• Advancements in seismic tomography with application to tunnel detection and volcano imaging

      Clippard, James Doyle; Christensen, Douglas H.; Pulpan, Hans; Barry, Ronald P.; Harisen, Roger A.; Eichelberger, John C. (1998)
      Practical geotomography is an inverse problem with no unique solution. A priori information must be imposed for a stable solution to exist. Commonly used types of a priori information smooth and attenuate anomalies, resulting in 'blurred' tomographic images. Small or discrete anomalies, such as tunnels, magma conduits, or buried channels are extremely difficult imaging objectives. Composite distribution inversion (CDI) is introduced as a theory seeking physically simple, rather than distributionally simple, solutions of non-unique problems. Parameters are assumed to be members of a composite population, including both well-known and anomalous components. Discrete and large amplitude anomalies are allowed, while a well-conditioned inverse is maintained. Tunnel detection is demonstrated using CDI tomography and data collected near the northern border of South Korea. Accurate source and receiver location information is necessary. Borehole deviation corrections are estimated by minimizing the difference between empirical distributions of apparent parameter values as a function of location correction. Improved images result. Traveltime computation and raytracing are the most computationally intensive components of seismic tomography when imaging structurally complex media. Efficient, accurate, and robust raytracing is possible by first recovering approximate raypaths from traveltime fields, and then refining the raypaths to a desired accuracy level. Dynamically binned queuing is introduced. The approach optimizes graph-theoretic traveltime computation costs. Pseudo-bending is modified to efficiently refine raypaths in general media. Hypocentral location density functions and relative phase arrival population analysis are used to investigate the Spring, 1996, earthquake swarm at Akutan Volcano, Alaska. The main swarm is postulated to have been associated with a 0.2 km$\sp3$ intrusion at a depth of less than four kilometers. Decay sequence seismicity is postulated to be a passive response to the stress transient caused by the intrusion. Tomograms are computed for Mt. Spurr, Augustine, and Redoubt Volcanoes, Alaska. Relatively large amplitude, shallow anomalies explain most of the traveltime residual. No large amplitude anomalies are found at depth, and no magma storage areas are imaged. A large amplitude low-velocity anomaly is coincident with a previously proposed geothermal region on the southeast flank of Mt. Spurr. Mt. St. Augustine is found to have a high velocity core.
    • Brine Percolation, Flooding And Snow Ice Formation On Antarctic Sea Ice

      Maksym, Ted; Jeffries, Martin O. (2001)
      Modelling studies of brine percolation, flooding, and snow ice formation on Antarctic sea ice were undertaken to (1) determine the influence of brine transport processes on the salinity, porosity, and stable isotopic composition of snow ice and the underlying ice, (2) explain the range of salinities and isotopic composition observed in ice cores, and to provide a better estimate of the contribution of snow ice to the thickness of the winter pack ice, (3) better understand the microstructural controls on brine percolation and its effects on the properties of sea ice, and (4) understand the effects of meteorological forcing on snow ice formation and development of the ice cover. Snow ice thickness is most dependent on snow accumulation rates. Once snow ice begins to form on a floe, most of the subsequent thickening is due to snow ice formation. Results show that percolation in winter sea ice is most likely an inhomogeneous process. Flooding most likely occurs rapidly through localized regions of high permeability, such as in large, open brine drainage channels or cracks. Simulations of the freezing of a flooded slush layer show that focussing of thermohaline convection may form porous drainage channels in the ice and snow. These channels allow rapid desalination of the slush and exchange of H218O depleted brine with sea water. Significant positive shifts in delta18O are possible in the slush layer. This process can explain the range of delta18O observed in ice cores. Based on these results, a cutoff of delta18O < -2� is recommended for snow ice identification in the Ross, Amundsen, and Bellingshausen seas. Such a cutoff puts the amount of snow ice observed at 6--18% of the ice thickness. Although flooding appears to occur through spatially restricted regions of the ice, the precise nature of the flow and factors controlling onset of percolation are unclear.
    • Crustal Deformation Along The San Andreas Fault And Within The Tibetan Plateau Measured Using Gps

