Summary of the current activity of Mauna Loa Volcano, Hawai`i

Current Activity

0540 October 15
Friday morning sees no changes at Mauna Loa. Since early July 2004, an increased number of earthquakes has been recorded from beneath Mauna Loa. From week to week, the numbers fluctuate but remain well above the norm. During the week ending October 13, 110 earthquakes were located under the summit, up from 47 for the week ending October 6. Through October 13, more than 730 earthquakes related to the ongoing seismic activity have been centered beneath Mauna Loa’s summit caldera and the adjacent part of the southwest rift zone. Most of these earthquakes are quite deep, from 35 to 50 km below the ground surface and small, less than magnitude 3. They are "long-period" (LP) earthquakes, which means that their signals gradually rise out of the  background rather than appearing abruptly. Such a concentrated number of deep LP earthquakes from this part of Mauna Loa is unprecedented, at least in our modern earthquake record dating back to the 1960s.

Inflation continued at the summit through the start of the earthquake swarm. In late August, however, distances across the summit caldera began to contract significantly. The contraction was apparently caused by a shift in the center of inflation a little to the south, rather than by deflation. This was the first contraction since inflation started in late April or early May 2002. Toward the end of September, the contraction ended and the line once again began to lengthen.

The GPS network first showed definite lengthening of the lines across the summit caldera in late April or early May 2002, after nearly 10 years of slight deflation. The summit expansion tailed off and perhaps stopped in mid-winter 2002-2003. It then resumed, starting on about February 15, 2003, slackened, and once more began in late April or early May. The lengthening again slowed during summer 2003 but accelerated as summer ended. During 2004, the inflation had been at a fairly steady to slightly increasing rate until the contraction in late August. In general, we interpret the lengthening, uplift, and tilting to indicate swelling of the magma reservoir within the volcano.

Starting on July 20, 2004, we began showing the tilt at station MLO rather than at MLK; the latter station has been having instrumental spikes that distort the record.

Current Mauna Loa Data

Global Positioning System (GPS)
Electronic Tilt
Seismicity

Activity Summary (1974-2002)

Long-term monitoring data

Global Positioning System (GPS) Data

Map of GPS stations at Mauna Loa summit caldera

We recently expanded our network of continuously recording global positioning system (GPS) stations on Mauna Loa, shown on the map above. The current network can detect changes in the shape of the ground surface due to inflation of both the relatively shallow magma reservoir at the summit and a deeper magmatic system. Several of the new GPS stations are positioned to monitor the potential movement of magma into the northeast and southwest rift zones and into the area of radial vents on the northwest flank. The network is also designed to monitor stress changes in the seismically active Ka`oiki fault zone on the southeast flank.

GPS data are usually processed about 24 hours after collection using the rapid satellite orbit information distributed by the Jet Propulsion Laboratory. Final orbits are released about two weeks later and the GPS data are then re-processed. Consequently, the data on the graph for the past two weeks or so should be considered preliminary.

Plotted below is the change in distance between the stations (MOKP and MLSP) located on opposite sides of Moku`aweoweo, Mauna Loa's caldera. Over the last several decades, various surveying techniques have shown that an increase in the distance between these two stations is a good indicator of inflation of the summit magma reservoir. In May 2002, the distance between these two stations abruptly began extending, reversing an almost decade-long trend of contraction. Survey GPS measurements of a large network of benchmarks on Mauna Loa, as well as tilt surveys at the summit, have confirmed that this extension is due to influx of magma into the summit reservoir.

A fairly rapid increase in distance across the summit caldera, 5-6 cm/year, started in May 2002. The rate started to slow in late October 2002 and had flattened by year's end. Starting in early February 2003, the distance began to lengthen again. The extension continued until late August 2004, when, rather abruptly, the distance began to contract. In general, data for the past two weeks shown on this graph are subject to greater error than older data owing to the use of rapid orbit corrections.

For more information on how GPS systems help monitor the deformation of Mauna Loa and Kilauea volcanoes, see a summary of the inflation-deflation of summit magma chambers.

Electronic Tiltmeter Data

We maintain several continuously recording electronic tiltmeters on Mauna Loa. The graph below shows two components of tilt recorded at MLO over the last month. Positive changes on the plot indicate tilting of the ground at the tiltmeter site toward the north and east. The tiltmeter is located northwest of the summit magma chamber, so tilt toward the northwest would indicate inflation of the magma reservoir. Electronic tiltmeters are highly sensitive instruments designed to detect minute changes in a volcano's shape. Unfortunately, this sensitivity makes electronic tiltmeters respond to signals that have nothing to do with volcanic processes -- temperature, for example. As the ground heats and cools it deforms slightly, resulting in a measurable tilt. Consequently, the tilt record contains a diurnal signal corresponding to the daily temperature fluctuations, and an annual signal corresponding to seasonal temperature changes. This makes tiltmeters less useful than GPS receivers for tracking relatively slow, long-lived processes. Rather, tiltmeters excel at imaging fast, short-term events in real-time, giving us the earliest possible warning of changes that could lead to eruption.

Tilt as recorded by MLO tiltmeter. No significant volcanic tilt was recorded over the time period shown in this graph. The tilt shown by the two components is seasonal, related to changes in ground temperature at this high (4 km) station. We are looking for abrupt departures from this seasonal trend that will indicate volcanic tilt. Note: Until July 20, 2004, this plot showed the tilt at MLK; instrumental spikes distorted the record frequently, so we decided to show the MLO tilt instead.

For more information on how electronic tiltmeters help monitor the deformation of Mauna Loa and Kilauea volcanoes, see a summary of the inflation-deflation of summit magma chambers.

Current Seismicity

Since a swarm of deep earthquakes centered just south of the caldera in late April 2002, seismicity was at a barely elevated level until July 2004, far lower than those of the months prior to the 1975 and 1984 eruptions. Starting in July 2004, however, a swarm of small (magnitude less than 3), deep (>40 km), long-period earthquakes has been taking place below the southern part of the caldera and adjacent areas. Neither the depth nor the magnitude of the earthquakes has changed during the swarm. In other words, the earthquakes are not becoming shallower or larger.

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Updated: 21 September 2004 (DAS)