G13B-0652
Comparing C and L band InSAR observations of volcanic deformation in South America
volcanic regions of the central and southern volcanic zones of South America incorporate about 2500 volcanic edifices. Only 10-20% of these are considered "potentially active," but this classification is incomplete as volcanoes that are not believed to be active can erupt or show seismic or deformation activity. To perform a more synoptic survey, we use satellite-based interferometric synthetic aperture radar (InSAR) to observe nearly all 2500 volcanoes for surface displacements. We compare results from the European ERS and Envisat satellites which use C-band radars (roughly 6 cm wavelength) with data from the Japanese ALOS satellite which has an L-band radar (24 cm wavelength). All satellites produce coherent interferograms in the generally arid central Andes. In the more humid climate of the southern Andes, the L-band interferograms are more coherent than the C-band ones. Our combined observations reveal previously undocumented volcanic/hydrothermal deformation. Further, we compare the differing styles of ground deformation observed during several recent volcanic unrest episodes or eruptions, including: Ticsani, Peru; Ubinas, Peru; Llaima, Chile; and Chaiten, Chile. Finally, our ALOS observations provide insights into the future potential of NASA's proposed DESDynI mission. For example, while the ALOS interferograms are of high quality even in the southern Andes, increased frequency of observations, consistent observation modes, and better baseline control would facilitate measurements of magma movements and response to volcanic crises.
G13B-0653
A Survey of Volcanic Deformation on Java Using ALOS InSAR
Of the hundreds of volcanic centers throughout the Indonesian archipelago, few are adequately monitored for pre-eruptive activity due to socioeconomic and logistical barriers, with the result that volcanic hazards in the region are not well quantified. The advent of satellite-borne L-band synthetic aperture radar, which due to its long wavelength is less sensitive to changes in vegetation, provides an opportunity for detection and measurement of volcanic deformation over broad regions in heavily vegetated tropical island arcs. We use data from the Japanese ALOS PALSAR instrument to conduct a comprehensive survey of volcanic deformation on the Indonesian island of Java, over a time period of 1-2 years. Consistent with previous results from other regions, our preliminary results suggest that volcanoes experiencing frequent, small eruptions are typically fed by magma bodies too small and/or too shallow or deep to produce a recognizable InSAR signal. However, we identified at least one deformation event on Java which is likely linked to a magmatic intrusion at 5-10 km depth. This initial test of a broad application of L-band data allowed us to better define the satellite imaging criteria required for successful observation, as well as necessary improvements to the InSAR processing algorithms originally designed for C-band data.
G13B-0654
Multi-scale analysis of InSAR time series to estimate variations in topographically correlated propagation delays with application to the Makran Subduction Zone
Many InSAR observations are plagued by propagation delays that correlate with topographic variations. These delays are frequently termed tropostatic delays and are assumed to result from temporal variations in horizontal stratification of the troposphere. Assuming a linear model between topography and phase, we present a robust approach to estimating the transfer function (K) that is relatively insensitive to confounding processes (e.g., earthquake deformation, phase ramps from orbit errors, etc). Our approach takes advantage of a multiscale perspective by adopting wavelet decomposition of both topography and observed phase. By decomposing topography and observed phase in a given interferogram into several spatial scales, we determine the bands spanning different characteristic length scales wherein correlation between topography and phase is significant and stable. Our approach also uses the inherent redundancy provided by multiple interferograms constructed with common scenes. We define a unique set of component time intervals, Tint, using a suit of interferometric pairs. The ensemble of pair-based Kpair are then combined to estimate a temporally consistent K for each time interval (Kint). The ensemble of Kint are then recombined to make a final consistent set of Kscene in order to correct each interferogram. We are testing our approach in the region of the Makran subduction zone, located in western Pakistan and eastern Iran, within the influence zone of South Asian monsoon. We use twenty-nine ENVISAT images to develop the time series. Preliminary results find large variations in estimates of Kpair. Generally, the tropostatic correction accounts for a relatively small portion of the observed phase, although significant effects are found for selected pairs. The typically small impact of the tropostatic correction implies that in the future we must consider more complex dynamic atmospheric models.
