The crust is the outer-most layer of the earth with thickness up to 80 km. Massive seismic waveform data have enabled imaging fine crustal structures with the aid of new imaging techniques. In this thesis, I develop seismic imaging techniques to take full advantage of the expanding dataset as well as apply the imaging techniques to understand crustal seismic structures. First, I apply receiver function techniques to image the crustal thickness and average Vp/Vs in Northeast China. I found an uplifted Moho in eastern flank of the Songliao Basin and the Changbaishan region and suggest that dynamic mantle upwelling might be the cause of the observed uplift. With accumulated waveform data available, it becomes possible to extract more subtle structural information from receiver function. Second, I develop a new technique to robustly estimate seismic azimuthal anisotropy with radial and transverse receiver functions. I apply this technique to estimate the crustal anisotropy in Southeast Yunnan region and found that the significant crustal anisotropy may be caused by lower crust flow in this region. Full-wave based imaging techniques such as reverse time migration and full-wave inversion does not assume flat interfaces or infinite frequency rays as that the receiver function techniques do and are desirable in imaging more complex crustal structures. However, their high computational cost is one of the issues that prevent their practical applications. In the last part, I developed an effective waveform modeling technique to efficiently simulate wave propagation in acoustic media. With this novel modeling technique, the full-wave based imaging techniques are accelerated by a factor up to 400%.

Ocean-continent convergence and subsequent continental collision are responsible for continental growth, mountain building, and severe tectonic events including volcanic eruptions and earthquake activity. They are also key driving forces behind the extensive thermal and compositional heterogeneities at crustal and mantle depths. Active subduction along the Calabrian Arc in southern Italy and the Hellenic Arc are examples of such collisional tectonics. The first part of this thesis examines the subduction related deformations within the crust beneath the southern Apennines. By modeling regional surface wave recordings of the largest temporary deployment in the southern Apennines, a lower-crustal/upper-mantle low-velocity volume extending down to 50 km beneath the mountain chain is identified. The magnitude (~ 0.4 km/s slower) and anisotropic nature (~ 10%) of the anomaly suggest the presence of hot and partially molten emplacement that may extend into the upper-crust towards Mt. Vulture, a once active volcano. Since the Apulian basement units are deformed during the compressional and consequent extensional events, our observations favor the ``thick-skin'' tectonic growth model for the region. In the deeper mantle, active processes are thermodynamically imprinted on the depth and strength of the phase transitions. This thesis examines more than 15000 SS precursors and provides the present-day reflectivity structure and topography associated with these phase transitions. Through case studies I present ample evidence for both slab penetration into the lower mantle (beneath the Hellenic Arc, Kurile Island and South America) and slab stagnation at the bottom of the Mantle Transition Zone (beneath the Tyrrhenian Sea and eastern China). Key findings include (1) thermal anomalies (~ 200 K) at the base of the MTZ, which represent the deep source for Cenozoic European Rift Zone, Mount Etna and Mount Cameroon volcanism, (2) significant depressions (by 20-40 km) at the bottom of the Mantle Transition Zone beneath subducting slabs, (3) a strong 520-km reflector near subducting slabs, (4) a weak and elevated (15-25 km) 410-km reflector within active deformation zones, (5) strong lower mantle reflectors (~ 900 km) while slabs penetrate into the lower mantle, and (6) consistency between the topography of a 300-km reflector and an exothermic phase transformation. / Geophysics

No description available.

Northern Central America is a tectonically complex zone defined by its borders with Cocos and North America plates. The Middle America subduction zone and the strike-slip motion along the North America-Caribbean plate boundary, in that order, control most of its deformation. The interaction between the different elements of the studied area is evident from the high seismicity in the region, especially along plate boundaries. Also in the interior of the region, seismicity shows that deformation takes place, though in lesser degree. In a time window of 30 years, three earthquakes with moment magnitude larger than 7 struck northern Central America evincing the need to estimate the seismic hazard for the zone. To tackle the problem, we compiled a catalogue of hypocenters commencing in 1964, defined seismogenic sources and described the evolution of earthquake activity through a Poisson model. Probabilistic seismic hazard (PSH) calculations for the next 50 years were performed. The highest estimate of seismic hazard was obtained for the zone adjacent to the subduction zone. Because of the fundamental importance of demarcating seismogenic sources in the PSH analysis, i.e. defining the seismotectonic model, we extended the catalogue to cover 102 years for the whole northern Central America. We have studied the North America-Caribbean plate boundary in order to refine the fault representation. Different techniques were used, like that of body-waveform modeling, allowing us to limit the extent of depth of faulting to 20 km. The seismic moment tensor was used to estimate the deformation velocities on known tectonic structures, including those of the Honduras depression and borderland faults. Finally, we made use of the Coulomb stress criterion to determine the relation between earthquake occurrence and static stress changes following major earthquakes.

