Quiescent Deformation of the Aniakchak Caldera, Alaska, Mapped by Insar
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Quiescent deformation of the Aniakchak Caldera, Alaska, mapped by InSAR Oh-Ig Kwoun* Science Applications International Corporation (SAIC), U.S. Geological Survey (USGS) Center for Earth Zhong Lu* Resources Observation and Science (EROS), Sioux Falls, South Dakota 57198, USA Christina Neal* U.S. Geological Survey, Alaska Volcano Observatory, Anchorage, Alaska 99508, USA Charles Wicks Jr.* U.S. Geological Survey, Earthquake and Volcano Hazards Program, Menlo Park, California 94025, USA ABSTRACT of unrest, although occasional bursts of vol- The 10-km-wide caldera of the historically active Aniakchak volcano, Alaska, subsides canic seismicity have been recorded (Jolly et ;13 mm/yr, based on data from 19 European Remote Sensing Satellite (ERS-1 and ERS- al., 2001; Neal et al., 2001; Dixon et al., 2002, 2) interferometric synthetic aperture radar (InSAR) images from 1992 through 2002. The 2003, 2004). pattern of subsidence does not re¯ect the distribution of pyroclastic deposits from the last Most volcanic processes are associated with eruption in 1931 and therefore is not related to compaction of fragmental debris. Weighted measurable surface deformation patterns that least-squares inversion of the deformation maps indicates a relatively constant subsidence can provide important insights into the struc- rate. Modeling the deformation with a Mogi point source locates the source of subsidence ture, plumbing, and state of restless volcanoes at ;4 km below the central caldera ¯oor, which is consistent with the inferred depth of (Dzurisin, 2003). Changes in the shape of a magma storage before the 1931 eruption. Magmatic CO2 and He have been measured at volcano can be precursory to an eruption; a warm soda spring within the caldera, and several sub-boiling fumaroles persist elsewhere therefore, monitoring surface deformation of in the caldera. These observations suggest that recent subsidence can be explained by the an active volcano is important even during cooling or degassing of a shallow magma body (;4 km deep), and/or the reduction of the quiescence (Lu et al., 2003a). The remote lo- pore-¯uid pressure of a cooling hydrothermal system. Ongoing deformation of the volcano cation of Aniakchak volcano and the harsh cli- detected by InSAR, in combination with magmatic gas output from at least one warm mate of the region result in expensive and spring, and infrequent low-level bursts of seismicity below the caldera, indicate that the complex logistics that hinder routine ground- volcanic system is still active and requires close attention for the timely detection of pos- based deformation surveys. Remote sensing sible hazards. techniques, especially spaceborne InSAR (in- terferometric synthetic aperture radar), pro- Keywords: volcanic processes, deformation, SAR, interferometry. vide a unique opportunity to cost-effectively monitor volcanic deformation under such cir- INTRODUCTION thick are distributed mostly on the northwest- cumstances (e.g., Lu et al., 2003b). Here, we Aniakchak volcano (Alaska, United States), ern part of the caldera ¯oor and were erupted applied an InSAR technique with satellite ra- a voluminous Pleistocene-Holocene stratovol- primarily from Main Crater (Fig. 1) (Nichol- dar images to study deformation of Aniakchak cano with a summit caldera ;10 km in di- son, 2003). Seismic monitoring of Aniakchak volcano from 1992 through 2002. ameter, is located 670 km southwest of An- volcano, initiated in 1997 by Alaska Volcano chorage on the Alaska Peninsula (Fig. 1). The Observatory, has revealed no signi®cant signs DATA PROCESSING AND ANALYSIS caldera was formed by catastrophic eruption We use the two-pass InSAR approach (e.g., and collapse ca. 3.5 ka during an eruption of Massonnet and Feigl, 1998) to produce 19 de- more than 50 km3 of andesite-dacite pyroclas- formation interferograms from 19 European tic debris (Neal et al., 2001). Eruptions since Remote Sensing Satellite (ERS-1 and ERS-2) this caldera-forming event have produced nu- synthetic aperture radar (SAR) images, with merous vents inside the caldera, including tuff interferometric baselines ranging from 12 to cones and a 1-km-high cinder and spatter 282 m (Data Repository Figure DR1; Table cone, Vent Mountain (Fig. 1). The eruption of DR11). The SAR data were collected during Half Cone within Aniakchak ;400 yr ago subarctic summers and autumns to avoid loss produced 0.75±1.0 km3 of material in a vol- of coherence due to snow and ice accumula- canic explosivity index (VEI) 3 or 4 eruption tion. The 19 SAR images are from two dif- (Neal et al., 2001; Browne et al., 2004). An- ferent descending passes with center incidence iakchak's most recent eruption occurred in angles of 21.38 and 23.58, respectively. The 