Seismic tomography of the southern California crust based on spectral-element and adjoint methods

We iteratively improve a 3-D tomographic model of the southern California crust using numerical simulations of seismic wave propagation based on a spectral-element method (SEM) in combination with an adjoint method. The initial 3-D model is provided by the Southern California Earthquake Center. The data set comprises three-component seismic waveforms (i.e. both body and surface waves), filtered over the period range 2–30 s, from 143 local earthquakes recorded by a network of 203 stations. Time windows for measurements are automatically selected by the FLEXWIN algorithm. The misfit function in the tomographic inversion is based on frequency-dependent multitaper traveltime differences. The gradient of the misfit function and related finite-frequency sensitivity kernels for each earthquake are computed using an adjoint technique. The kernels are combined using a source subspace projection method to compute a model update at each iteration of a gradient-based minimization algorithm. The inversion involved 16 iterations, which required 6800 wavefield simulations. The new crustal model, m_(16), is described in terms of independent shear (V_S) and bulk-sound (V_B) wave speed variations. It exhibits strong heterogeneity, including local changes of ±30 per cent with respect to the initial 3-D model. The model reveals several features that relate to geological observations, such as sedimentary basins, exhumed batholiths, and contrasting lithologies across faults. The quality of the new model is validated by quantifying waveform misfits of full-length seismograms from 91 earthquakes that were not used in the tomographic inversion. The new model provides more accurate synthetic seismograms that will benefit seismic hazard assessment

Flow current field observation with underwater moving acoustic tomography

Underwater environment observation with underwater acoustic tomography has been considerably developed in recent years. Moving sound transmission can obtain the observation of entire spatial area with sound station moving. Various internal structures, unique surface and submarine boundaries and changing environment constitutes a complex acoustic propagation channel. This paper focus on the inversion method and signal resampling for sound moving transmission. Also, the current field in three-dimensional (3D) scale is also studied. A five-station sound transmission experiment with four moored station and one moving station that conducted in range of 500m×500m at Huangcai reservoir, Changsha, China is presented. Signal resampling is performed to get correlation of received acoustic data. The vertical layer-averaged flow current results between moving station and moored station are inversed with 2D grid method. 3D flow current field result is composed by grid-averaged inversion current of vertical profile via moving station at different moment. The received results of reciprocal signal transmission between two moored stations and one moving station were used for layer-averaged current variations at vertical scale and grid-averaged current. The feasibility of the method in underwater moving acoustic tomography research is proved and its applicability is discussed. The proposed underwater acoustic tomography technology develops an innovative idea for the further development of temporal- spatial grided tomography observation

Elevation and Deformation Extraction from TomoSAR

3D SAR tomography (TomoSAR) and 4D SAR differential tomography (Diff-TomoSAR) exploit multi-baseline SAR data stacks to provide an essential innovation of SAR Interferometry for many applications, sensing complex scenes with multiple scatterers mapped into the same SAR pixel cell. However, these are still influenced by DEM uncertainty, temporal decorrelation, orbital, tropospheric and ionospheric phase distortion and height blurring. In this thesis, these techniques are explored. As part of this exploration, the systematic procedures for DEM generation, DEM quality assessment, DEM quality improvement and DEM applications are first studied. Besides, this thesis focuses on the whole cycle of systematic methods for 3D & 4D TomoSAR imaging for height and deformation retrieval, from the problem formation phase, through the development of methods to testing on real SAR data. After DEM generation introduction from spaceborne bistatic InSAR (TanDEM-X) and airborne photogrammetry (Bluesky), a new DEM co-registration method with line feature validation (river network line, ridgeline, valley line, crater boundary feature and so on) is developed and demonstrated to assist the study of a wide area DEM data quality. This DEM co-registration method aligns two DEMs irrespective of the linear distortion model, which improves the quality of DEM vertical comparison accuracy significantly and is suitable and helpful for DEM quality assessment. A systematic TomoSAR algorithm and method have been established, tested, analysed and demonstrated for various applications (urban buildings, bridges, dams) to achieve better 3D & 4D tomographic SAR imaging results. These include applying Cosmo-Skymed X band single-polarisation data over the Zipingpu dam, Dujiangyan, Sichuan, China, to map topography; and using ALOS L band data in the San Francisco Bay region to map urban building and bridge. A new ionospheric correction method based on the tile method employing IGS TEC data, a split-spectrum and an ionospheric model via least squares are developed to correct ionospheric distortion to improve the accuracy of 3D & 4D tomographic SAR imaging. Meanwhile, a pixel by pixel orbit baseline estimation method is developed to address the research gaps of baseline estimation for 3D & 4D spaceborne SAR tomography imaging. Moreover, a SAR tomography imaging algorithm and a differential tomography four-dimensional SAR imaging algorithm based on compressive sensing, SAR interferometry phase (InSAR) calibration reference to DEM with DEM error correction, a new phase error calibration and compensation algorithm, based on PS, SVD, PGA, weighted least squares and minimum entropy, are developed to obtain accurate 3D & 4D tomographic SAR imaging results. The new baseline estimation method and consequent TomoSAR processing results showed that an accurate baseline estimation is essential to build up the TomoSAR model. After baseline estimation, phase calibration experiments (via FFT and Capon method) indicate that a phase calibration step is indispensable for TomoSAR imaging, which eventually influences the inversion results. A super-resolution reconstruction CS based study demonstrates X band data with the CS method does not fit for forest reconstruction but works for reconstruction of large civil engineering structures such as dams and urban buildings. Meanwhile, the L band data with FFT, Capon and the CS method are shown to work for the reconstruction of large manmade structures (such as bridges) and urban buildings

