Long-term Monitoring Data | deformation | seismicity | gas | 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. Deformation 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 1974 The 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. This map shows horizontal velocities measured with GPS from 2004 to 2005, the period of fastest motion since inflation began in 2002. The arrows represent the speed and direction of motion at both continuously recording and survey GPS stations on Mauna Loa. The ellipses at the arrow tips provide information about the uncertainty associated with the measurements; the tips of the arrows actually lie somewhere within the ellipses. The radial velocity pattern results from inflation of a complex magma reservoir beneath Mauna Loa's summit area. Interferometric Synthetic Aperture Radar (InSAR) InSAR is a remote-sensing technique in which radar images of the Earth's surface acquired by orbiting satellites are combined to show subtle movements of the ground surface that occurred between image acquisition times. The result is a very detailed map of the ground motion in the direction toward (or away from) the satellite. InSAR maps can be modeled to reveal the subsurface structures causing the ground motion. The main advantage of InSAR over other techniques is the incredible spatial resolution it provides. However, it is limited by the repeat interval of satellite passes over the area of interest; for Mauna Loa, satellites repeat their measurements about once a month. Mauna Loa is a nearly ideal setting for this technique, as most of the rapidly deforming areas are above the tree line (InSAR measurements are hampered by heavy vegetation). However, snow and occasional atmospheric changes can interfere with the ability to map the ground motion. InSAR image of ground surface motion near the summit of Mauna Loa during 2004-2005, the period of fastest deformation. Each cycle of colored fringes represents about 3 cm of motion toward the satellite. The butterfly-shaped pattern of fringes centered on Moku`aweoweo indicates inflation of a complex magma reservoir beneath Mauna Loa. Seismicity HVO's seismic network recorded significant changes in seismicity before the Mauna Loa eruptions in 1975 and 1984. Our short-term forecasts of these eruptions were based in large part on such precursory seismicity. In April 1974, following more than two decades of quiet at Mauna Loa, HVO seismologists recognized and reported increasing numbers of earthquakes beneath the volcano. In August 1974, a swarm of earthquakes occurred northwest of Moku`aweoweo and centered at intermediate depths of 5 to 8 km. In December 1974, a shallower earthquake swarm (depths less than 5 km) occurred beneath Mauna Loa's summit. In February 1975, after a brief lull in seismicity, the August 1974 source region became active again, and the numbers of earthquakes steadily rose until the eruption began on July 5, 1975. A period of quiescence followed the July 1975 eruption. Regional seismic activity gradually resumed, and in 1978, rates of seismicity at both shallow and intermediate depths increased. A swarm of intermediate-depth earthquakes that occurred in the same region as the August 1974 earthquake swarm was possibly the strongest indicator of volcanic unrest. Shallow earthquake activity dramatically increased beneath the summit caldera in March 1984, three weeks before the eruption started on March 25. After the 1984 eruption, HVO located approximately 30 earthquakes per year beneath the summit and upper flanks of Mauna Loa until late April 2002, when a swarm of small, deep earthquakes occurred beneath the volcano's summit. Following this swarm, seismicity returned to low levels until July 2004, when the numbers of earthquakes increased markedly. From July through December 2004, about 1,700 locatable earthquakes occurred beneath Mauna Loa. These long-period earthquakes were very deep-greater than 40 km below the surface-and created a type of seismic swarm that had never before been recorded beneath Mauna Loa. This swarm coincided with the increased rate of magma influx into the shallower (about 4 km deep) reservoir beneath the summit area. The increased inflation rate continued for another year after the deep seismic swarm ceased at the end of 2004. Since 2005, seismic activity has been near background levels, with an average of two earthquakes per month. Before Mauna Loa's next eruption becomes imminent, we expect that rates of shallow seismicity will elevate to levels much higher than those currently observed. Cumulative numbers of located earthquakes beneath Mauna Loa, 1970-2008 Cumulative numbers of located earthquakes beneath Mauna Loa, 1970-2008. Top graph shows all earthquakes that occurred beneath Mauna Loa (same area as shown in maps below) from 1970 through 2008. The lower graphs, from top to bottom, show earthquakes that occurred in three depth ranges: shallow (0 to 5 km deep), intermediate (5 to 15 km deep), and deep (more than 15 km deep). Note the increase in shallow and intermediate depth seismicity preceding the 1975 and 1984 eruptions. The current network of seismometers provide greater sensitivity than was possible in the past, so in order to directly compare the rates of seismicity, these graphs plot only earthquakes with magnitudes greater than 1.8. Locations of earthquakes with magnitudes greater than 1.8. During the two years preceding the 1975 (left) and 1984 (right) Mauna Loa eruptions, shallow seismicity beneath Moku`aweoweo, the summit caldera, greatly increased (shown in green). A cluster of intermediate-depth earthquakes northwest of the caldera also occurred prior to both eruptions (shown in red). During the past two years, shallow and intermediate-depth seismicity indicative of an impending eruption has not occurred beneath Mauna Loa. This plot is updated about once a month. For more information on current seismicity, go to HVO's map of recent earthquakes in Hawai`i. Gas Monitoring Monitoring volcanic gases can provide clues about the internal workings of an active volcano. So, in 2005, HVO installed two gas monitors atop Mauna Loa: a fixed-view ultraviolet spectrometer system and a real-time ambient gas monitor. The fixed-view spectrometer "looks" south along the 1984 fissure within Moku`aweoweo, Mauna Loa's summit caldera. Continuous, real-time data from this instrument is telemetered to HVO. The small amount of SO2 currently released from Mauna Loa's summit area is below the detection limit of the instrument. A real-time ambient gas monitoring station (see photo below), also in Moku`aweoweo, measures fumarole (volcanic gas vent) and ambient air temperatures, as well as sulfur dioxide (SO2) and carbon dioxide (CO2) concentrations adjacent to the fumarole. Because changes in gas emissions can signal a change in eruptive status, gathering SO2 and CO2 data while Mauna Loa is quiet is important to establish normal background levels for gases emitted from the volcano. A real-time ambient gas monitoring station. The URL of this page is http://hvo.wr.usgs.gov/maunaloa/current/longterm.html Contact: hvowebmaster@usgs.gov Updated: 17 March 2009 (pnf)