# Volcano Watch — The 20th anniversary of the Mauna Loa eruption of 1984

Release Date:

March 25 marks the 20th anniversary of the most recent eruption of Mauna Loa. The eruption was preceded by roughly three years of increasing numbers of shallow- and intermediate-depth earthquakes. A marked increase in earthquake activity occurred nearly six months before the eruption.

Even before the increase in earthquakes, the summit region of Mauna Loa inflated as magma accumulated inside the volcano.

The immediate precursors to the eruption were recorded by the seismic network on March 24, 1984. Small earthquakes began at a rate of 2-3 per minute. By 11:30 p.m., the seismic background signal increased, marking the onset of volcanic tremor. At 1:30 a.m. March 25, residents of the island reported glow above the volcano.

The eruption began in Mokuaweoweo, the summit caldera. The eruptive fissures migrated rapidly across the southern half of Mokuaweoweo and down the southwest rift zone to the 3,886-m (12,750-foot) elevation. By 4:00 a.m., fountains extended across the northeast half of Mokuaweoweo and into the upper reaches of the northeast rift zone, eventually moving down the rift zone and reaching the 2,850-m (9,350-foot) elevation at 4:41 p.m.

Lava flows quickly advanced downslope. At one point, Kulani Prison was put on alert to evacuate its inmates. Eventually, the flows slowed as lava moved away from the steep slopes of the rift zone onto the gentler slopes of the Saddle Road region. Slowing of the flow front caused lava in the channel to stagnate and eventually led to the creation of new flow lobes adjacent to the existing lobes. By diverting lava from a single channel into several smaller flows, the advance of the front was arrested. This process continued from March 29 until April 5.

Concurrently, lava production at the source vents diminished slightly. By April 14, no active flows extended more than 2 km (1.25 miles) from the vents. On April 15, the eruption ended, with the longest flow extending to within 6 km (4 miles) of Kaumana City.

What has the Hawaiian Volcano Observatory been doing about Mauna Loa since 1984? HVO has installed new monitoring equipment, including 8 Global Positioning System receivers, 5 tiltmeters, and 4 seismometers, so that we can better monitor the volcano. In addition, we have deployed 3 dilatometers--sensitive pressure gauges--to monitor magma accumulation.

Technology has enhanced our ability to monitor the volcano in real-time, 24/7. If systems detect a change, alarms go off and scientists remotely log into the computer system, evaluate the situation, and take action if warranted.

From geologic investigations and the historical record, we now recognize that a combination of steep slopes and high eruption rates leads to increased lava flow hazard. For example, during the southwest rift eruption in 1950, flows were in the ocean in 3 to 24 hours because of high eruption rates and steep slopes. During the 1984 eruption, the flows took weeks to advance down gentler slopes only part-way to Hilo.

We have prepared lava inundation maps (http://geopubs.wr.usgs.gov/map-mf/mf2401/) based on detailed geologic mapping. These maps identify zones that could be overrun by lava from specified source regions. The maps were created to assist emergency managers and decision makers during an eruption. Island residents can use the maps to educate themselves regarding which segment of the rift zone or summit can shed flows into their area and to take appropriate action if lava flow inundation becomes imminent.

We calculate that, since 1995, over \$1.9 billion dollars (current replacement cost) of improvements have been made on the flanks of the volcano. Our continuing assessment looks at long-term impacts associated with development on Mauna Loa. The risk from lava flow hazards is real and potentially costly.

Mauna Loa will undoubtedly erupt again. It continues to inflate, but seismic activity has not begun to increase as it did preceding both the 1975 and the 1984 eruptions. The low seismic activity suggests that the next eruption is unlikely to happen imminently.

The primary goal of HVO is to reduce risks due to volcanic activity. We assess volcanic hazards and inform the public about them. To this end, the observatory is vigilant, and the public should know that HVO is monitoring Mauna Loa for everyone's welfare.

Volcano Activity Update

Eruptive activity at the Puu Oo vent of Kīlauea Volcano continued during the past week. Most lava flows have been at the lower and upper ends of the rootless shield complex along the Mother's Day lava tube south of Puu Oo. Such flows have been small and short-lived. On March 20 a new lava flow erupted from the south base of Puu Oo; it remains active. Vents within the crater of Puu Oo are incandescent and sometimes visible from Mountain View and Glenwood. No active flows have reached Pulama pali or the coastal flat below Paliuli, and no lava is entering the ocean.

Four earthquakes were felt in the week ending March 25. The largest, a magnitude-3.4, occurred at 12:13 p.m. March 18 and was felt at Hakalau. The earthquake was located 11 km (7 miles) southwest of Kaena Point at a depth of 39 km (24 miles). The next felt earthquake took place at 11:35 a.m. March 19 and was felt at Leilani Estates. It came from 2 km (1 mile) north of Puulena Crater at a depth of 4 km (2 miles) and had a magnitude of 2.7. Residents of Volcano Village felt a magnitude 2.9 earthquake at 1:01 p.m. March 20. The quake occurred 4 km (3 miles) southeast of Kīlauea's summit at a depth of 3 km (2 miles). The fourth felt earthquake took place at 9:17 March 21 about 5 km (3 miles) north-northwest of Honokaa at a depth of 26 km (16 miles). It was felt by residents of Kukuihaele, Papa`aloa, Waikolo, and Waimea.

Mauna Loa is not erupting. The summit region continues to inflate slowly. Seismic activity remains very low, with 2 earthquakes located in the summit area during the past week.