Detailed monitoring data is available only for Mauna Loa's last two eruptions, which occurred in 1975 and 1984. Earlier eruptions of the volcano preceded the invention and deployment of modern volcano- monitoring instruments.
The 1975 eruption was preceded by more than 12 months of irregular, but increased, seismic unrest and notable inflation of the summit magma reservoir. Late in 1974, HVO alerted Hawai`i residents to the possibility of a Mauna Loa eruption through extensive media reports. The volcano erupted in July 1975.
Prior to the 1984 eruption, the rate of intermediate-depth seismicity began to increase as early as 1980. This unrest led HVO scientists to forecast in 1983 that Mauna Loa was likely to erupt within the next two years. The eruption began in March 1984.
Since 1984, HVO's capability to detect unrest on Mauna Loa has increased markedly. Monitoring instruments on the volcano now include digital seismic stations, Global Positioning System (GPS) receivers, electronic tiltmeters, an ultraviolet spectrometer, fumarole temperature sensor, SO2 and CO2 gas sensors, and webcams. These remotely located instruments transmit real-time data via radio signals to HVO 24 hours a day, seven days a week.
The plot below shows changes in distance across Mokuʻāweoweo, Mauna Loa's summit caldera, since 1974, as measured by continuously recording GPS stations. Increases in distance often indicate increased pressure in the magma reservoir beneath the summit area (inflation). For more information about the inflation-deflation cycles of magma chambers, see How Hawaiian Volcanoes Work.
Distance changes and earthquake counts in the summit area
Distance changes across the summit caldera of Mauna Loa, as measured between MOKP and MSLP GPS stations (see map inset). Vertical thin red lines indicate eruptions in 1975 and 1984. Monthly earthquake rates are shown in black. Plots will be updated as necessary.
The large extensions associated with the 1975 and 1984 eruptions were caused by magma rising from the summit reservoir to the volcano's surface and accompanied by increased rates of seismicity. During the 1984 eruption, the summit area contracted and subsided rapidly as magma left the reservoir to feed the eruption along the northeast rift zone. When the eruption stopped, the summit magma reservoir immediately began to re-inflate. The inflation ceased in 1993; distances across the caldera decreased and the ground surface subsided from then until 2002.
In May 2002, years of slow contraction and subsidence abruptly changed to extension and uplift. GPS measurements and remote imaging revealed patterns of ground-surface motion that indicated renewed influx of magma into a complex shallow magma system below. The extension and uplift rates increased dramatically in July 2004, as a swarm of very deep earthquakes started. The high rates of inflation slowed dramatically in 2005 and continued to slow until completely ceasing by mid-2013. Inflation resumed in mid-2014. For information on the current status of Mauna Loa click here.
Spatial pattern of deformation