G13B-0655
Analysis of the 2000 2002 earthquake sequence along the Sultan Dag front (southern Turkey): Fault interactions and incremental growth of a fault-block mountain range
The Sultan Dag range of southern Turkey is a fault-block mountain range that has developed within or at the edge of the Isparta Angle a region of the Anatolian plate situated near a tear in the subducting slab of the African plate. The mountain range is bounded on the northeast by the Sultandagi-Aksehir fault and the Aksehir-Afyon graben. A sequence of moderate earthquakes that occurred between 2000 and 2002 provides insight into the incremental growth of the mountain front through individual seismic events. The earthquake sequence consists of three moderate size events: a Mw 5.1 and a Mw 6.0 earthquake (December 15, 2000) and a Mw 6.5 earthquake (February 3, 2002). Surface deformations corresponding with the individual earthquakes were imaged using Interferometric Synthetic Aperture Radar (InSAR). The resulting displacement maps provided a basis for estimating faulting parameters associated with each earthquake (fault orientation and slip) using elastic dislocation models. These fault models were subsequently used to estimate static coulomb stress changes resulting from each of the earthquakes. The findings suggest that the two events of December 15, 2000, were not directly related to one another in terms of stress triggering. However, both events from 2000 appear to influence the extent of the 2002 earthquake. The larger event increased the stress on the segment ruptured in 2002, whereas the stress shadow from smaller event, which occurred on an antithetic fault, appears to have unloaded the stress at the termination of the 2002 rupture. Furthermore, the uplift and subsidence patterns associated with the earthquakes closely mimic overall geologic structure and general topography. Hence, these results permit exploring the extrapolation of coseismic displacements to understand the long-term growth of the mountain front.
G13B-0656
Crustal Deformation due to Ice Mass Unloading at Jakobshavn Isbrae, Greenland, Measured With ERS/Envisat/Radarsat SAR Interferometry
Glaciers are among the most visible indicators of the effects of climate change. Jakobshavn Isbrae, the largest outlet glacier in Greenland, is observed to have been undergoing dramatic thinning and acceleration in speed in recent years. The changes in Jakobshavn Isbrae are likely important indicators of the dynamic response of the Greenland ice sheet to warming temperatures. And the rapid response may have a significant impact on the mass imbalance of the entire Greenland ice sheet. Using InSAR (Interferometric Synthetic Aperture Radar) processing, we study the crustal deformation near Jakobshavn Isbrae, with the goal of placing constraints on the ice mass loss of that glacier and the surrounding region. The biggest challenge of applying InSAR to this unloading problem is to distinguish the deformation signals, which are predominantly long-wavelength in the spatial domain, from the satellite orbit errors (or InSAR baseline errors). By stacking ERS and ENVISAT SAR interferograms, we can reduce the orbit errors and the atmospheric artifacts, since they both vary randomly in the time domain. For RADARSAT- 1 SAR data, whose orbit errors are significantly larger than the deformation signals, we estimate interferogram baselines empirically and study the short-wavelength components instead. Using the Lagrange Multiplier method, we incorporate several ground control points, including observations from one permanent GPS station (KAGA), into the baseline fitting in order to reconstruct some of the longer-wavelength components. We find good agreement between a variety of InSAR estimates of the secular deformation rates at different wavelengths and the corresponding deformation models based on independent ice elevation change measurements from NASA's Airborne Topographic Mapper (ATM).
G13B-0657
Determining glacier flow with novel polar GPS systems
At the junction between ice sheets, oceans, and atmosphere, fast-moving outlet glaciers can respond to environmental changes operating over a wide range of timescales, thus providing critical insight into the state of those systems. We have developed a high-accuracy, low-cost, L1-only GPS receiving system that enables determination of glacier flow with both high temporal sampling and high spatial density. The system includes two-way communications, thus also enabling real-time monitoring and remote GPS data retrieval. We deployed two networks of such GPS instruments, at Helheim Glacier and Kangerdlugssuaq Glacier, Greenland, during the summer of 2008. Although these two large outlet glaciers have been shown to exhibit the rapid motions associated with glacial earthquakes as well as large interannual variations in flow speed, little is known about their seasonal behavior. We will describe the GPS systems, which could be adapted to other applications in which the risk of instrument loss is large, and present preliminary results pertaining to glacier flow from the kinematic analysis of GPS data collected by those systems.