<p>Northern Central America is a tectonically complex zone defined by its borders with Cocos and North America plates. The Middle America subduction zone and the strike-slip motion along the North America-Caribbean plate boundary, in that order, control most of its deformation. The interaction between the different elements of the studied area is evident from the high seismicity in the region, especially along plate boundaries. Also in the interior of the region, seismicity shows that deformation takes place, though in lesser degree. In a time window of 30 years, three earthquakes with moment magnitude larger than 7 struck northern Central America evincing the need to estimate the seismic hazard for the zone. To tackle the problem, we compiled a catalogue of hypocenters commencing in 1964, defined seismogenic sources and described the evolution of earthquake activity through a Poisson model. Probabilistic seismic hazard (PSH) calculations for the next 50 years were performed. The highest estimate of seismic hazard was obtained for the zone adjacent to the subduction zone. Because of the fundamental importance of demarcating seismogenic sources in the PSH analysis, i.e. defining the seismotectonic model, we extended the catalogue to cover 102 years for the whole northern Central America. We have studied the North America-Caribbean plate boundary in order to refine the fault representation. Different techniques were used, like that of body-waveform modeling, allowing us to limit the extent of depth of faulting to 20 km. The seismic moment tensor was used to estimate the deformation velocities on known tectonic structures, including those of the Honduras depression and borderland faults. Finally, we made use of the Coulomb stress criterion to determine the relation between earthquake occurrence and static stress changes following major earthquakes.</p>

This dissertation reports on a new methodology to characterize and simulate a standard cell library to be used for statistical static timing analysis. A compact variation-aware timing model for a standard cell in a cell library has been developed. The model incorporates variations in the input waveform and loading, process parameters, and the environment into the cell timing model. Principal component analysis (PCA) has been used to form a compact model of a set of waveforms impacted by these sources of variation. Cell characterization involves determining equations describing how waveforms are transformed by a cell as a function of the input waveforms, process parameters, and the environment. Different versions of factorial designs and Latin hypercube sampling have been explored to model cells, and their complexity and accuracy have been compared. The models have been evaluated by calculating the delay of paths. The results demonstrate improved accuracy in comparison with table-based static timing analysis at comparable computational cost. Our methodology has been expanded to adapt to interconnect dominant circuits by including a resistive-capacitive load model. The results show the feasibility of using the new load model in our methodology. We have explored comprehensive accuracy improvement methods to tune the methodology for the best possible results. The following is a summary of the main contributions of this work to the statistical static timing analysis: (a) accurate waveform modeling for standard cells using statistical waveform models based on principal components; (b) compact performance modeling of standard cells using experimental design statistical techniques; and (c) variation-aware performance modeling of standard cells considering the effect of variation parameters on performance, where variation parameters include loading, waveform shape, process parameters (gate length and threshold voltage of NMOS and PMOS transistors), and environmental parameters (supply voltage and temperature); and (f) extending our methodology to support resistive-capacitive loads to be applicable to interconnect dominant circuits; and (e) classifying the sources of error for our variational waveform model and cell models and introducing of the related accuracy improvement methods; and (f) introducing our fast block-based variation-aware statistical dynamic timing analysis framework and showing that (i) using compiler-compiler techniques, we can generate our timing models, test benches, and data analysis for each circuit, which are compiled to machine-code to reduce the overhead of dynamic timing simulation, and (ii) using the simulation engine, we can perform statistical timing analysis to measure the performance distribution of a circuit using a high-level model for gate delay changes, which can be linked to their parameter variation.

xiv, 103 p. : ill. (some col.) / Results from a recent mid-ocean ridge tomography study along the fast-spreading, northern East Pacific Rise (EPR) reveal that the axis of mantle upwelling beneath the ridge is skewed with respect to the spreading axis, giving rise to regions of both rise-centered and off-axis mantle melt accumulation. Here, we investigate the effects of off-axis melt accumulation on the architecture of overlying crust as well as off-axis melt delivery on crustal construction along the ridge axis. We first present evidence for off-axis magmatism 20 km from the spreading center in 300-ka-old crust overlying a region of off-axis melt supply. Seismic data reveal an intrusive complex ∼2 km beneath the seafloor that is limited in lateral extent (<5 km) and comprises a melt lens underlain by low-velocity, high-attenuation crust, which provides the necessary conditions to drive off-axis volcanic and hydrothermal activity. We next present results from thermodynamic modeling that show systematic, along-axis variations in the depth of crystallization and degree of differentiation of magma produce crustal density variations of ∼0.1 g/cm 3 . These density anomalies are on the order inferred from a recent study that shows increasing axial depth along the northern EPR correlates with an increase in crustal density and offset of mantle upwelling with respect to the ridge axis. Our results, along with geophysical and geochemical data from the 9°-10°N region of the EPR, suggest that along-axis deeps correspond with magmatic systems that have significant near-Moho (i.e., crust-mantle transition) crystallization, which we attribute to off-axis delivery of mantle melt. As this investigation is motivated by the EPR tomography results, we conclude with a numerical study that examines the travel time sensitivity of Pn , a sub-crustal head wave commonly used in local travel time tomography, to crustal and mantle heterogeneity. Our results indicate that Pn travel times and Fresnel zones are insensitive to normal sub-axial crustal thickness anomalies, mantle velocity gradients and crust-mantle velocity contrast variations and that mantle low-velocity zones must be at least 3 km thick to produce significant, near-constant Pn delay times. Our data support the validity and interpretation of the EPR tomography results. This dissertation includes both previously published and unpublished co-authored material. / Committee in charge: Dr. Douglas R. Toomey, Chairperson; Dr. Paul J. Wallace, Member; Dr. Eugene Humphreys, Member; Dr. James Isenberg, Outside Member