Tectonic and sedimentary processes of the southeast Alaska margin

Strike-slip tectonics dominate the southeast Alaska margin. The Queen Charlotte-Fairweather Fault transform system extends ~1200 km from southern Canada north to Yakutat Bay, Alaska, accommodating nearly 4.5 cm/yr of dextral offset between the Pacific and North American plates. This dissertation aims to better characterize the transform plate boundary by examining the accommodation of oblique transpression, crustal structure, seismogenic faults, and tectonic influence on regional sedimentary processes. We address fundamental tectonic questions utilizing a suite of geophysical data including multichannel seismic (MCS) reflection, bathymetry, magnetics, gravity, and earthquake data. A 2011 MCS survey reveals subsurface channel deposits related to the development of the deep-sea Baranof sedimentary fan in the Gulf of Alaska. We find that Baranof Fan channels have avulsed consistently southward, affected by the changing position of channel heads relative to sediment sources along the shelf edge due to strike-slip motion along the Queen Charlotte Fault (QCF). Baranof Fan sediments sit atop a flexural depression in the Pacific crust near the QCF, which developed between ~6 Ma and ~2 Ma. We interpret the flexure to be an artifact of oblique convergence along the southern QCF, preserved by sedimentary loading in part from the Baranof Fan. ~150 km of the QCF near the Pacific flexural depression ruptured in January 2013, producing a M [subscript]w 7.5 earthquake near Craig, Alaska. A tomographic inversion of Craig aftershock data reveals a low velocity zone on the Pacific side of the plate boundary at seismogenic depths, which may indicate the contrast of a warm, young Pacific crust along the older, colder North American crust. These results have relevance for rupture directionality and future seismic hazard along the QCF. Finally, we revisit seismic hazard associated with the 10 September 1899 Mw 8.2 earthquake at the northern termination of the transform system near Yakutat Bay, Alaska. We quantify uncertainty on coseismic uplift measurements and integrate various geophysical data, including a 2012 MCS survey, to provide an updated fault map and tectonic model of the Yakutat Bay region. Our results support a subduction-related rupture of the 10 September event with limited slip along the transpressive termination of the Fairweather Fault.Geological Science

Earth's structure from a bayesian analysis of seismic signals and noise

The prevailing drive of modern seismology is to improve our knowledge of the Earth's structure, composition, and dynamics through an analysis of seismic waveforms. With increasing computing power, number and quality of seismic stations, and length of data records, the resolution and spatial coverage of current Earth models has improved substantially over the past few decades. Yet many limitations remain. The advent of ambient noise seismology has provided the solution to many issues, such as the irregular distribution of earthquakes, biases from structures outside the model region, earthquake location errors, and lack of near-surface resolution. Despite improvements to data quality and quantity and the introduction of unconventional datasets such as ambient seismic noise, a persisting shortcoming of many tomographic inversions is ad-hoc error estimation, parameterization, and regularization, which prevent a meaningful portrayal of model complexity and uncertainty. With the rapid increase in computing power, non-linear techniques based on densely sampling favorable regions of model space are now becoming tractable for real-world tomographic problems and directly address these shortcomings. One such recently introduced and promising method is transdimensional and hierarchical Bayesian inference. This alternate approach allows model parameterization and resolution to be driven by the data. This thesis presents a collection of seismic inverse problems using real world datasets, some of which are tackled using fully non-linear Bayesian statistics. The benefits of a probabilistic approach are demonstrated for datasets targeting the uppermost crust down to the core through the development of novel methods of inversion and uncertainty quantification. To begin, an unconventional methodology for studying earthquake focal mechanisms in intraplate settings is presented through the inversion of ambient noise, receiver functions, and dispersion curves. The ambient seismic noise imaging approach of this study is subsequently applied to Tasmania - to which it is highly suited - and the resulting group and phase velocity maps help decipher Tasmania's enigmatic tectonic history. The same ambient noise dataset is further manipulated to yield a 3D shear velocity model of the region using a two-step transdimensional, hierarchical ensemble inference approach. Two prominent low-velocity anomalies offer insight into the Paleozoic evolution of the east Gondwana margin and support a connection between Tasmania and mainland Australia since the Cambrian. This approach is also applied to a larger dataset encompassing much of mainland southeast Australia. The Bayesian approach is also applied to a global dataset of differential body wave travel times in an effort to reveal P-wave velocity heterogeneity in the lowermost mantle. Another deep Earth application is demonstrated through an inversion for the time-dependent differential rotation of the inner core with respect to the rest of the mantle using careful measurements of earthquake doublets. The transdimensional nature of the inversion problem means that the data drive the number of free parameters constraining the differential rotation pattern, which exhibits much more complexity than the simple linear trend long-promoted by previous studies. The contents of this thesis help augment the diverse and wide-reaching applications for Bayesian statistics, which will continue to improve with future increases in computational power

