Studying Aleutian Volcanoes With InSAR

by Zhong Lu, USGS, EROS Data Center, SAIC; Chuck Wicks, USGS, Earthquake & Volcano Hazards Programs;
Dan Dzurisin, USGS, Cascades Volcano Observatory; John Power, USGS, Alaska Volcano Observatory

Satellite InSAR has proven a powerful spaceborne geodetic tool to study varied volcanic processes by analyzing surface deformation patterns. With the implementation of InSAR technology, volcano monitoring has entered an exciting phase wherein magma accumulation in the middle to upper crust can be observed long before the onset of short-term eruption precursors.

Ultimately, more widespread use of InSAR for volcano monitoring could shed light on a part of the eruption cycle—the time period between eruptions when a volcano seems to be doing essentially nothing. Combining InSAR technique with observations from continuous GPS, gravity, strainmeters, tiltmeters, seismometers and volcanic gas studies will improve our capability to forecast future eruptions and lead to improved volcano hazard assessments and better eruption preparedness.

Kiska Volcano

Kiska volcano is the westernmost historically active volcano in the Aleutian arc. InSAR images of Kiska show a circular area about 3 km in diameter centered near the summit subsided by as much as 10 cm from 1995 to 2001. The volcano emitted copious amounts of steam during recent eruptions. Field reports document vigorous steaming and persistent ground shaking near the summit. Based on these reports and the shallow source depth (

Seguam Volcano

Seguam volcano, often referred to as Pyre Peak, erupted in 1901, 1927, 1977 and 1992-93. InSAR images, spanning various intervals during 1992-2000, document co-eruptive and post-eruptive deformation of the 1992-93 eruption. A model that combines magma influx, thermoelastic relaxation, and poroelastic effects accounts for the observed deformation. This example demonstrates that spatial and temporal coverage with InSAR data reveals dynamic processes within a volcano.

Okmok Volcano

Okmok volcano, a broad shield topped with a 10-km-wide caldera, produced blocky basaltic flows during relatively large effusive eruptions in 1945, 1958 and 1997. Multiple InSAR images mapped: 1) the surface inflation of more than 18 cm during 1992-95 and subsidence of 1-2 cm during 1995-96, 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 1997-2003, after the 1997 eruption.

Numerical modeling suggested the magma reservoir responsible for the observed deformation resided at a depth of about 3 km beneath the center of the caldera and about 5 km away from the eruptive vent. This example demonstrates how InSAR is capable of measuring preeruptive, co-eruptive, and post-eruptive deformation in the subarctic environment.

Makushin Volcano

Makushin volcano, a broad, ice-capped, truncated stratovolcano, is one of the more active volcanoes in the Aleutians, producing at least 17 explosive, relatively small eruptions since the late 1700s. Additional smaller eruptions probably occurred during this period but were unrecorded, either because they occurred when the volcano was obscured by clouds or because the eruptive products did not extend beyond the volcano’s flanks.

Several independent InSAR images that each span the time period from October 1993 to September 1995 show evidence of ~7 cm of uplift centered on the volcano’s east flank. The uplift was interpreted as pre-eruptive inflation of a small explosive, but unsubstantially reported, eruption on January 30, 1995. This example demonstrates that ground deformation of a few cm can be unambiguously identified with InSAR images over a rugged terrain where geometric distortion of radar images is severe.

Akutan Volcano

Akutan, the second most active volcano in Alaska, was shaken in 1996 by an intense earthquake swarm accompanied by extensive ground cracking, but no eruption of the volcano. Both L-band JERS-1 and C-band ERS-1/2 InSAR images show uplift of as much as 60 cm on the western part of the island associated with the swarm. Our JERS-1 interferogram, displaying greater coherence, especially in areas with loose surface material or thick vegetation, also shows subsidence of similar magnitude on the eastern part of the island, and displacements along faults reactivated during the swarm.

The axis of uplift and subsidence strikes about N70°W and is roughly parallel to: 1) a zone of fresh cracks on the volcano’s northwest flank, 2) normal faults that cut the island, and 3) the inferred maximum compressive stress direction. Both before and after the swarm, the northwest flank uplifted 5-20 mm/year relative to the southwest flank, probably by magma intrusion. This example demonstrates that InSAR can provide a basis not only for interpreting and modeling movement of shallow magma bodies that feed eruptions, but also for detecting intrusive activities that do not result in an eruption.

Westdahl Volcano

Westdahl volcano, a young glacier-clad shield volcano had documented eruptions in 1964, 1978-79 and 1991-92. The background level of seismic activity since the last eruption has been generally low (about five M

The rates of post-eruptive inflation and co-eruptive deflation are approximated by exponential decay functions with time constants of about 6 years and a few days, respectively. This behavior is consistent with a deep, constant-pressure magma source connected to a shallow reservoir by a magma-filled conduit where the magma flow rate is governed by the pressure gradient between the deep source and the shallow reservoir.

This example demonstrates that: 1) InSAR is becoming the best tool available for detecting deep, aseismic magma accumulation by measuring broad, subtle deformation of the ground surface to identify restless volcanoes long before they become active and before seismic and other precursors emerge and 2) multi-temporal InSAR images enable construction of a virtual magma plumbing system that can be used to constrain magma accumulation at the shallow reservoir and shed light on the time window of the next eruption.

Peulik Volcano

Peulik volcano, a stratovolcano located on the Alaska Peninsula, is known to have erupted in 1814 and 1852. InSAR images that collectively span the time interval from July 1992 to August 2000 reveal that a presumed magma body located 6.6 km beneath the Peulik volcano inflated 0.051 km3 between October 1996 and September 1998.

The average inflation rate of the magma body was about 0.003 km3/month from October 1996 to September 1997; peaked at 0.005 km3/month from June 26 to October 9, 1997; and dropped to 0.001 km3/month from October 1997 to September 1998. Deformation before October 1996 or after September 1998 is not significant. An intense earthquake swarm occurred about 30-km northwest of Peulik from May to October 1998, around the end of the inflation period.

The 1996-98 inflation episode at Peulik confirms that InSAR can be used to detect magma accumulation beneath dormant volcanoes at least several months before other signs of unrest are apparent. This application represents a first step toward understanding the eruption cycle at Peulik and other stratovolcanoes with characteristically long repose periods.

Augustine Volcano

Augustine volcano, an 8-by-11 km island, has had six significant eruptions in the last two centuries: 1812, 1883, 1935, 1963-64, 1976, and 1986. InSAR images show the pyroclastic flows from the 1986 eruption have been experiencing subsidence/compaction at a rate of about 3 cm/year, and no sign of significant volcano-wide deformation was observed during 1992-2000. The observed deformation can be used to study the characteristics of the pyroclastic flows.

For more information about this volcano monitoring project, please visit http://edc.usgs.gov/Geo_Apps or contact lu@usgs.gov.

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