# Volcano Watch - Mauna Loa signals a change as Spring arrives

Release Date:

Each week since October 2002, seismologists at the U.S. Geological Survey's Hawaiʻian Volcano Observatory (HVO) have provided brief synopses of the ongoing rumblings beneath Mauna Loa. Our last detailed discussion of the activity occurred last Thanksgiving, when we proclaimed the continuation of deep, long-period (3-5 cycles per second) Mauna Loa seismic events.

At that time, we had located over 1,300 earthquakes in just over five months.

What has happened since Thanksgiving? Another burst of activity, mostly of the long-period type, occurred between December 14 and Christmas, generally originating at depths between 35 and 50 km (22 and 31 miles) in the upper mantle. It pushed the cumulative number of cataloged earthquakes to over 1,700 since July. By the end of the year, it appeared that we would reach 2,000 earthquakes in short order. The extension rate across the summit caldera had risen to about 6 centimeters (2.4 inches) per year between November and January, with even higher rates of up to 19 centimeters (7.5 inches) per year on northwest-southeast trending lines just crossing the northern edge of the summit caldera.

Just as the thought of 2,000 earthquakes crossed the minds of HVO seismologists, seismic activity beneath Mauna Loa dropped dramatically at the beginning of 2005, to the lowest level seen since the onset of the long-period activity. Only 34 earthquakes were cataloged in the month of January, as compared with 365 in December. From August 2004 to December 2004, 1,650 earthquakes, representing 98 per cent of the total catalogued in the Mauna Loa area, were classified as deep (greater than 13 km {8 miles}). In January, the number of deep events plummeted to 21, comprising 62 per cent of the total.

As seismic activity beneath Mauna Loa changed, Kīlauea began stirring, with a significant increase in horizontal extension across its summit, from 8 centimeters (3.1 inches) per year to over 40 centimeters (15.7 inches) per year. An increase in shallow seismic activity beneath Kīlauea and its upper east rift accompanied this extension, startling residents in the Volcano area, who felt several of the larger events in late January and early February.

During much of January, Puu Oo, the tube system downrift, and associated surface flows showed a dramatic increase in activity, resulting in several spectacular new ocean entries. By the third week of February, extension across Kīlauea caldera ceased, and line lengths have since contracted. Seismic activity beneath Kīlauea summit dropped considerably by the end of February, although the east rift zone remains active.

Meanwhile, extension across Mauna Loa's summit became erratic around the beginning of February. In fact, during most of the month, summit inflation slowed considerably, and horizontal line lengths remained fairly constant. Earthquake activity remained low, with just 25 earthquakes cataloged during the month of February. Of these, 60 per cent fell into the deep-focus category.

Over the past two weeks, Mauna Loa has begun to extend once again, although seismic activity continues at a low level, averaging less than one catalogued event per day. More notably, none of the earthquakes since February 27 have been of the deep, long-period type that dominated the 2004 activity. To date, since last July, just over 1,800 earthquakes have been cataloged in the Mauna Loa region. It could be awhile before we catalog our 2,000th earthquake.

Scientists at the Hawaiʻian Volcano Observatory continue to vigilantly monitor Mauna Loa. A significant increase in earthquake activity northwest of Mokuaweoweo at intermediate depths between 5 and 13 km (3 and 8 miles) has yet to occur, as it did prior to both the 1975 and 1984 eruptions. Monthly totals of all catalogued Mauna Loa earthquakes shallower than 13 km (8 miles) have remained fairly constant since July, numbering anywhere from 6 (in October) to 13 (in September and January). Deformation instruments have not yet shown the large, rapid changes indicating that an eruption is imminent.

As changes occur in Mauna Loa's behavior, we will inform you through these weekly updates, our web pages, and through the Hawaii County Civil Defense, as needed. HVO's current Mauna Loa activity web page.

### Volcano Activity Update

Eruptive activity at Puu Oo continues. The Puu Oo crater camera is offline for repair.

The PKK flow continues to host substantial breakouts from atop Pulama pali to the coastal plain. Open channels have been visible intermittently on Pulama pali, particulary on the east branch of the PKK flow. Three ocean entries are currently active at Highcastle, East Laeapuki and Kailiili. Much of the lava delta at Ka 'iliili collapsed into the sea on March 19. Highcastle, East Laeapuki and Kailiili entries are about 3.7 km (2.3 miles), 4.9 km (3 miles) and 7.5 km (4.7 miles), respectively from the ranger shed on Chain of Craters Road. Expect a 1.5- to 2- hour walk each way to the Highcastle entry. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

During the week ending March 23, only one earthquake was felt on Hawai`i Island. The magnitude-3.2 quake occurred 5 km (3 miles) southwest of Pahala at a depth of 38 km (24 miles) at 10:38 a.m. on Wednesday, March 23. The earthquake was felt at Pahala.

Mauna Loa is not erupting. During the week ending March 23, only two earthquakes were recorded beneath the summit area. Inflation continues at rates of 6 cm/yr (2.4 in/yr) at the summit and near 20 cm/yr (8 in/yr) on the flanks.