive activity generally last for only a few days, so it would not be surprising if activity has restarted by the time that this article is printed on Sunday.

One earthquake was reported felt during the week ending on June 17. A magnitude-3.9 earthquake was felt by residents of Hilo and Glenwood at 9:32 a.m. on June 16. The earthquake was located 21 km (12.6 mi) southeast of the summit of Kilauea Volcano at a depth of 6 km (3.6 mi).

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Updated: 23 June 1999