Juliet Biggs

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.

Interseismic slip rate of the northwestern Xianshuihe fault from InSAR data

[1]xa0The Xianshuihe fault is a highly active strike-slip fault system near the eastern margin of the Tibetan plateau. We use a multi-interferogram method to construct a map of line-of-sight deformation rate due to interseismic strain accumulation on the northwestern Xianshuihe fault from a decade of SAR data acquired by the ERS-1/2 and Envisat satellites. The rate map shows a clear deformation gradient associated with the fault, but slightly offset to the northeast from the surface trace. Joint inversion of InSAR rate map and GPS data using Monte-Carlo method, we estimate a slip rate of 9-12 mm/yr with a locking depth of 3-6 km at the 90% confidence level. The slip rate agrees with the Holocene fault slip rate and the historical earthquakes. Our results also suggest no significant across-fault extension. In the future, InSAR data from both ascending and descending orbits may further constrain the 3-D fault slip rate on this fault.

Applicability of InSAR to tropical volcanoes: insights from Central America

Abstract Measuring volcano deformation is key to understanding the behaviour of erupting volcanoes and detecting those in periods of unrest. Satellite techniques provide the opportunity to do so on a global scale but, with some notable exceptions, the deformation of volcanoes in the tropics has been understudied relative to those at higher latitudes, largely due to technical difficulties in applying Interferometric Synthetic Aperture Radar (InSAR). We perform a systematic survey of the Central American Volcanic Arc to investigate the applicability of Interferometric Synthetic Aperture Radar (InSAR) to volcanoes in the tropics. Volcano characteristics that may prevent InSAR measurement include: (1) dense vegetation cover; (2) persistent activity; and (3) steep slopes. Measurements of deformation are further inhibited by atmospheric artefacts associated with: (4) large changes in topographical relief. We present a systematic method for distinguishing between water vapour artefacts and true deformation. Our data show a linear relationship (c. 2 cm/km) between the magnitudes of water vapour artefacts and volcano edifice height. For high relief volcanoes (e.g. Fuego, Guatemala, 3763 m a.s.l. (above sea level)) errors are of the order of 4–5 cm across the volcanos edifice but are less than 2 cm for lower relief (e.g. Masaya, Nicaragua, 635 m a.s.l.). Examples such as Arenal, Atitlan and Fuego illustrate that satellite acquisition strategies incorporating ascending and descending tracks are particularly important for studying steep-sided volcanoes. Poor coherence is primarily associated with temporal decorrelation, which is typically more rapid in southern Central America where Evergreen broadleaf vegetation dominates. Land-use classification is a better predictor of decorrelation rate than vegetation index. Comparison of coherence for different radar wavelengths match expectations; high resolution X-band radar is best suited to local studies where high-resolution digital elevation models (DEMs) exist, while L-band wavelengths are necessary for regional surveys. However, this is the first time that relationships between phase coherence and time, perpendicular baseline, radar wavelength, and land use have been quantified on the scale of a whole volcanic arc.

From quiescence to unrest: 20 years of satellite geodetic measurements at Santorini volcano, Greece

Periods of unrest at caldera-forming volcanic systems characterized by increased rates of seismicity and deformation are well documented. Some can be linked to eventual eruptive activity, while others are followed by a return to quiescence. Here we use a 20u2009year record of interferometric synthetic aperture radar (InSAR) and GPS measurements from Santorini volcano to further our understanding of geodetic signals at a caldera-forming volcano during the periods of both quiescence and unrest, with measurements spanning a phase of quiescence and slow subsidence (1993–2010), followed by a phase of unrest (January 2011 to April 2012) with caldera-wide inflation and seismicity. Mean InSAR velocity maps from 1993–2010 indicate an average subsidence rate of ~6u2009mm/yr over the southern half of the intracaldera island Nea Kameni. This subsidence can be accounted for by a combination of thermal contraction of the 1866–1870 lava flows and load-induced relaxation of the substrate. For the period of unrest, we use a joint inversion technique to convert InSAR measurements from three separate satellite tracks and GPS observations from 10 continuous sites into a time series of subsurface volume change. The optimal location of the inflating source is consistent with previous studies, situated north of Nea Kameni at a depth of ~4u2009km. However, the time series reveals two distinct pressure pulses. The first pulse corresponds to a volume change (ΔV) within the shallow magma chamber of (11.56u2009±u20090.14)u2009×u2009106u2009m3, and the second pulse has a ΔV of (9.73u2009±u20090.10)u2009×u2009106u2009m3. The relationship between the timing of these pulses and microseismicity observations suggests that these pulses may be driven by two separate batches of magma supplied to a shallow reservoir. We find no evidence suggesting a change in source location between the two pulses. The decline in the rates of volume change at the end of both pulses and the observed lag of the deformation signal behind cumulative seismicity, suggest a viscoelastic response. We use a simple model to show that two separate pulses of magma intruding into a shallow magma chamber surrounded by a viscoelastic shell can account for the observed temporal variation in cumulative volume change and seismicity throughout the period of unrest. Given the similarities between the geodetic signals observed here and at other systems, this viscoelastic model has potential use for understanding behavior at other caldera systems.