G13B-0658
Geodetic Fault Modelling of the 1997 Zirkuh Earthquake (Iran) using InSAR data from the ERS-1/2 and JERS-1 Satellites
We present source modelling results of the Mw=7.2 Zirkuh Earthquake based on the surface deformation measured with InSAR. This large strike-slip event occurred on 10 May 1997 and caused a 125 km superficial rupture along the Abiz Fault in Northeast Iran. Despite the seismic hazard of the Abiz Fault is the highest in Iran and the surface rupture of this event is the longest observed in Iran so far, the Zirkuh earthquake has not been studied extensively. Field observations along the spatially complex surface trace have revealed large variations in both slip and rake along the fault trace. In addition, an existing Moment-Tensor solution, based on teleseismic data excited by the uni-lateral rupture, shows that the data can be better explained when assuming four subsequent ruptures rather than assuming only one dominant mechanism. To learn more details about the Zirkuh event we image its source using InSAR observations of the coseismic surface deformation. The archived InSAR data of the study region are relatively limited so that the coherence in C-band interferograms suffers from long temporal and/or orbital baselines. When combining C-band ERS data with L- band JERS data, which are less sensitive to small changes on the ground, we obtain a much better data coverage. Moreover, the L-band JERS data help to measure large surface deformation and they therefore provide data very close to the fault. We use three descending interferograms and one ascending interferogram in our source modelling. The data quality in terms of coherence, atmospheric and orbital errors varies strongly between the data sets. Therefore, to combine the data in the fault modelling in a meaningful way, we analyze the noise structure in each interferogram and estimate the corresponding auto-covariance weighting functions. The ascending InSAR data cover only the southernmost part of the fault whereas two descending ERS interferograms and one JERS interferogram completely cover the coseismic surface deformation. The main features of our preliminary fault model are consistent with the four moment tensors from teleseismic inversion. Along the curved fault trace we find a change in the fault dip from a west-dipping fault in the north to an east-dipping fault in the south. This change in dip is associated with changes in faulting style from predominantly strike-slip along the northern half of the fault to highly oblique mechanisms further south, to reverse faulting along the southernmost part of the fault.
G13B-0659
New Global Bathymetry and Topography Model Grids
A new version of the "Smith and Sandwell" global marine topography model is available in two formats. A one-arc-minute Mercator projected grid covering latitudes to +/- 80.738 degrees is available in the "img" file format. Also available is a 30-arc-second version in latitude and longitude coordinates from pole to pole, supplied as tiles covering the same areas as the SRTM30 land topography data set. The new effort follows the Smith and Sandwell recipe, using publicly available and quality controlled single- and multi-beam echo soundings where possible and filling the gaps in the oceans with estimates derived from marine gravity anomalies observed by satellite altimetry. The altimeter data have been reprocessed to reduce the noise level and improve the spatial resolution [see Sandwell and Smith, this meeting]. The echo soundings database has grown enormously with new infusions of data from the U.S. Naval Oceanographic Office (NAVO), the National Geospatial-intelligence Agency (NGA), hydrographic offices around the world volunteering through the International Hydrographic Organization (IHO), and many other agencies and academic sources worldwide. These new data contributions have filled many holes: 50% of ocean grid points are within 8 km of a sounding point, 75% are within 24 km, and 90% are within 57 km. However, in the remote ocean basins some gaps still remain: 5% of the ocean grid points are more than 85 km from the nearest sounding control, and 1% are more than 173 km away. Both versions of the grid include a companion grid of source file numbers, so that control points may be mapped and traced to sources. We have compared the new model to multi-beam data not used in the compilation and find that 50% of differences are less than 25 m, 95% of differences are less than 130 m, but a few large differences remain in areas of poor sounding control and large-amplitude gravity anomalies. Land values in the solution are taken from SRTM30v2, GTOPO30 and ICESAT data. GEBCO has agreed to adopt this model and begin updating it in 2009. Ongoing tasks include building an uncertainty model and including information from the latest IBCAO map of the Arctic Ocean.
