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Comparison of GPS and InSAR deformation measurements at Okmok volcano, Alaska

Dr. Zhong Lu, Science Applications International Corporation (SAIC), U.S. Geological Survey Center for Earth Resources Observation and Science, Sioux Falls, SD 57198; email: lu@usgs.gov; http://edc.usgs.gov/Geo_Apps/ Yousuke Miyagi, Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University; email: yousuke@eos.hokudai.ac.jp

Since the 1980s, two modern geodetic techniques, namely the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR), have revolutionized the study and monitoring of active volcanoes. These techniques have documented patterns of deformation before, during, and after eruptions of volcanoes, and enabled exploration of quantitative physical models to understand the magmatic processes.

The two techniques work in a remarkably similar fashion, both utilizing phases of electromagnetic waves to resolve the precise distance between the satellites and ground targets. The capabilities of the two techniques compliment each other where GPS can provide a 3-D deformation vector at each GPS station with an accuracy of a few millimeters, while InSAR can image the line-of-sight component of ground deformation over a large area at spatial resolution of tens of meters with an accuracy of centimeters to sub-centimeters.

In GPS observations, dual-frequency waves and temporal averaging enable the removal of ionospheric anomalies and the reduction of tropospheric artifacts, both of which plague InSAR deformation measurements. Furthermore, InSAR measurements suffer from the loss of interferometric coherence due to modification of the imaged surface characteristics caused by vegetation, snow, ice, and other environmental factors. Regardless, the combination of both GPS measurements and InSAR deformation images can enhance mapping, modeling and interpretation of ground deformation at active volcanoes.

Okmok volcano is among a limited number of volcanoes in the world being monitored by both GPS and InSAR techniques. Okmok is a dominantly basaltic central volcanic complex that occupies most of the northeastern end of Umnak Island, Alaska (Figure 1). Catastrophic pyroclastic eruptions circa 12.0 and 2.05 ka resulted in the formation of two overlapping summit calderas. Subsequent eruptions produced a field of small cones and lava flows, including several historically active vents within the younger caldera [Grey, 2003; Miller et al., 1998]. Most of the volcano’s historical eruptions are poorly documented owing to its remote location. Minor explosive eruptions occurred in 1931, 1936, 1938, 1943, 1960-1961, 1981, 1983, and 1986-1988; blocky basaltic flows were extruded during relatively large effusive eruptions in 1945, 1958, and 1997. These eruptions originated from Cone A, located on the southern edge of the caldera floor (Figure 1), which formed almost entirely during the 20th century.

InSAR images were used to study transient deformation of the volcano before, during, and after the 1997 eruption at Okmok volcano. Spherical point-source models suggest that a magma reservoir, residing at a depth of 3-4 km below sea level and located beneath the center of the caldera and about 5 km northeast of the 1997 vent, is responsible for observed volcano-wide deformation: 1) surface inflation of more than 18 cm during 1992-1995 and subsidence of 1-2 cm during 1995-1996, prior to the 1997 eruption; 2) more than 140 cm of surface deflation during the 1997 eruption; and 3) 5-15 cm/year inflation during 1997-2004, after the 1997 eruption.

Figure 1, an interferogram spanning 2000-2002, is one of the interferograms with the greatest coherence. From this InSAR image one can infer several distinct deformation processes: 1) volcano-wide inflation due to replenishment of the shallow magma reservoir (i.e., the broad fringes across the whole caldera), and 2) deformation of the 1997 lava flows (i.e., the localized fringes over and around the 1997 lava flows).

Campaign GPS surveys were also carried out during 2000-2002 (Figure 1): the uplift of the caldera center relative to the caldera rim was about 9 cm during 2000-2002. These GPS surveys allow us to compare the GPS displacement vectors with InSAR deformation measurements.

To compare GPS displacements with InSAR measurements, we first referenced both GPS displacement vectors and InSAR observations to a GPS station FTGL (Figure 1). We projected the 3-D GPS displacement vectors into the deformations along InSAR line-of-sight direction. We then unwrapped the interferometric phase values and converted them into line-ofsight displacements. Figure 2 compares the line-of-sight displacements between GPS and InSAR measurements. In general, the two sets of observations agree with each other, with a correlation value of r2 = 0.91. As only a single InSAR image is used in the comparison, atmospheric delay anomalies in the InSAR image may be one cause for the dispersion between GPS and InSAR observations.

Of course, the integration of GPS and InSAR goes beyond comparing the deformation values from each dataset. First, precise GPS positions can be used to improve InSAR baseline estimates (i.e., estimates of the spatial separation between satellite vantage points when the two images comprising an interferogram were acquired) and therefore, enhance deformation accuracy of InSAR images. Second, the perceptible water-vapor content retrieved from Continuous GPS (CGPS) networks presents an appealing opportunity for estimating atmospheric water-vapor content as a means to correct atmospheric delay anomalies in InSAR deformation measurements. By modeling and interpolating water-vapor values from CGPS measurements, the measurement accuracy of InSAR images will be improved. Innovative methods of comparing and integrating GPS and InSAR measurements will facilitate enhanced volcanic deformation mapping and provide a better understanding of volcanic processes.

More information about this work can be found at: Lu, Z., T. Masterlark, and D. Dzurisin, Interferometric Synthetic Aperture Radar (InSAR) Study of Okmok volcano, Alaska, 1992-2003: Magma Supply Dynamics and Post-emplacement Lava Flow Deformation, Journal of Geophysical Research, 110, B2, B02403, DOI:10.1029/2004JB003148, 2005; Yousuke M., J. Freymueller, F. Kimata, T. Sato, and D. Mann, Surface deformation caused by shallow magmatic activity at Okmok volcano, Alaska, detected by GPS campaigns 2000–2002, Earth Planets Space, 56, e290e32, 2004.

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