      Chen, Qizhi; Freymueller, Jeffrey T. (2002)
      Using the Global Positioning System (GPS), we study crustal deformation along the San Andreas Fault (SAF) in the San Francisco Bay area and within the Tibetan Plateau, and provide new constraints for the kinematics of these actively deforming plate boundaries. GPS measurements in 1996 and 1997 and Electronic Distance Measuring (EDM) data from the 1970s and 1980s at sites along the SAF in northern California were used to determine the near-fault strain rate and to investigate the slip rate, locking depth, and rheology. We found a pronounced high near-fault shear strain rate that can be explained by a 2-D inhomogeneous model in which a low-rigidity compliant zone concentrates strain near the fault. We suggest that the materials on either side of the fault and the cumulative fault offset play a role in the development of the compliant zone. If such a compliant zone is present but unmodeled, the geodetic estimates of slip rate and locking depth (seismogenic depth) would be biased. This would lead to a miscalculated seismic hazard. Thirteen GPS sites in southern Tibet, surveyed in 1995, 1998 and 2000, were merged with other data from China and Nepal into a single, self-consistent velocity field. The Himalaya and southern Tibet was modeled using a kinematically-consistent block model and elastic dislocation theory. We show a significantly lower convergence rate between India and Eurasia in central Himalaya than that previously estimated. We observe that southern Tibet undergoes non-uniform (spatial) east-west extension with one-half of the extension across the Yadong-Gulu rift. We infer that spatially non-uniform extension in southern Tibet results in variation of the arc-normal convergence rates along the Himalaya, and that the Yarlung-Zangbo suture or adjacent structure may be active as a right-lateral strike slip fault. From 44 GPS sites in the Tibetan Plateau, we show that deformation of Tibet is distributed and strain accumulation is spatially uniform across the entire plateau. We propose a kinematic model for the Tibetan Plateau to be a combination of rigid block motion, pure shear and uniaxial contraction in the direction of about N32�E, comparable to the convergence direction between India and Eurasia.
    • Crustal Deformation In Alaska Measured Using The Global Positioning System

      Fletcher, Hilary Jane; Freymueller, Jeffrey T. (2002)
      Repeat observations using the Global Positioning System at sites on the Earth's surface enable the velocity of those sites to be estimated. These velocity estimates can be used to model the processes of the crust's deformation by faulting and folding. The focus of this study is crustal deformation in Alaska and in particular the region of interior Alaska within 300km of Fairbanks, including the Denali fault; the Fairweather fault and Yakutat block in southern Alaska; and the Semidi region of the Aleutian arc. This deformation is driven by the relentless northwestward motion of the Pacific plate relative to North America. The Yakutat block, an allocthonous terrane located in the 'armpit' of southern Alaska is shown to be moving at neither the Pacific Plate rate nor is it attached to North America. Instead it has a velocity parallel to the Fairweather fault, which means that some offshore structure, possibly the Transition Zone, must accommodate some of the Pacific-North American relative motion. The slip on the Fairweather fault is estimated to be 44 +/- 3 mm/yr with a locking depth of 8 +/- 1 km, which implies a recurrence time of ~80 years for an MS 7.9 earthquake. Using a model of southern Alaska block rotation with the Denali fault as the northern boundary, the slip rate on the McKinley segment of the Denali fault is estimated to be ~6--9 mm/yr for a locking depth of 12 km. Moving to the southwest, data from sites in the Semidi segment of the Alaska subduction zone, between the fully-coupled segment to the northeast and the slipping Shumagin segment to the southwest are studied. This region, which sustained a magnitude 8.2 earthquake in 1938, is determined to be highly coupled and accumulating strain. Finally, all of these pieces are connected in a quantitative model for southern Alaska. This model involves three crustal blocks, the Yakutat block, Fairweather block and southern Alaska block, which lie between North America and the Pacific plate and move relative to these major plates.
    • Deformation Of Alaskan Volcanoes Measured Using Sar Interferometry And Gps

