Hawai'i Volcano Blog

Volcano Watch: People and Jobs at HVO, Part 1: What on Earth is a Geodesist?

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Geophysicist Sarah Conway, a member of the USGS Hawaiian Volcano Observatory’s deformation team, readies a temporary GPS station during a campaign survey of benchmarks on Kīlauea Volcano to monitor changes in the ground surface. USGS photo by I. Johanson.

The USGS Hawaiian Volcano Observatory (HVO) is responsible for monitoring active volcanoes in Hawai‘i, which involves assessing hazards and communicating with interagency partners and the public. Our mission also includes advancing our knowledge of volcanic processes through scientific research.

It takes many people in diverse roles to accomplish these goals. During January’s Volcano Awareness Month, our Volcano Watch articles will introduce readers to some of the people and jobs at HVO.

One role at HVO is that of a geodesist, which is someone who studies the sub-field of geophysics called geodesy. I usually avoid calling myself a geodesist when meeting new people because it often results in puzzled looks. Instead, I use HVO’s terminology for our team, and say that I’m part of “the deformation group.”

This informal team name gets at the heart of what we do: We study how the surface of a volcano deforms as a way to determine what is happening underground.

Geodesy is primarily concerned with precise measurements of the earth, such as might happen during a geodetic survey. Results from surveys after the 1906 magnitude-7.9 San Francisco earthquake, which offset fence lines and property boundaries, had a profound impact on our understanding of how faults move — and ultimately brought geodesists into the earth sciences.


A geodesist’s tools are similar to those of a surveyor. In the past century, triangulation and leveling were popular techniques. Today, Global Positioning System (GPS) instruments form the backbone of our monitoring program, which also includes borehole tiltmeters and satellite radar (InSAR).

The general approach to using geodetic data on a volcano is to perform multiple surveys to determine how benchmark positions have changed. As magma moves into a volcano, the surrounding rock is pushed outward. When we measure positions of benchmarks on the surface of the volcano, we find that they have also been pushed away from the magma source. Today, permanently installed instruments constantly monitor benchmark positions so we can see ground motion within minutes.

Growing and maintaining HVO’s permanent geodetic instrument network is one of the deformation group’s most important jobs. This permanent network consists of over 60 GPS stations and 16 tiltmeters, and data from it are critical for hazard assessment. In particular, tiltmeters, which are incredibly sensitive to changes in ground slope, are often the first indicator of inflation as a volcano pressurizes.

While HVO’s deformation group is responsible for analyzing and interpreting the data, it takes many others to keep the network running. HVO’s field engineers build, install, and maintain our field instruments. IT staff ensure that our computers can communicate with remote sites from which data are transmitted and that we are equipped to analyze the data.

We supplement the permanent geodetic network with annual campaigns to collect additional benchmark data using temporary GPS stations. Around 80 benchmarks are surveyed each year for 2-3 days to determine yearly changes in position. These surveys provide a higher density of measurements in certain areas, enabling us to more precisely determine deformation patterns over many years.


To help interpret geodetic data, I use computer models that calculate the expected motion at the earth’s surface due to expansion or contraction of magma bodies with simplified shapes, such as spheres or ellipsoids. Simple shapes are used because they adequately match the data and are less time-consuming to calculate than irregularly shaped bodies. Time is important because I run many thousands of calculations to test different models to discover the position, length, width, depth and volume changes that best match the data we’ve collected.

The best-fitting model shows us the most likely place that magma is moving into or out of the volcano, as well as where magma is accumulating and how close it is to the surface. However, no single type of data gives the whole picture of a volcano, so we must interpret our geodetic data along with geologic, seismic and gas data. HVO’s different teams come together as a whole to develop sound hypotheses for current activity, hazard levels, and future scenarios.

Next week, another HVO team writes about its work. Until then, we hope to see you at one or more of our Volcano Awareness Month programs. The schedule is posted on HVO’s homepage.

Volcano Activity Updates

Kīlauea Volcano is not erupting and its USGS Volcano Alert level remains at NORMAL. Updates for Kīlauea are now issued monthly.


Kīlauea monitoring data showed no significant changes in activity over the past month. Seismicity was relatively consistent. Sulfur dioxide emission rates are low at the summit and below detection limits at Pu‘u ‘Ō‘ō and the lower East Rift Zone (LERZ). The water lake at the bottom of Halema‘uma‘u continues to slowly expand and deepen.

Mauna Loa is not erupting. Its USGS Volcano Alert level remains at ADVISORY. This alert level does not mean that an eruption is imminent or that progression to an eruption is certain.

This past week, 56 small-magnitude earthquakes were recorded beneath the upper elevations of Mauna Loa, most at depths of less than about three miles. Deformation measurements show continued slow summit inflation. Fumarole temperature and gas concentrations on the Southwest Rift Zone remain stable.

Mauna Loa updates are issued weekly.

Six earthquakes with three or more felt reports occurred on the Island of Hawai‘i this past week:

  • A M2.8 quake eight miles south of Volcano at a four-mile depth on Jan. 7 at 6:08 a.m. HST.
  • A M3.6 quake 10 miles north of Pāhala at a 6-mile depth on Jan. 7 at 2:04 a.m. HST.
  • A M2.4 quake eight miles southeast of Volcano at a one-mile depth on Jan. 6 at 11:33 a.m. HST.
  • A M3.5 quake three miles northeast of Pāhala at a 22-mile depth on Jan. 6 at 11:19 a.m. HST.
  • A M2.3 quake four miles south of Leilani Estates at a one-mile depth on Jan. 4 at 8:05 p.m. HST.
  • A M2.9 quake eight miles southwest of Hōnaunau-Nāpō‘opo‘o at a seven-mile depth on Jan. 4 at 6:46 p.m. HST.

HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.

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