Causes of unrest at silicic calderas in the East African Rift: New constraints from InSAR and soil‐gas chemistry at Aluto volcano, Ethiopia

Restless silicic calderas present major geological hazards, and yet many also host significant untapped geothermal resources. In East Africa, this poses a major challenge, although the calderas are largely unmonitored their geothermal resources could provide substantial economic benefits to the region. Understanding what causes unrest at these volcanoes is vital for weighing up the opportunities against the potential risks. Here we bring together new field and remote sensing observations to evaluate causes of ground deformation at Aluto, a restless silicic volcano located in the Main Ethiopian Rift (MER). Interferometric Synthetic Aperture Radar (InSAR) data reveal the temporal and spatial characteristics of a ground deformation episode that took place between 2008 and 2010. Deformation time series reveal pulses of accelerating uplift that transition to gradual long-term subsidence, and analytical models support inflation source depths of ∼5 km. Gases escaping along the major fault zone of Aluto show high CO2 flux, and a clear magmatic carbon signature (CO2-δ13C of −4.2‰ to −4.5‰). This provides compelling evidence that the magmatic and hydrothermal reservoirs of the complex are physically connected. We suggest that a coupled magmatic-hydrothermal system can explain the uplift-subsidence signals. We hypothesize that magmatic fluid injection and/or intrusion in the cap of the magmatic reservoir drives edifice-wide inflation while subsequent deflation is related to magmatic degassing and depressurization of the hydrothermal system. These new constraints on the plumbing of Aluto yield important insights into the behavior of rift volcanic systems and will be crucial for interpreting future patterns of unrest.

A Tremor and Slip Event on the Cocos-Caribbean Subduction zone as measured by a GPS and Seismic Network on the Nicoya Peninsula, Costa Rica

In May 2007 a network of global positioning systems (GPS) and seismic stations on the Nicoya Peninsula, of northern Costa Rica, recorded a slow-slip event accompanied by seismic tremor. The close proximity of the Nicoya Peninsula to the seismogenic part of the Cocos-Caribbean subduction plate boundary makes it a good location to study such events. Several centimeters of southwest motion were recorded by the GPS stations over a period of several days to several weeks, and the seismic stations recorded three distinct episodes of tremor during the same time span. Inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25–30 km, downdip of the main seismogenic zone. Estimated temperatures here are ∼250°–300°C, lower than in other subduction zones where events of this nature have been previously identified. There may also be a shallower patch of slip at ∼6 km depth. These results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding, and that a critical temperature threshold is not required for slow slip. Tremor and low-frequency earthquake locations are more difficult to determine. Our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip, but not spatially colocated.

InSAR phase unwrapping based on extended Kalman filtering

InSAR phase measurements are relative, ambiguous due to 2π wrapping, and affected by atmospheric and other noise sources. For many real-world applications unambiguous displacement measurements are necessary. Currently available methods for extracting time series of surface displacement, start with unwrapping in time or space domain. We present a new extended Kalman filtering approach to phase unwrapping, which resolves the 2π ambiguity in a computationally efficient way. Also this algorithm can easily be expanded to use all the data available in space and time dimensions. Several modifications are made to the standard EKF for phase unwrapping. Hence we call our new algorithm as optimized Kalman (OK) filter.

A slow slip and tremor event in May 2007, Costa Rica margin

In May 2007 a network of global positioning systems (GPS) and seismic stations on the Nicoya Peninsula, of northern Costa Rica, recorded a slow-slip event accompanied by seismic tremor. The close proximity of the Nicoya Peninsula to the seismogenic part of the Cocos-Caribbean subduction plate boundary makes it a good location to study such events. Several centimeters of southwest motion were recorded by the GPS stations over a period of several days to several weeks, and the seismic stations recorded three distinct episodes of tremor during the same time span. Inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25–30 km, downdip of the main seismogenic zone. Estimated temperatures here are ∼250°–300°C, lower than in other subduction zones where events of this nature have been previously identified. There may also be a shallower patch of slip at ∼6 km depth. These results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding, and that a critical temperature threshold is not required for slow slip. Tremor and low-frequency earthquake locations are more difficult to determine. Our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip, but not spatially colocated.