Sizing up volcanoes from space

Volcano monitoring utilizes a wide range of methods, including the composition and emission rate of gases, earthquake activity, and ground deformation. In the past decade, observations from orbiting satellites have also become important because of their ability to detect such important volcanic activity indicators as changes in surface temperature, the presence of ash in the atmosphere, and even rock composition from space. Among the most useful recent advances in space-based volcano surveillance is radar interferometry, also known as InSAR.

Radar is certainly not a new technology. Radar systems have been in use for over 100 years and are well known for their ability to track ships and aircraft (which has obvious military and navigation applications), as well as more recent civilian uses, like monitoring weather patterns. The principles behind radar operation are quite simple. A radar instrument emits an electromagnetic pulse and detects any reflections from, for example, rain clouds or airplanes. The time difference between pulse transmission and reception of reflected energy gives the range to the detected target. Further, the characteristics of the reflected signal can be used to determine certain properties of the target, for example, size or texture – rough, like `a`a, or smooth, like pahoehoe.

InSAR makes use of a pair of radar images of the same area on Earth taken from about the same location in space at different times. By differencing – or “interfering” – the distance information in the two images, changes in the shape of the ground surface can be detected. For example, if a volcano were inflating due to the underground accumulation of magma, the ground above the magma chamber would appear to move closer to the satellite over time as the surface is uplifted. This technology has some obvious advantages over traditional deformation monitoring techniques. A large area (usually more than 3,600 square miles) can be imaged at once, and people don’t even have to be in the field when data are being collected by the satellite! This being the case, you might think that we could pack up all our gear and use InSAR to monitor a deforming volcano from an office thousands of miles away.

Unfortunately, InSAR has some substantial drawbacks. Most of the radar satellites currently orbiting the Earth use a wavelength that is unable to penetrate vegetation. As a result, InSAR doesn’t work in heavily forested areas like the rainforests of Hawai`i. Steep slopes and ice- or snow-covered regions are also difficult to monitor with InSAR. Atmospheric conditions, especially moisture, can produce unreliable results. Finally, satellites generally take repeat images of the same place on Earth only about once each month, so we might miss important deformation events if we relied solely on InSAR. That is why is it important to use InSAR in combination with other proven techniques, like GPS and tilt measurements, thus providing the most data possible to help scientists understand the cause of any volcanic activity.

InSAR has been applied with notable success at numerous volcanoes around the world. For example, the lack of earthquake activity and gas emissions in the Three Sisters region of central Oregon led volcanologists to believe that the area was currently inactive. However, InSAR results show that the ground just west of South Sister volcano has been inflating since 1997, probably due to magma accumulating in the subsurface. Now, South Sister is the site of intense earthquake, deformation, and gas-emission monitoring, and valuable new data regarding volcanic unrest are being collected.

Scientists are also using InSAR to monitor the volcanoes of Hawai`i, which present several challenges. The recent inflation of Mauna Loa is readily apparent in InSAR data of the Big Island; however, dense vegetation and highly variable climatic conditions over parts of Kilauea cause InSAR results from that volcano to be unreliable. For these reasons, the area is an ideal natural laboratory for researching ways to remove such problems. Efforts are currently underway at universities and laboratories around the world, including right here in Hawai`i, to compensate for atmospheric conditions and extract signals hidden in vegetated areas. Stay tuned for new results, and, for more information on monitoring volcanoes with InSAR, check out

Activity Update

Eruptive activity at Pu`u `O`o continues. All vents inside Pu`u `O`o crater were incandescent this past week, producing bright glow on clear nights. Some vents sporadically produced spatter.

The PKK flow continues to produce substantial breakouts from atop Pulama pali to the coastal plain. Open lava channels have been intermittently visible on Pulama pali, particularly on the east branch of the PKK flow. Two ocean entries are currently active at East Lae`apuki and Ka`ili`ili. East Lae`apuki and Ka`ili`ili entries are about 3.5 km (2 miles) and 7 km (4.5 miles) from the ranger shed. Expect a 2- to 3-hour walk each way and remember to bring lots of water. Stay well back from the sea cliff, regardless of whether there is an active ocean entry or not. Heed the National Park warning signs.

During the week ending March 17, only one earthquake was felt on Hawai`i Island. The magnitude-2.9 quake occurred 14 km (9 miles) west of Kailua-Kona at a depth of 12 km (7 miles) at 8:06 a.m. on Monday, March 14. The earthquake was felt at Kailua and Kalaoa.

The accelerated inflation and earthquake swarm beneath Kilauea summit that started in mid-January has ceased. The accelerated inflation stopped about February 21, and the earthquake swarm stopped a few days afterwards.

Mauna Loa is not erupting. Since July 2004, the rate of inflation and number of deep earthquakes beneath the summit increased until the early part of 2005. Weekly earthquake counts have varied from 5 to over 150 in the last half of 2004 – nearly all the quakes were of the long-period type and 30 km (18 mi) or more deep. Since the beginning of 2005, weekly counts have been less than 10. Most of the recent quakes are of the short-period type and shallower. During the week ending March 17, seven earthquakes were recorded beneath the summit area.

Visit our web site ( for daily volcano updates and nearly real-time Hawai`i earthquake information.

This article was written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.

This article was written by scientists at the U.S. Geological Survey’s Hawaii Volcano Observatory and is republished by with permission.

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