G13A-01 INVITED
Taking Measure of Natural Hazards with Satellite InSAR Imagery
Interferometric synthetic aperture radar (InSAR) imaging is a new, all-weather geodetic technique capable of measuring ground-surface deformation with centimeter to subcentimeter vertical precision and spatial resolution of tens-of-meter over a relatively large region. The spatial distribution of surface deformation data, derived from InSAR images acquired by European Remote Sensing Satellite (ERS)-1, ERS-2, Environmental Satellite (Envisat), Japanese Earth Resources Satellite (JERS)-1, and Canadian Radar Satellite (Radarsat)-1 enables the construction of detailed mechanical models to enhance the study of natural hazards induced by volcanic, seismic, landslide, and hydrogeologic processes. Measuring how a volcano's surface deforms before, during, and after eruptions provides essential information about magma dynamics and a basis for mitigating volcanic hazards. Measuring spatial and temporal patterns of surface deformation in seismically active regions is extraordinarily useful for understanding rupture dynamics and estimating seismic risks. Measuring how landslides develop and activate is a prerequisite to minimize associated hazards. Mapping surface subsidence and uplift related to extraction and injection of fluids in groundwater aquifers provides fundamental data on aquifer properties and processes and improves our ability to mitigate undesired consequences. Monitoring dynamic water-level changes beneath wetlands improves hydrological modeling predictions and enhances the assessment of future flood events over wetlands. We conclude that evolving satellite radar imagery combined with InSAR techniques are powerful tools for studying ground surface deformation associated with various natural hazards and will play an increasingly important role in better understanding, and eventually forecasting, natural hazards.
G13A-02
Assessing Fuel Moisture With Satellite Imaging Radar for Improved Fire Danger Prediction in Boreal Alaska
Wildfire is a common occurrence in boreal regions and Alaskan natural resource management agencies devote considerable resources to fire management and suppression. Currently these agencies rely on the Canadian Forest Fire Danger Rating System's Fire Weather Index (FWI) for the assessment of the potential for wildfire. FWI is based solely on point source weather data collected daily in a sparse network across the state of Alaska. There are problems with the current FWI system, particularly in the determination of the spring start up values and problems mid-summer within permafrost regions. Melting permafrost causes increased moisture not accounted for in the weather-based system. The drought code (DC), which is an estimate of moisture in the deep compact duff layers, is the most affected by the default start up values because it has a 52 day lag period. Research has been conducted to improve the prediction of wildfire potential in Alaska using satellite c-band (5.3 cm wavelength) imaging radar. Imaging radar is sensitive to the moisture content of the features being imaged including vegetation and soils. We have been investigating the relationship between in situ soil moisture, c-band backscatter and fire danger codes for several years at a variety of burned and unburned sites in interior Alaska. Focus has been on recently burned (0-7 years) boreal forests because they allow moisture in the ground layer to be measured directly from a satellite sensor without interference of the forest canopy, and because they are a common feature across the Alaskan landscape. Studies of unburned forests adjacent to burned forests have revealed similarities in the temporal patterns of in situ moisture monitored throughout a fire season. Our research has resulted in the development of algorithms to predict DC from c-band backscatter. This will improve current weather-based estimates by providing a means for calibration of the DC throughout the season, and add additional point-sources of fuel moisture estimation. While the FWI codes provide good indicators of general fuel moisture, they do not depict the spatially varying patterns of fuel moisture across a landscape. Knowing the spatial variation in fuel moisture is important when land managers determine fire danger, prescribe a burn, or predict fire behavior. Techniques have been developed to map spatially varying soil moisture across a burned landscape using a combination of Landsat and c-band imaging radar. Further analysis of radar data acquired in unburned boreal forests is also underway. Time series analysis is proving to be instrumental in deriving spatial soil moisture information across an entire landscape from radar satellite imagery, since each location is compared to itself through time rather than to other surrounding locations. This allows the time-variant feature of soil moisture to be revealed while minimizing the time-invariant features that confound radar backscatter such as biomass and surface roughness.
G13A-03 INVITED
Rising through the Stack: Assessing the Scientific Value of High-Rate GPS
It has been demonstrated that high-rate GPS provides useful measurements of seismic waves in cases where seismic instrumentation clips. Even so, intrinsic questions about high-rate GPS remain to be answered. Are there any specific scientific questions that can be answered better with high-rate GPS data than with other instruments? What is the error spectrum of high-rate GPS positions? How degraded are real-time GPS positions relative to the best post-processed positions? Examples from our analysis of high-rate GPS data from recent earthquakes will be discussed, including the impact of modified sidereal filtering and stacking.
