Zhong Lu

Synthetic aperture radar interferometry of Okmok volcano, Alaska: Radar observations

ERS-1/ERS-2 synthetic aperture radar interferometry was used to study the 1997 eruption of Okmok volcano in Alaska. First, we derived an accurate digital elevation model (DEM) using a tandem ERS-1/ERS-2 image pair and the preexisting DEM. Second, by studying changes in interferometric coherence we found that the newly erupted lava lost radar coherence for 5–17 months after the eruption. This suggests changes in the surface backscattering characteristics and was probably related to cooling and compaction processes. Third, the atmospheric delay anomalies in the deformation interferograms were quantitatively assessed. Atmospheric delay anomalies in some of the interferograms were significant and consistently smaller than one to two fringes in magnitude. For this reason, repeat observations are important to confidently interpret small geophysical signals related to volcanic activities. Finally, using two-pass differential interferometry, we analyzed the preemptive inflation, coeruptive deflation, and posteruptive inflation and confirmed the observations using independent image pairs. We observed more than 140 cm of subsidence associated with the 1997 eruption. This subsidence occurred between 16 months before the eruption and 5 months after the eruption, was preceded by ∼18 cm of uplift between 1992 and 1995 centered in the same location, and was followed by ∼10 cm of uplift between September 1997 and 1998. The best fitting model suggests the magma reservoir resided at 2.7 km depth beneath the center of the caldera, which was ∼5 km from the eruptive vent. We estimated the volume of the erupted material to be 0.055 km3 and the average thickness of the erupted lava to be ∼7.4 m.

Ground surface deformation patterns, magma supply, and magma storage at Okmok volcano, Alaska, from InSAR analysis: 1. Intereruption deformation, 1997–2008

eruption, magma storage had increased by 3.7– 5.2 ×1 0 7 m 3 or 85–100% of the 1997 eruption volume. We propose that the supply rate decreased in response to the diminishing pressure gradient between the shallow storage zone and a deeper magma source region. Eventuallytheeffectsofcontinuingmagmasupplyandvesiculationofstoredmagmacaused acriticalpressurethresholdtobeexceeded,triggeringthe2008eruption.Asimilarpatternof initially rapid inflation followed by oscillatory but generally slowing inflation was observed prior to the 1997 eruption. In both cases, withdrawal of magma during the eruptions depressurized the shallow storage zone, causing significant volcano‐wide subsidence and initiating a new intereruption deformation cycle.

Estimating lava volume by precision combination of multiple baseline spaceborne and airborne interferometric synthetic aperture radar: the 1997 eruption of Okmok volcano, Alaska

Interferometric synthetic aperture radar (InSAR) techniques are used to calculate the volume of extrusion at Okmok volcano, Alaska by constructing precise digital elevation models (DEMs) that represent volcano topography before and after the 1997 eruption. The posteruption DEM is generated using airborne topographic synthetic aperture radar (TOPSAR) data where a three-dimensional affine transformation is used to account for the misalignments between different DEM patches. The preeruption DEM is produced using repeat-pass European Remote Sensing satellite data; multiple interferograms are combined to reduce errors due to atmospheric variations, and deformation rates are estimated independently and removed from the interferograms used for DEM generation. The extrusive flow volume associated with the 1997 eruption of Okmok volcano is 0.154/spl plusmn/0.025 km/sup 3/. The thickest portion is approximately 50 m, although field measurements of the flow margins height do not exceed 20 m. The in situ measurements at lava edges are not representative of the total thickness, and precise DEM data are absolutely essential to calculate eruption volume based on lava thickness estimations. This study is an example that demonstrates how InSAR will play a significant role in studying volcanoes in remote areas.

Mapping Three-Dimensional Surface Deformation by Combining Multiple-Aperture Interferometry and Conventional Interferometry: Application to the June 2007 Eruption of Kilauea Volcano, Hawaii