1931, the only reported eruption in the past corresponding line-of-sight (LOS) vectors (de- 250 yr. The 1931 eruption (VEI 3) was explo- ®ned as east, north, up) are 0.353, 20.087, sive and produced both lava and tephra from 0.932 and 0.387, 20.095, 0.917 for interfer- a series of vents near the western caldera wall. Figure 1. Aniakchak volcano, Alaska, a stra- ograms from these two passes. The digital el- tovolcano with circular summit caldera ~10 evation model (DEM) used to produce the de- Although the main source of the eruption was km in diameter, is located ~670 km south- a crater ;1 km in diameter (Main Crater in west of Anchorage on the Alaska Peninsula. Fig. 1), blocky dacite lava ¯ows issued from During 1931 eruption, lava was emitted from 1GSA Data Repository item 2006002, Figure Slag Heap, Doublet Crater, and Main Crater Main Crater, Slag Heap, and Doublet Crater. DR1 and Table DR1, SAR image acquisition dates (Fig. 1). Tephra deposits as much as 40 m Solid lines indicate isopachs of tephra; and baseline information, are available online at dashed lines, uncertain thickness (Nichol- www.geosociety.org/pubs/ft2006.htm, or on request son, 2003). This isopach map accounts for from [email protected] or Documents Secre- *E-mails: [email protected]; [email protected]; most lithic-rich tephra of 1931 eruption, con- tary, GSA, P.O. Box 9140, Boulder, CO 80301- [email protected]; [email protected]. stituting over 70% of total tephra volume. 9140, USA. q 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; January 2006; v. 34; no. 1; p. 5±8; doi: 10.1130/G22015.1; 5 ®gures; Data Repository item 2006002. 5 Figure 3. Mean deformation rate (black), modeled deformation rate by Mogi modeling (black dotted), and topographic pro®le (gray) along pro®le AB in Figure 2. Missing Figure 4. Maximum subsidence of caldera data in deformation pro®le are due to loss center (location C in Fig. 2) as function of of coherence in associated interferograms. time. Subsidence is relative to presumed Observed subsidence exhibits no correla- nondeforming reference point (D in Fig. 2) tion with topography, excluding possibility near caldera rim. F test on deviations from of any strong topography related tropo- mean velocity suggests that line-of-sight spheric artifacts. LOSÐline of sight. (LOS) deformation is linear with time at 90% Figure 2. Stacked deformation interferogram con®dence level. Deviation from mean dur- of Aniakchak caldera showing mean subsi- ing 2000±2002 might be due to possible at- dence rate from 1992 to 2002. Each fringe ; mospheric artifacts (only one interferogram less than 5% of the total range displacement. spanning that interval was used in (full color cycle) represents 14 mm/yr of Therefore, we ignore the differences in LOS range change between ground and satellite inversion). in line-of-sight direction. Areas that lack in- displacement implied by the difference in im- terferometric coherence are uncolored. aging geometry. The displacement maps from where v is the unknown deformation velocity both tracks are stacked together to estimate a corresponding to each of the time-adjacent ac- mean deformation rate along the LOS direc- f formation interferograms is the 1-arc-second quisitions, j is the cumulative deformation tion as follows: df (;30 m posting) Shuttle Radar Topography phase at tj with respect to t0, i is the defor- 3 Mission DEM, which has a relative vertical mation phase of interferogram i,B is an M O f(i) 5 accuracy better than 10 m and an absolute ver- N matrix whose (i,j) element will be B(i,j) f5˙¯ i , (1) 2 tical accuracy better than 16 m. DEM errors O D tk11 tk within the time span covered by ith t(i) df 5 of this magnitude would result in no more i deformation phase ( i)orB(i,j) 0 outside 3 than 5 mm of LOS range error in the inter- the time span, and W is an M M diagonal ferograms (e.g., Massonnet and Feigl, 1998). where f(i) is the deformation phase and Dt(i) matrix whose diagonal elements are phase The 19 interferograms indicate a continuing is the time span of the ith interferogram. variances of individual interferograms. Equa- subsidence inside the 10-km-wide circular cal- Figure 2 (also Figure DR2) is the stacked tion (2) is solved for v by applying the sin- dera, with the maximum subsidence near the deformation interferogram, and Figure 3 is the gular value decomposition to the matrix of center of the caldera ¯oor. mean deformation rate in the LOS direction equation (5) without temporal smoothing or The 19 interferograms are stacked in order along the pro®le AB in Figure 2. This pro®le adjustments of small singular values. The ®nal f to examine the mean deformation rate while indicates that the center of the caldera subsid- solution j is determined by an integration reducing the effect of atmospheric delay ed ;13 mm/yr in the LOS direction from step, i.e., anomalies.