Hawaiian volcanoes - Part 8: Volcano Monitoring

At the beginning of the 20th century, the way people treated the threats that volcanoes represent changed. Lead by the efforts of Thomas A. Jaggar, Jr. people came to the understanding that short-lived expeditions as response to volcanic eruptions are not enough. To understand the volcanic activity and to be able to avoid catastrophes, one must study the volcanoes continuously. He inspired the founding of the Hawaiian Volcano Research Association (HVRA) and with their support established the Hawaiian Volcano Observatory (HVO) which is run by the US Geological Survey. From there people study the activity of Mauna Loa and Kïlauea on a permanent, scientific basis. From there volcanology was molded as a modern science.
Since its foundation, scientists at the HVO have developed and refined most of the surveillance techniques now commonly used at volcano observatories worldwide.

_{Picture of the Hawaiian Volcano Observatory.
commons.wikimedia.org}

To understand the behavior of a volcano and to increase the chances to be able to predict eruptions, constant volcano monitoring is required. As the name might suggest, it comprises the observations (visual or instrumental) that document the changes of the state of a volcano during and between eruptions. Through the work at the HVO these changes are well understood for Kïlauea. Mauna Loa has been less studied, but certain patterns have been recognized.

By actively going into the field, geologists monitor volcanoes by visiting the active vents and lava flows. This includes in person observations, as well as observations made from aircrafts and webcams.
On their webpage the USGS says:

Geologists require "boots on the ground" and "eyes in the sky" to observe volcanic eruptions and their impacts. ^[]

No matter how good all machine observations can get, direct observations are crucial for a comprehensive understanding of volcanic activity and its impact on the people and the infrastructure.

Observations made in the field are mapping the areal extend of lava flows and their effusion rate. By walking along the edges of the lava flows, mapping can be done in the most detailed way, but observations made from the air are much faster, while some of the sacrificing accuracy. The estimates of effusion rates (volume of lava erupted per unit of time) are used to characterize the intensity of an eruption. Adding the average thickness of a lava flow to the areal extends determined by the flow mapping creates those estimates. Another way of estimating effusion rates is to look at lava tubes. The width of the lava inside the tubes can be measured by with radio waves and the velocity of the lava stream can be measured through little openings inside the lava tube. A further technique to estimate effusion rates is by measuring sulfur dioxide (SO₂) concentrations. Amounts of SO₂ are proportional to the amount of lava effused.

Constant observation is not always possible and geologists have installed a network of webcams that record time-lapses of Kīlauea and Mauna Loa and help understand their volcanic activity. Selected time lapses recorded by the USGS are freely available on the Multimedia section of the USGS website. The direct link is here.

<sub>HVO geologist mapping a Pāhoehoe lava flow of Kīlauea.
volcanoes.usgs.gov

HVO geologists measure the rate of lava flowing through a lava tube.
volcanoes.usgs.org</sub>

Additionally the physical changes the volcano itself undergoes are monitored. The first change observable at Hawaiian volcanoes is the inflation (swelling) of the whole volcano. This occurs when new magma is entering the shallow reservoirs causing the surrounding material to stretch. This swelling changes the slope of the volcano. During times of inflation, the slopes of the volcano will become slightly steeper. The inflation before an eruption on Hawaii usually develops slow and gradually over the course of weeks or years. Once the eruption begins, deflating occurs more rapidly, due to the swift release of pressure from the magma chamber.

_{Sketch of a cross section of a Hawaiian volcano. Additonal magma input into the shallow reservoir results in earthquakes and a change in volcano shape from the dotted to the solid line.
pubs.usgs.gov}

This change in slope is not visible to the naked eye. The changes are monitored using tiltmeters, unique pendulum-type instruments to track long-term changes in the ground slope over time. Multiple tiltmeters are installed along the flanks of volcanoes and changes of their position against a fixed reference point are recorded. Tiltmeters are able to detect changes in angles below 0.0001 degree.

_{A tiltmeter on Mauna Loa, used to predict eruptions by measuring very small changes in the profile of the mountain.
en.wikipedia.org}

In recent years the global positioning system (GPS) is more and more used to observe the ground movements of active volcanoes. High quality GPS receivers are able to identify tiny changes in latitude, longitude and elevation, with extremely high precision. This makes it possible to now observe the complete movement of a volcano. Additionally to the swelling and deflating of the volcano, the GPS data also shows that the southern flanks of Kïlauea and Mauna Loa are slowly moving southeastward (the direction of the sea) on an average speed of a few cm per year. In the future it is possible and very likely that GPS will replace any other ground measuring technique, as data can be obtained around the clock and are not dependent on environmental conditions (e.g., weather).