G13B-0660
Accuracy estimation of SRTM and Map-based DEMs using ICESat elevation data in Tibetan Plateau
The Geoscience Laser Altimeter System (GLAS) instrument onboard the Ice, Cloud, and land Elevation
Satellite (ICESat) provides elevation data with very high accuracy and can be used as ground truth to
evaluate the vertical accuracy of an existing Digital Elevation Model (DEM). In this paper, we examine the
differences between ICESat elevation data (from the 1064 nm channel) and Shuttle Radar Topography
Mission (SRTM) 3-arc second resolution (90 m) and Map-based DEMs in the Qinghai-Tibet Plateau, China.
Both DEMs are linearly correlated with ICESat elevation for different land covers, while the SRTM DEM shows
a stronger correlation than that of Map-based DEM on each land cover type. The statistics indicated that
land cover, surface slope and roughness do influence the vertical accuracy of the two DEMs. The standard
deviation of the elevation differences between the two DEMs and ICESat elevation gradually increases as
vegetation stands, terrain slope or surface roughness increases. The SRTM DEM consistently shows lower
vertical error than that of the Map-based DEM. The overall means and standard deviations of the elevation
differences between ICESat and SRTM and between ICESat and Map-based DEMs over the study area are
0.63 ?0?6?067673?0?6?067698 9.67 m and 4.44 ?0?6?067673?0?6?067698 21.19 m,
respectively. Our results suggest that the SRTM DEM has higher accuracy than the Map-based DEM that we
collected for the region. It is found that ICESat elevation increases as snow falling and decreases as snow or
glacier melting, while the SRTM DEM gives a relative stable elevation of snow/land interface or a glacier
elevation where the C-band can penetrate through or reach at. Therefore, this makes the S-DEM a promise
dataset (baseline) in monitoring the glacier volume change since 2000.
http://www.utsa.edu/LRSG/
G13B-0661
Sea Level Budget over 2003-2008: a reevaluation from GRACE space gravimetry, satellite altimetry and Argo
From the IPCC 4th Assessment Report published in 2007, ocean thermal expansion contributed by ~50 percent to the 3.1 mm/yr observed global mean sea level rise during the 1993-2003 decade, the remaining rate of rise being essentially explained by shrinking of land ice. Recently published results suggest that since about 2003, ocean thermal expansion change, based on the newly deployed Argo system, is stalling while sea level is still rising, although at a reduced rate (2.5 mm/yr). Using space gravimetry observations from GRACE, we show that recent years sea level rise can be mostly explained by an increase of the mass of the oceans. Estimating GRACE-based ice sheet mass balance and using published estimates for glaciers melting, we further show that ocean mass increase since 2003 results by about half from an enhanced contribution of the polar ice sheets -compared to the previous decade- and half from mountain glaciers melting. Taking also into account the small GRACE-based contribution from continental waters (0.2 mm/yr), we find a total ocean mass contribution of ~2 mm/yr over 2003-2008. Such a value represents ~80 percent of the altimetry-based rate of sea level rise over that period. We next estimate the steric sea level (i.e., ocean thermal expansion plus salinity effects) contribution from: (1) the difference between altimetry-based sea level and ocean mass change and (2) Argo data. Inferred steric sea level rate from (1) (~ 0.3 mm/yr over 2003-2008) agrees well with the Argo-based value also estimated here (0.37 mm/yr over 2004-2008). Furthermore, the sea level budget approach presented in this study allows us to constrain independent estimates of the Glacial Isostatic Adjustment (GIA) correction applied to GRACE-based ocean and ice sheet mass changes, as well as of glaciers melting. Values for the GIA correction and glacier contribution needed to close the sea level budget and explain GRACE-based mass estimates over the recent years agree well with totally independent determinations.