The summit of Kilauea began inflating in the fall of 2003. This was a major change, because, for the most part, it had been doing the opposite-gradually deflating-since the start of the current eruption, way back in January 1983. During those two decades, the summit was operating on a deficit, with more magma going out-moving down the east rift zone to erupt at Pu`u `O`o-than coming in.
When a volcano starts to inflate, we generally interpret it as a sign that it is more likely to erupt. Of course, Kilauea had already been erupting along its east rift zone for 20 years by the time the summit inflation started. The summit continued to inflate at variable rates for the last two years.
Then, in January 2006, the rate of inflation increased dramatically, and, through mid-March, the summit area was rattled by several hundred small earthquakes (magnitude 1 to 3.5). This period of unrest began with a rather common “tilt event” at Kilauea’s summit on January 10, 2006. Over the span of a few hours, the summit tiltmeter recorded rapid deflation, immediately followed by rapid inflation. Such events are related to brief changes in magma-pressure beneath the summit, and often the tiltmeter at Pu`u `O`o shows the same pattern, with a slight time delay.
This time, however, Pu`u `O`o seemed unaffected. Instead, this tilt event seemed to trigger an acceleration of the inflation that had started in 2003.
Less than a week later, on January 16, an unusual number of earthquakes started to occur, both in the upper east rift zone and in the area south of Halema`uma`u Crater. By January 23, earthquakes were also occurring near Namakani Paio campground. Most of these earthquakes were less than 5 km (2 miles) deep.
On January 26, the rate of inflation increased yet again. A continuously recording GPS station in the southern caldera recorded abrupt upward motion, and, by March 11, the site had risen 10 cm (4 inches).
Earthquakes continued to shake the upper east rift zone and south caldera, with two brief excursions back to the Namakani Paio area on February 23 and March 1-3. The earthquakes quieted dramatically after another summit tilt event on March 11. Since then, summit inflation has continued at a slower rate.
The two largest earthquakes in the sequence had magnitudes of 4.6 (on February 16 south of the summit) and 4.0 (on March 1 near Namakani Paio campground). Thirty-five of these earthquakes were reported felt, and we at HVO certainly felt many of them at our office on the rim of Kilauea’s caldera.
All this occurred without any significant change in the ongoing eruption at Pu`u `O`o. Lava tubes continue to deliver lava to the ocean at East Lae`apuki, with surface flows breaking out along the way.
What exactly happened? Elevation measurements over last two years show that the ground surface rose in the summit area, with the maximum uplift occurring near Keanakako`i Crater. The changes were measured both by traditional surveying and by GPS. Ground tilt changes also showed inflation in approximately the same area.
Precisely measured changes in gravity through the end of 2005 showed that gravity was increasing at a rate greater than expected in the south caldera. These changes suggest that mass is being added, possibly as magma fills voids left open by the previous years of deflation.
The elevation and gravity changes both suggest that magma is intruding beneath the summit area of Kilauea. The continuation of the current eruption at Pu`u `O`o further suggests that the supply of magma to Kilauea must have increased in order to supply both the eruption and the summit intrusion.
This is consistent with another piece of information: both sulfur dioxide and carbon dioxide gas emission rates in the summit area increased over the past year. More magma at shallow levels means more gas is released.
All’s quiet at Kilauea’s summit for now, but as its recent behavior has demonstrated, we should never take the status quo for granted on an active volcano.
Eruptive activity at Pu`u `O`o continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Pu`u `O`o to the ocean, with occasional surface flows breaking out of the tube. In the past week, surface flows were active on the coastal plain in about the same location as the last several weeks: 0.4 km (0.3 mi) inland of the coast at Kamoamoa, about 5.5 km (3.4 mi) from the end of Chain of Craters Road.
As of April 6, lava is entering the ocean at East Lae`apuki, in Hawai`i Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 1,100 m (3,600 ft) long by 250 m (820 ft) wide. Access to the ocean entry and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.
No earthquakes were reported felt on Hawai`i Island in the past week.
Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano’s summit, with four deep earthquakes (>40 km or 25 mi) located. Inflation continues, but at a rate that has slowed since early October 2005.
This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by HawaiiNews.com with permission.