      Mann, Doerte; Freymueller, Jeffrey (2002)
      Geodetic measurements using the Global Positioning System (GPS) and synthetic aperture radar interferometry (InSAR) show deformation of Okmok, Westdahl, and Fisher volcanoes in the Alaska-Aleutian arc. This thesis shows the variety of deformation signals observed, presents models for the observations, and interprets them in terms of underlying processes. InSAR data show deflation of Okmok caldera during its last eruption in 1997, preceded and followed by inflation of smaller magnitude. Modeling shows that the main deformation source, interpreted as a central magma reservoir, is located at 2.5 to 5.0 km depth beneath the approximate center of the caldera, and 5 km away from the active vent. Mass balance calculations and comparison with the long-term eruptive frequency indicate that Okmok may be supplied with magma continuously from a deep source. GPS measurements between 1998 and 2001 show inflation of Westdahl volcano, with a source located about 7 km beneath the summit. The combined subsurface volume increase measured during the GPS and an earlier InSAR observation period [Lu et al., 2000a] accounts for at least 15% more than the volume erupted from Westdahl in 1991--92, suggesting that an eruption of that size could occur at any time. Neighboring Fisher caldera shows subsidence and contraction across the caldera center that is not related to any eruptive activity. The main mechanisms to explain this deformation are degassing and contractional cooling of a shallow magma body, or depressurization of Fisher's hydrothermal system, possibly triggered by an earthquake in the vicinity of the caldera in 1999. A systematic coherence analysis of SAR interferograms documents the cooling history of the 1997 Okmok lava flow. The flow is incoherent directly after emplacement, but coherence increases as more time has passed since the eruption, and also the shorter the period spanned by the interferogram. Coherence is regained three years after the eruption. This corresponds to the time when the 20 m thick flow has solidified, indicating that flow mobility is the dominant factor degrading coherence on young lava flows. Based on these results, InSAR coherence analysis can be used to derive the minimum thickness of a lava flow.
    • Design And Implementation Of A Relative Plasma Density Probe For The Hex Sounding Rocket Mission

      Johnson, Bruce A.; Hawkins, Joseph (2003)
      This thesis describes the design and implementation of a relative plasma density probe for the Horizontal E-region eXperiment (HEX) sounding rocket mission conducted by the University of Alaska Fairbanks (UAF). The purpose of the plasma probe in the HEX experiment is to confirm the anticipated plasma depletion resulting from the upwelling of ionospheric gases theorized to exist poleward of an active auroral arc. The instrument uses a conducting band wrapped around the circumference of the rocket payload to collect and measure positively charged ions in the vicinity of an auroral arc while following a nearly horizontal trajectory. The probe collectors are biased to repel free thermal electrons while the ions are "swept up" by the cross sectional area of the probe's motion. A separate electrode detects the presence of high-energy electrons within the aurora. The resulting collector currents are measured and transmitted to ground equipment for archiving and later analysis.*<p> *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Microsoft Office; QuickTime; Adobe Acrobat; Windows MediaPlayer or RealPlayer; Internet browser.<p>
    • Ice -wedge networks and "whale-hole" ponds in frozen ground

      Plug, Lawrence J.; Hopkins, David M. (2000)
      The patterns of ice-wedge networks and of whale-hole ponds in frozen ground self-organize by strong interactions between pattern elements. Mechanisms for the consistent spacing (15--25 m) and orientation between ice wedges are examined in a model encapsulating the opening of fractures under a combination of thermally-induced tensile stress, stress reduction near open fractures, and heterogeneity of frozen ground and insulating snow. Modeled networks are similar to ice-wedge networks on the Espenberg coastal plain, Bering Land-Bridge National Park, Alaska, at the level of variation among Espenberg networks, as indicated by: (i) comparisons of distributions of relative orientation and spacing between wedges; and (ii) application of nonlinear spatial forecasting to modeled and Espenberg network patterns. Spacing in modeled networks is sensitive to fracture depth and weakly sensitive to thermally-induced tensile stress and substrate strength, consistent with the narrow range of spacing between natural ice wedges in different regions. In an extended model that includes recurring fractures over thousands of winters, networks similar to natural ice-wedge networks form. The annual pattern of fractures diverges from the ice-wedge pattern, with only &frac12;--&frac34; of wedges fracturing in a single year at a steady-state reached after approximately 103 y. Short-lived sequences of extreme stress from cooling can permanently alter the spacing between and the fracture frequency of modeled ice wedges, suggesting that the existence and characteristics of existing and relic natural ice-wedge networks reflect extreme, not mean, climate conditions. Ponds on the Espenberg beach-ridge plain, approximately 2 m across and 1 m deep and surrounded by raised rings of ice-rich permafrost 2 m across and 0.5 m high, form through an interplay between localized bacterial decomposition of peat, thawing of frozen ground and frost heaving of peat in rings. Groups of hundreds of ponds at Espenberg assemble through time because new ponds are favored to form adjacent to raised rings around existing ponds. The nonlinear behavior that results from strong interactions in patterns of ice-wedge networks and in ponds suggests general limitations in the application of linear approaches to inferring the response of geomorphic systems to changes in forcing, such as climate change.
    • Massive ice interactions with offshore structures