Surface deformation caused by an intrusion and small eruption during June 17-19, 2007, along the East Rift Zone of Kilauea Volcano, Hawaii, was three-dimensionally reconstructed from radar interferograms acquired by the Advanced Land Observing Satellite (ALOS) phased-array type L-band synthetic aperture radar (SAR) (PALSAR) instrument. To retrieve the 3-D surface deformation, a method that combines multiple-aperture interferometry (MAI) and conventional interferometric SAR (InSAR) techniques was applied to one ascending and one descending ALOS PALSAR interferometric pair. The maximum displacements as a result of the intrusion and eruption are about 0.8, 2, and 0.7 m in the east, north, and up components, respectively. The radar-measured 3-D surface deformation agrees with GPS data from 24 sites on the volcano, and the root-mean-square errors in the east, north, and up components of the displacement are 1.6, 3.6, and 2.1 cm, respectively. Since a horizontal deformation of more than 1 m was dominantly in the north-northwest-south-southeast direction, a significant improvement of the north-south component measurement was achieved by the inclusion of MAI measurements that can reach a standard deviation of 3.6 cm. A 3-D deformation reconstruction through the combination of conventional InSAR and MAI will allow for better modeling, and hence, a more comprehensive understanding, of the source geometry associated with volcanic, seismic, and other processes that are manifested by surface deformation.

Constraining the Slip Distribution and Fault Geometry of the Mw 7.9, 3 November 2002, Denali Fault Earthquake with Interferometric Synthetic Aperture Radar and Global Positioning System Data

The M w 7.9, Denali fault earthquake (dfe) is the largest continental strike-slip earthquake to occur since the development of Interferometric Synthetic Aperture Radar (Insar). We use five interferograms, constructed using radar images from the Canadian Radarsat-1 satellite, to map the surface deformation at the western end of the fault rupture. Additional geodetic data are provided by displacements observed at 40 campaign and continuous Global Positioning System (gps) sites. We use the data to determine the geometry of the Susitna Glacier fault, thrusting on which initiated the dfe, and to determine a slip model for the entire event that is consistent with both the Insar and gps data. We find there was an average of 7.3 ± 0.4 m slip on the Susitna Glacier fault, between 1 and 9.5 km depth on a 29 km long fault that dips north at 41 ± 0.7° and has a surface projection close to the mapped rupture. On the Denali fault, a simple model with large slip patches finds a maximum of 8.7 ± 0.7 m of slip between the surface and 14.3 ± 0.2 km depth. A more complex distributed slip model finds a peak of 12.5 ± 0.8 m in the upper 4 km, significantly higher than the observed surface slip. We estimate a geodetic moment of 670 ± 10 × 10 18 N m ( M w 7.9), consistent with seismic estimates. Lack of preseismic data resulted in an absence of Insar coverage for the eastern half of the dfe rupture. A dedicated geodetic Insar mission could obviate coverage problems in the future.

Ground deformation associated with the March 1996 earthquake swarm at Akutan volcano, Alaska, revealed by satellite radar interferometry

In March 1996 an intense swarm of volcano-tectonic earthquakes (∼3000 felt by local residents, Mmax = 5.1, cumulative moment of 2.7×1018 N m) beneath Akutan Island in the Aleutian volcanic arc, Alaska, produced extensive ground cracks but no eruption of Akutan volcano. Synthetic aperture radar interferograms that span the time of the swarm reveal complex island-wide deformation: the western part of the island including Akutan volcano moved upward, while the eastern part moved downward. The axis of the deformation approximately aligns with new ground cracks on the western part of the island and with Holocene normal faults that were reactivated during the swarm on the eastern part of the island. The axis is also roughly parallel to the direction of greatest compressional stress in the region. No ground movements greater than 2.83 cm were observed outside the volcanos summit caldera for periods of 4 years before or 2 years after the swarm. We modeled the deformation primarily as the emplacement of a shallow, east–west trending, north dipping dike plus inflation of a deep, Mogi-type magma body beneath the volcano. The pattern of subsidence on the eastern part of the island is poorly constrained. It might have been produced by extensional tectonic strain that both reactivated preexisting faults on the eastern part of the island and facilitated magma movement beneath the western part. Alternatively, magma intrusion beneath the volcano might have been the cause of extension and subsidence in the eastern part of the island. We attribute localized subsidence in an area of active fumaroles within the Akutan caldera, by as much as 10 cm during 1992–1993 and 1996–1998, to fluid withdrawal or depressurization of the shallow hydrothermal system.

