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 (see map) now include seismic stations, Global Positioning System (GPS) receivers, electronic tiltmeters, an ultraviolet spectrometer, fumarole temperature sensor, SO2 and CO2 gas sensors, and a Web camera. These remotely located instruments transmit real-time data via radio signals to HVO 24 hours a day, seven days a week.
Mauna Loa Monitoring Network
Continuously recording instruments monitor deformation and seismicity on Mauna Loa. In addition to these sites, many other benchmarks are used in GPS surveys; they are reoccupied yearly or whenever necessary.
Overview of changes on Mauna Loa since 2002
In May 2002, rates of ground motion on Mauna Loa abruptly increased. The direction of ground motion also changed—from a fairly uniform, slow southeastward movement to a predominantly radial pattern. Modeling of the data suggested that the ground motion was due to the influx of magma into a complex reservoir system 4 to 5 km below the summit caldera. The magma influx continues today, but the rate of inflation is not steady. There are periods of weeks to months in which inflation slows, or even stops, only to resume again.
Mauna Loa's seismic activity has also varied. In July 2004, a sustained flurry of deep, long-period earthquakes began. This seismicity was associated with the increased inflation of the shallow magma system, and consisted, on average, of one located earthquake per day for the first three weeks, and then increased to over 100 locatable events per week. The earthquake swarm ceased at the end of 2004. Since 2005, seismicity beneath Mauna Loa's summit area has been near background levels, with an average of two earthquakes per month.
During the past decade, HVO has greatly improved its capability to monitor ground deformation on Mauna Loa. In collaboration with Stanford University and the University of Hawai`i, the observatory has installed numerous continuously recording instruments, including Global Positioning System (GPS) receivers, electronic tiltmeters, and strainmeters on Mauna Loa.
In addition to these continuously recording instruments, HVO obtains deformation data on Mauna Loa through regular GPS surveys and occasional leveling and tilt surveys. HVO also uses new remote-sensing techniques, such as Interferometric Synthetic Aperture Radar (InSAR), to map ground deformation.
Summit deformation since 1974The plot below shows changes in distance across Moku`aweoweo, Mauna Loa's summit caldera, since 1974, as measured between two benchmarks, MOKP and MLSP. The distance changes between these two monitoring stations usually correspond to changes in reservoir pressure. Distance increases with inflation (magma reservoir pressure rises) and decreases with deflation (magma reservoir pressure declines). For more information about the inflation-deflation cycles of summit magma chambers, see How Hawaiian Volcanoes Work.
Distance measurements across Moku`aweoweo, 1974-2008
Distance changes between MOKP and MSLP benchmarks (see map inset) at the summit of Mauna Loa. Electronic distance measurements (EDM) are shown in blue. Measurements by GPS are shown in purple. Red lines indicate eruptions in 1975 and 1984.
Huge extensions associated with the 1975 and 1984 eruptions were caused by magma rising from the summit reservoir to the volcano's surface. During the 1984 eruption, after the summit area inflated, it contracted and subsided rapidly as lava erupted along the northeast rift zone. When the eruption stopped, the summit magma reservoir immediately began to re-inflate. The inflation stopped in 1993. From 1993 to 2002, distances across the caldera shortened by as much as 7 cm, and leveling surveys in 1996 and 2000 measured more than 7 cm of subsidence southeast of Moku`aweoweo.
Distance measurements across Moku`aweoweo, 2000-2009
Distance between MOKP and MSLP benchmarks (see map inset) measured with continuously recording GPS receivers since 2000. Note the abrupt change from contraction to extension in May 2002.
In May 2002, the slow contraction and subsidence abruptly changed to extension and uplift. GPS measurements and remote imaging revealed radial patterns of motion as the ground was pushed away from a complex reservoir system beneath the summit area. The initial inflation rate was high, but slowed in late 2002. The period of fastest inflation since 2002 occurred from July 2004 through 2005. Since 2006, inflation has continued at a fairly steady, moderate rate.