      Lu, Mingchi (1992)
      The interaction between a multiyear sea ice floe of variable thickness, and an offshore structure, has been examined using a 3-dimensional finite element method. Elastic response within the ice floe was assumed initially, and a uniform loading of the ice floe by the adjacent pack ice was used. As an example of the results for a frozen boundary condition at the ice/structure contact zone, with a central region of the ice floe having its thickness reduced to 50% as compared to the floe thickness at the structure ($\Delta$t/t = 0.5), tensile cracks first form at the top surface in the thinnest area of the floe. The total force on the structure was 108 MN, as compared with 1500 MN which would be present in the case of an ice floe of uniform thickness. Parameters varied were ice/structure contact zone (located in the centric or the eccentric region), the sliding boundary condition, two-dimensional ice thickness variation, variable ice elastic modulus as a function of depth, and viscoelastic ice behavior. Cases of rigid and of compliant structure and foundation were included. In a second part of the study, the ice island loads acting upon a cylindrical rigid structure were analyzed by this 3-dimensional finite element method. A force of 6600 MN was computed to be acting on the structure with a maximum penetration distance of 8.2 m. A different theoretical method based upon multiyear ice field data resulted in a force of 336 MN and a maximum penetration distance of 75 m. The ice forces on the structure are reduced by ice floe thickness variations, and also are affected by the geometries at the ice floe/structure and ice floe/pack ice boundaries. The reduced elastic modulus in the warmer. lower part of an ice sheet promotes ice bending failure and causes lower structure loads.
    • Modeling of Arctic stratus cloud formation and the maintenance of the cloudy Arctic boundary layer

      Zhang, Qiuqing; Stamnes, Knut; Harrington, Jerry; Sentman, Davis; Watkins, Brenton (1999)
      The formation of Arctic stratus clouds (ASCs) and the maintenance of the cloudy Arctic boundary layer are studied with two models: a one-dimensional radiative-convective model and a three-dimensional large eddy simulation (LES) model. The one-dimensional radiative-convective model consists of a comprehensive radiative module, a cloud parameterization with detailed microphysics and a convective adjustment scheme. The model is designed specifically for studying ASC formation. With this model, the roles of radiation and cloud microphysics in the formation of ASCs and multiple cloud layers are investigated. The simulations reproduce both single and multiple cloud layers that were observed with inversions of temperature and humidity occurring near the cloud top. The detailed cloud microstructure produced by the model also compares well with the observations. The physics of the formation of both single and multiple cloud layers is investigated. Radiative cooling plays a key role during the initial stage of cloud formation in a atmosphere. It leads to a continual temperature decrease promoting water vapor condensation on available cloud condensation nuclei. The vertical distribution of humidity and temperature determines the radiative cooling and eventually where and when the cloud forms. The observed temperature inversion may also be explained by radiative cooling. The three-dimensional LES model is adopted to evaluate the one-dimensional model, especially the convective adjustment scheme. The advantages and limitations of the one-dimensional model are discussed. The LES results suggest that the convective adjustment scheme is capable of capturing the main features of the vertical heat and moisture fluxes in the cloudy Arctic boundary layer. The LES model is also used to investigate the maintenance of the cloudy Arctic boundary layer. The turbulence in the cloudy Arctic boundary layer is primarily maintained by the buoyancy effect due to the cloud top cooling. It is found that weak large scale downward motion aids in cloud development and maintenance.