G13A-04
Impact of GPS-Based Water-Vapor Data on Quantitative Precipitation Forecasts in Southern California
Cold season flash flooding is one of the leading natural hazards in southern California. Although overall precipitation is relatively small, this region is characterized by a high frequency of heavy downpours during the wet season that, in conjunction with steep topography, often causes massive flash flooding. As many of the densely-populated areas are located in valleys and on surrounding hillsides, heavy precipitation events that last only a few hours can cause substantial amounts of damage. In fact, of the 10 highest 24-hour rainfall totals in California, all 10 have occurred in southern California. Hence, accurate quantitative precipitation forecasts (QPF) are crucial for mitigating hazards due to flash flooding in this region. For improving QPF in southern California, a fine-resolution numerical weather prediction model and accurate initial water vapor data are crucial. The Southern California Integrated GPS Network (SCIGN) can provide local precipitable water vapor (PWV) data at high spatial and temporal resolution and is a cost-effective source of water vapor data for improving regional QPF. In this study, we examine the impact of additional water vapor data obtained from the SCIGN array on regional QPF in southern California. Comparison of regional simulations generated with and without GPS-PWV data in a number of case studies shows that the additional GPS-derived water vapor data tends to improve QPF compared to simulations initialized with only NCEP/ETA operational forecast data. It is also found that due to the limited spatial coverage of the SCIGN array, in particular the lack of GPS receivers over the adjacent Pacific, the impact of the GPS-PWV data on regional QPF lasts for only the first 6 hours after initialization.
G13A-05
A Canadian Pilot Project for a GPS-Augmented Tsunami Warning System
Whether a tsunami has been generated from a large earthquake immediately offshore cannot be determined within a crucial time window from seismic or tide gauge data alone. Geological and geodetic data show that coseismic motions of the Earth's surface even hundreds of kilometers from the fault can be used to determine the nature of the rupture and whether the earthquake is tsunamigenic. High-rate (1 sps or greater), totally autonomous GPS stations located along the coast can provide on-line streamed data that can be analyzed in real time to provide an update of relative positions to an accuracy of 1-2 cm horizontally and 3-5 cm vertically with a latency of a few seconds. Regional ground displacements along the coast at the time of a major offshore earthquake could therefore point to the certainty of a tsunami within less than a minute. The Geological Survey of Canada is currently setting up a prototype network to facilitate real-time positioning along the coast of the Canadian segment of the Cascadia subduction zone. The aim is to evaluate the realizability and effectiveness of automatically determining major vertical and horizontal motion at coastal versus inland GPS stations that would unambiguously and rapidly indicate tsunami generation. A network of GPS receivers purchased by the Canadian Hydrographic Service is currently being deployed at geodetic quality installations with continuous on-line communications. As part of this system, we are implementing real-time GPS technology previously applied to seismic (Bock et al., 2004; Langbein and Bock, 2004; Yamagiwa et al., 2004) and volcanic (Mattia et al., 2004) monitoring applications. The target date to have a prototype system operating is December 2005. It is hoped that this relatively low cost technique can become a mainstream tool of tsunami warning systems worldwide.
G13A-06 INVITED
Enhancing Tsunami Warning With Real Time GPS Magnitudes for the Largest Earthquakes
Rapid determination of earthquake magnitude from seismic data alone is straightforward for most earthquakes, but remains problematic for the largest events. In large part this is due to the increase in low-frequency energy for the largest earthquakes, which means longer recording times are needed to reliably estimate the moment (proportional to the static slip) from dynamic seismic waveforms. Although new techniques are being developed now to rapidly estimate rupture duration and rupture length (a reasonable proxy for magnitude for the largest events), the near-real-time magnitude estimate for the disastrous December 2004 Sumatra-Andaman earthquake, the initial magnitude estimate was dramatically underestimated. Although the most critical failure in that event was the lack of a regional and local tsunami warning system, this event shows that we need to improve rapid magnitude estimation for the largest events. Real time GPS data from near-regional distances can reliably determine magnitude for the largest events within minutes of the earthquake, with no upper limit on magnitude. In fact, the larger the earthquake, the better GPS will perform because the signal to noise ratio increases. This makes real time GPS recordings highly complementary to seismic methods. Displacement records from large earthquakes show that a reliable estimate of the static displacement can be made as soon as the large dynamic displacements at a site die down. Even an extremely simple point source inversion, requiring a hypocenter location but no knowledge of fault mechanism, can provide an estimate of moment to within about 20-30%, and can provide starting values for a finite fault inversion that can determine moment to better than 10%. Simple inversions work because no possible earthquake with a small moment, regardless of mechanism, can cause large displacements a few hundred km from the fault. The critical requirement is to have real-time GPS data from multiple sites that are close enough to the rupture to have several cm to a few decimeters of static displacement. This can be accomplished by having a network of real-time GPS sites at a spacing of 100-200 km across potentially tsunamigenic subduction zones. For rapid magnitude determination for the largest events, sites farther from the trench than the volcanic arc simplify the inversion, but sites closer to the trench are valuable for resolving details of the slip and may constrain uplift related to tsunamigenesis.
G13A-07 INVITED
High Rate GPS on Volcanoes
The high rate GPS data processing can be considered as the "new deal" in geodetic monitoring of active volcanoes. Before an eruption, infact, transient episodes of ground displacements related to the dynamics of magmatic fluids can be revealed through a careful analysis of high rate GPS data. In the very first phases of an eruption the real time processing of high rate GPS data can be used by the authorities of Civil Protection to follow the opening of fractures field on the slopes of the volcanoes. During an eruption large explosions, opening of vents, migration of fractures fields, landslides and other dangerous phenomena can be followed and their potential of damage estimated by authorities. Examples from the recent eruption of Stromboli volcano and from the current activities of high rate GPS monitoring on Mt. Etna are reported, with the aim to show the great potential and the perspectives of this technique.