Summary of monitoring data (1970-May 2005)
| Summit deformation | Earthquake activity | Instrument map |
The 1975 and 1984 eruptions of Mauna Loa are the only events for which we have detailed seismic and deformation data. Earlier eruptions of the volcano preceded the invention and deployment of modern volcano-monitoring instruments, including telemetered seismometers, electronic tiltmeters, electronic distance meters, GPS receivers, and gas-measuring devices.
The July 1975 eruption (photo) was preceded by more than 12 months of uneven but increased seismic unrest and notable inflation of the summit magma reservoir. Civil authorities were briefed on the activity in the fall of 1974, and Hawai`i residents were alerted to the possibility of an eruption through extensive news media reports.
In 1980, the rate of intermediate-depth seismicity began to increase. This unrest led to a forecast in 1983 for an eruption of Mauna Loa within the next two years. The eruption began in March 1984.
Changes start in 2002
The current unrest at Mauna Loa began with an abrupt increase in rates of ground motion in May 2002. The direction of ground motion also changed, from a fairly uniform, slow southeastward movement to a predominantly radial pattern. Modeling of the data suggests that the ground motion is due to influx of magma into a complex reservoir system about 4 to 5 km below the summit caldera. This influx of magma continues as of May 2005, though at variable rates.
Seismic activity has also been variable. In early July 2004, a sustained flurry of deep, long-period seismicity began, averaging one located earthquake per day for the first three weeks, and increasing to over 100 locatable events per week. This swarm of earthquakes stopped at the end of 2004. As of May 2005, seismicity beneath the summit area of Mauna Loa is slightly higher than background levels, but not nearly as intense as in the several years before the two most recent eruptions.
The July to December 2004 earthquake swarm was associated with increased inflation of the shallow magma system, and this increased rate of magma influx continued after the swarm stopped. However, the rate of inflation has not been steady. There are numerous periods of weeks to months in which the inflation rate has slowed or even stopped, only to resume again.
The capability to detect unrest at Mauna Loa has increased
markedly in the past decade with the installation of continuously recording electronic
tiltmeters, GPS receivers, and strainmeters (see map of station locations). In response to the recent
unrest on Mauna Loa, HVO, in collaboration with Stanford University and the University of Hawai`i, has
installed numerous additional GPS receivers and tiltmeters. In addition, the
observatory conducts regular surveys both with GPS and traditional monitoring methods, such as EDM
(electronic distance measurement), leveling, and dry tilt.
Summit deformation since 1974
Measured distance across Moku`aweoweo, 1974-Present
This graph shows distance across Moku`aweoweo, Mauna Loa's caldera, between MOKP and MSLP benchmarks (see map inset) as measured by electronic distance meter (blue) and GPS receivers (red). Continuously operating GPS receivers were installed in 1999. Red lines indicate eruptions in 1975 and 1984. Note the abrupt change from contraction to extension in May 2002.
Changes in distance across the summit area usually correspond to changes in the pressure of the magma reservoir. Distances between points on opposite sides of the magma reservoir increase with inflation (increase in magma reservoir pressure) and decrease with deflation (decrease in magma reservoir pressure) (see a summary of the inflation-deflation of summit magma chambers).
On the plot of distance changes above, the huge extensions associated with the 1975 and 1984 eruptions were caused by the rise of magma from the summit reservoir to the volcano's surface. During the 1984 eruption, the summit area 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 with magma. 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.
In May 2002, the slow contraction across the summit caldera changed abruptly to extension. The inital extension rate was very high, about 5 cm/yr, but slowed to an average rate of about 1.5 cm/yr in the fall of 2002, then increased again in July 2004. Currently, the average extension rate across the caldera is about 5 cm/yr, but longer baselines are extending even faster. For example, the distance from stations on the northwest flank to stations on the southeast flank are increasing by over 15 cm/yr! Continuous GPS receivers also measure vertical movements; from May 2002 to May 2005, the summit rose at least 11 cm.
|Horizontal velocities measured by GPS from April 2002 through May 2005. The arrows represent the speed and direction of motion at both continuously recording and survey stations on Mauna Loa. The ellipses at the arrow tips provide information about the uncertainty associated with the measurements; the tips of the arrows actaully lie somewhere within the ellipses. The radial velocity pattern results from inflation of a magma reservoir beneath Mauna Loa's summit area.|
HVO's telemetered seismographic 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, centered at intermediate depths between 5 and 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 seismicity gradually resumed, and rates of seismicity at both shallow and intermediate depths increased from 1978. Possibly the strongest indicator of volcanic unrest was a swarm of intermediate-depth earthquakes that occurred in the same region as the August 1974 swarm. Shallow earthquake activity dramatically increased beneath the summit caldera in March 1984, three weeks before the eruption started on March 25.
From the 1984 eruption until late April 2002, we located approximately 30 earthquakes per year beneath the summit and upper flanks of Mauna Loa.
In late April 2002, a swarm of small earthquakes occured at great depth beneath the summit. After this swarm, seismicity returned to low levels until July 2004, when the numbers of earthquakes showed a marked increase. From July through December 2004, about 1,700 locatable earthquakes occurred beneath Mauna Loa. These were of the long-period type and very deep - greater than 40 km below the surface. This type of swarm had never been recorded beneath Mauna Loa Volcano before. Since the end of 2004, seismic activity has been slightly above background levels. Before an 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, separated into three depth ranges, within or beneath Mauna Loa (within the area of the maps below) between 1970 and May 25 2005. The four graphs are, from top to bottom, (1) all earthquakes, (2) shallow earthquakes (0 to 5 km deep), (3) intermediate earthquakes (5 to 15 km deep), and (4) deep earthquakes (more than 15 km deep). Note the difference in scale on the various graphs. These graphs plot only earthquakes with magnitudes greater than 1.8 to minimize the effects of increased sensitivty in more recent years.
Locations of earthquakes with magnitude greater than 1.8. The top two maps plot earthquakes during the two years preceding each of the two most recent eruptions of Mauna Loa. The bottom map plots two years of earthquakes through October 2004. A cluster of intermediate-depth (5-19 km deep, red color) earthquakes northwest of Mauna Loa's caldera has occurred prior to the 1975 and 1984 eruptions. As of May 2005, there has been little or no activity in this area. The seismicity beneath the summit area over the last two years has been very deep, mostly below 40-km depth.
Map showing locations of continuously recording instruments for measuring deformation and seismicity on Mauna Loa. Many more benchmarks are used in GPS, EDM and leveling surveys than are shown here; they are reoccupied yearly or whenever necessary.
Updated: 22 June 2005 (pnf)