Aseismic inflation of Westdahl volcano, Alaska, revealed by satellite radar interferometry

Westdahl volcano, located at the west end of Unimak Island in the central Aleutian volcanic arc, Alaska, is a broad shield that produced moderate-sized eruptions in 1964, 1978–79, and 1991–92. Satellite radar interferometry detected about 17 cm of volcano-wide inflation from September 1993 to October 1998. Multiple independent interferograms reveal that the deformation rate has not been steady; more inflation occurred from 1993 to 1995 than from 1995 to 1998. Numerical modeling indicates that a source located about 9 km beneath the center of the volcano inflated by about 0.05 km³ from 1993 to 1998. On the basis of the timing and volume of recent eruptions at Westdahl and the fact that it has been inflating for more than 5 years, the next eruption can be expected within the next several years.

Details of stress directions in the Alaska subduction zone from fault plane solutions

We used the cumulative misfit method to divide a data set with heterogeneous stress orientations into subsets with homogeneous stress directions. We propose that slope changes of the cumulative misfit as a function of earthquake number pinpoint the boundaries of homogeneous stresses. Using a synthetic data set of 50 fault plane solutions, composed of two halves corresponding to two incompatible stress tensors, we tested the validity and efficacy of the cumulative misfit method. Our acceptance criteria for results of stress tensor inversions are (1) the directions must be well constrained, that is, the 95% confidence regions of the greatest and least principal stresses do not overlap and (2) there must be evidence for homogeneity in the sample, that is, the average misfit of the inversion is less than 6°. We estimated stress directions in the Alaska Wadati-Benioff Zone (WBZ) using 470 fault plane solutions, which we determined for earthquakes with ML∼3, and 67 published fault plane solutions for earthquakes with Ms∼5. We succeeded in subdividing the data set of the small and large earthquakes into 25 and 3 subsets, with the average misfit ranging from 3.2° to 5.5°. The average misfits of the subsets we accept as satisfying the assumption of homogeneity are smaller than the average misfit of the overall data set (F∼10°) by factors of 2 to 3. We estimated the stress fields at two scales along the Alaska WBZ. The stress directions measured by the large earthquakes (Ms∼5) were homogeneous over large volumes, with extension downdip and the direction of greatest compression along strike. This unusual orientation of the greatest principal stress is attributed to the bend of the slab under central Alaska, which generates compressive stresses along strike. The stress orientations revealed by small earthquakes, in contrast, exhibited a great deal of heterogeneity as a function of space, although their general trend confirms the overall stress directions obtained from the large events. We propose that the ratio of the dimensions of the stress field sensed by earthquakes to the rupture dimensions is about 20 to 50. The estimated stress directions of the crustal earthquakes corresponded to the following mechanisms: (1) strike-slip faulting with the greatest principal stress oriented N-S near Fairbanks and (2) thrusting with the greatest principal stress oriented NW-SE near Mount McKinley.

Global link between deformation and volcanic eruption quantified by satellite imagery

A key challenge for volcanological science and hazard management is that few of the world’s volcanoes are effectively monitored. Satellite imagery covers volcanoes globally throughout their eruptive cycles, independent of ground-based monitoring, providing a multidecadal archive suitable for probabilistic analysis linking deformation with eruption. Here we show that, of the 198 volcanoes systematically observed for the past 18 years, 54 deformed, of which 25 also erupted. For assessing eruption potential, this high proportion of deforming volcanoes that also erupted (46%), together with the proportion of non-deforming volcanoes that did not erupt (94%), jointly represent indicators with ‘strong’ evidential worth. Using a larger catalogue of 540 volcanoes observed for 3 years, we demonstrate how this eruption–deformation relationship is influenced by tectonic, petrological and volcanic factors. Satellite technology is rapidly evolving and routine monitoring of the deformation status of all volcanoes from space is anticipated, meaning probabilistic approaches will increasingly inform hazard decisions and strategic development.

InSAR analysis of natural recharge to define structure of a ground‐water basin, San Bernardino, California

Using interferometric synthetic aperture radar (InSAR) analysis of ERS-1 and ERS-2 images, we detect several centimeters of uplift during the first half of 1993 in two areas of the San Bernardino ground-water basin of southern California. This uplift correlates with unusually high runoff from the surrounding mountains and increased ground-water levels in nearby wells. The deformation of the land surface identifies the location of faults that restrict ground-water flow, maps the location of recharge, and suggests the areal distribution of fine-grained aquifer materials. Our preliminary results demonstrate that naturally occurring runoff and resultant recharge can be used with interferometric deformation mapping to help define the structure and important hydrogeologic features of a ground-water basin. This approach may be particularly useful in investigations of remote areas with scant ground-based hydrogeologic data.