Nearshore Bathymetry Estimation from Drone Video Using PIV Technique

This research introduces a novel method to estimate nearshore bottom topography using an unmanned aerial vehicle (UAV), or drone. The UAV was manipulated over the area of interest to film video, and the Particle image velocimetry (PIV) technique was then applied to analyze the video frames in order to retrieve the wave speeds. Under the shallow water conditions, the wave dispersion relation can be simplified in a manner such that when the wave speed is known, the water depth can be inferred. In other words, when wave speed is known, water depth can be inferred. After combining the inferred water depths at multiple points from within the area of interest, the bathymetry was constructed. To validate the method, individual waves were recorded in the nearshore breaking zone during two trials at Freeport, Texas, USA. We measured the significant difference in intensity across the recorded images, as the intensity had a larger signal-to-noise ratio, and this improved the implementation of the PIV algorithm. We then compared the PIVestimated water depth with field measurement and observations, finding that the water depth was overestimated by 13.5%, which was primarily explained by non-linear wave breaking effects. We then introduced a correction factor, reducing the estimation error to within 6% of the true observed water depth. Though there are limitations, this new approach can lower the cost of developing bathymetric maps in the nearshore and result in greater flexibility across space and time. Further improvements in equipment and work on developing better correction factors may result in still greater precision

Urban Deformation Monitoring using Persistent Scatterer Interferometry and SAR tomography

This book focuses on remote sensing for urban deformation monitoring. In particular, it highlights how deformation monitoring in urban areas can be carried out using Persistent Scatterer Interferometry (PSI) and Synthetic Aperture Radar (SAR) Tomography (TomoSAR). Several contributions show the capabilities of Interferometric SAR (InSAR) and PSI techniques for urban deformation monitoring. Some of them show the advantages of TomoSAR in un-mixing multiple scatterers for urban mapping and monitoring. This book is dedicated to the technical and scientific community interested in urban applications. It is useful for choosing the appropriate technique and gaining an assessment of the expected performance. The book will also be useful to researchers, as it provides information on the state-of-the-art and new trends in this fiel

Four decades of geophysical research on Iberia and adjacent margins

The dimensions, the geographical position and the complex geological history of the Iberian Peninsula makes it a unique and singular target to study its crustal and upper mantle structure and geodynamical evolution using geophysical methods. The lithospheric structure beneath Iberia has been investigated since the 1970’s using deep multichannel seismic reflection and refraction/wide-angle reflection profiling. Gravimetric and magnetic data were acquired following the deployment of permanent and temporary stations, mostly during the 1990’s. Beginning in the late 1990’s, the progressive use of Global Navigation Satellite Systems (GNSS) instruments contributed to monitor the present-day motions. During the last decades, numerous geological and geophysical surveys have investigated the Iberian lithosphere and upper mantle in the onshore and offshore regions, the most recent ones related to the TopoIberia project. The approach developed in this contribution is twofold. Firstly, we summarize the available geophysical information over Iberia, from focusing on the upper crust to the lithosphere-asthenosphere boundary and the transition zone marking the bottom of the upper mantle. Results of GNSS data, potential fields, controlled source seismic profiles, magnetotelluric data, body and surface wave tomography, receiver functions and 2D and 3D lithospheric modeling are reviewed and compared. Secondly, we focus on the areas of greater geodynamic interest and the regions where inconsistencies within the geophysical results, or contradictions in their tectonic interpretation exist, identifying the major questions that are still under debate. Besides shedding light to the state of knowledge and pointing out present-day research challenges, this review provides a tool for the integration of the diverse geophysical datasets with the surface geology and geodynamical processes that are interpreted to have built the complex geology of the Iberian Peninsula.The authors acknowledge funding from the Generalitat de Catalunya, grant/awards number AGAUR 2017SGR1022, and AGAUR 2017SGR847, the Spanish Ministry of Science, Innovation, and Universities grant numbers RTI2018-095594-B-I00, PGC2018-095154-B-100 and PGC2018-094227-B-I00 and the Spanish Ministry of Economy and Competitiveness grant numbers CGL2017-84901-C2 and PIE-CSIC-201830E039. IP is funded by the Spanish Ministry of Science, Innovation, and Universities and University of Salamanca grant BEAGAL18/00090. AV acknowledges funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S).Peer reviewe

Summary of Research 1998, Department of Oceangraphy

The views expressed in this report are those of the authors and do not reflect the official policy or position of the Department of Defense or the U.S. Government.This report contains summaries of research projects in the Department of Oceanography. A list of recent publications is also included which consists of conference presentations and publications, books, contributions to books, published journal papers, technical reports, and thesis abstracts