The newest technique used for volcano monitoring is Interferometric Synthetic Aperture Radar (InSAR). It maps ground deformation using radar images. Satellites send radar signals towards the earth. These signals get reflected by the Earth’s surface and bounce back towards the satellites. Comparing different images over time can then monitor changes in topography. The reason why radar signals are used (instead of visible or infrared light) is that they are unaffected by clouds and therefore do not depend on weather conditions.
InSAR data is interpreted by overlapping data from the same point at two different times. This image then does not portray the surface in general but how the surface itself has changed. InSar has the advantage that it does not require any ground-based transmitters to receive the data. This makes it particularly useful in remote locations where hazardous conditions prevent detailed ground-based volcano monitoring.
Generally, InSAR data covers a wide overall area but the data is collected periodically, whereas GPS uses data that is collected at fixed points, but is transmitting data consistently. The two techniques complement each other and taken together give a great overview on the ground deformation.

_{InSAR image produced from overlapping two pictures 35 days apart in 2007 from Hawai’i. The picture shows subsidence at Kïlauea’s summit caldera (left) and a combination of uplift and subsidence at the Makahopuhi Crater (right). The pattern is caused by movement of magma from the reservoir beneath at Kïlauea towards the east rift zone. Each succession of the color spectrum (pink – blue – green – yellow – red) represents ca. 3 cm of deformation. If, moving towards the center, the order of color goes from pink to blue it represents downward deformation. I the color changes form pink to yellow, it demonstrates uplift.
commons.wikimedia.org}

The backbone of volcano monitoring is the persistent measurement of seismic activity. Increasing number of shallow earthquakes precedes and accompanies all Hawaiian eruptions. When new magma gets introduced into the volcanoe’s reservoir, the surrounding material is pushed away. This is causing the previously discussed inflation. While the material is being pushed outwards it rubs against itself and ruptures. These ruptures produce seismic waves. A network of seismometers is installed on the surface of the volcano. These seismometers transmit measurements of ground shaking to seismographs that record the intensity of the ground movement via pendulum on a piece of paper. Over time location, depth, and magnitude of earthquakes is recorded, enabling the scientists on Hawai’i to track the movement of magma below the surface. Besides the individual seismic shocks caused by rupturing, all Hawaiian eruptions are accompanied by volcanic tremor. It is a constant vibration in the ground, caused by moving magma. Usually, volcanic tremors are only detected by instruments and only in rare occasions are actually felt by humans on the surface.

_{Kinematics Seismograph.
en.wikipedia.org}

_{Common seismic signatures recorded before and during eruptions.
pubs.usgs.gov}

Conclusion

A combination of techniques is used to monitor the Hawaiian volcanoes in an effort to understand and predict their behavior. In the field, geologist monitor areal extend of lava flows and effusion rate. Using state of the art techniques, large scale, 3-dimensional maps are created for active Hawaiian volcanoes, monitoring changes not visible to the naked eye. These changes include the inflation of a volcano during the injection of new magma batches as well as their overall movement to the south-east. The last and most vital monitoring technique includes seismic measurements that help monitor the movement of magma underground.

Now that we have discussed all these different techniques, we can, in my next post, look at how effective the geologists at the HVO can predict eruptions to minimize damage and prevent casualty.

Previous Posts

Hawaiian volcanoes - Part 1: Introduction
Hawaiian volcanoes - Part 2: The Hawaiian Hotspot
Hawaiian volcanoes - Part 3: A growing volcano
Hawaiian volcanoes - Part 4: The Rejuvenation Stage
Hawaiian volcanoes - Part 5: Evolution to Atolls and Seamounts
Hawaiian volcanoes - Part 6: Mythology vs Geology
Hawaiian volcanoes – Part 7: Recent eruptions

References

Tilling, R.I., Heliker, C., and Swanson D.A., (2010). Eruptions of Hawaiian volcanoes - past, present, and future: U.S. Geological Survey General Information Product 117, 63 p.

https://en.wikipedia.org/wiki/Seismometer
https://en.wikipedia.org/wiki/Thomas_Jaggar
https://en.wikipedia.org/wiki/Tiltmeter
https://www.usgs.gov/
https://pubs.usgs.gov/ds/621/
https://pubs.usgs.gov/gip/hawaii/page17.html
https://pubs.usgs.gov/gip/hawaii/page19.html
https://volcanoes.usgs.gov/vhp/insar.html
https://volcanoes.usgs.gov/observatories/hvo/
https://volcanoes.usgs.gov/observatories/hvo/hvo_monitoring_deformation.html
https://volcanoes.usgs.gov/observatories/hvo/hvo_monitoring_earthquakes.html
https://volcanoes.usgs.gov/observatories/hvo/hvo_monitoring_geology.html
https://volcanoes.usgs.gov/observatories/hvo/hvo_multimedia.html