Typical reinforced concrete (RC) bridges built prior to 1970 were designed with minimum seismic consideration, leaving numerous bridges highly susceptible to damage following an earthquake. In order to improve the seismic behavior of substandard RC bridges, this study presents the seismic performance of reinforced concrete bridge bents retrofitted and repaired using Buckling-Restrained Braces (BRBs) while considering subduction zone earthquake demands. In order to reflect displacement demands from subduction ground motions, research studies were conducted to develop quasi-static loading protocols and then investigate their effect on structural bridge damage. Results suggested that subduction loading protocols may reduce the displacement ductility capacity of RC bridge columns and change their failure mode. The cyclic performance of reinforced concrete bridge bents retrofitted and repaired using BRBs was experimentally evaluated using large-scale specimens and the developed loading histories. Three BRB specimens were evaluated with the aim of assessing the influence of these components on the overall performance of the retrofitted and repaired bents. Additionally, subassemblage tests were conducted in an effort to study the response of these elements and to allow for refined nonlinear characterization in the analysis of the retrofitted and repaired systems. The results of the large-scale experiments and analytical studies successfully demonstrated the effectiveness of utilizing buckling-restrained braces for achieving high displacement ductility of the retrofitted and repaired structures, while also controlling the damage of the existing vulnerable reinforced concrete bent up to an operational performance level.

Arc magmatism is sustained by the complex interactions between the subducting slab, the overriding plate, and the mantle wedge. Partial melting of mantle peridotite is achieved by fluid-induced flux melting and decompression melting due to upward flow. The distribution of melting is sensitive to temperature, the pattern of flow, and the pressure in the mantle wedge. The arc front is the surface manifestation of partial melting in the mantle wedge and is characterized by a narrow chain of active volcanoes that migrate in time. The conventional interpretation is that changes in slab dip angle lead to changes in the arc front position relative to the trench. We explore an alternative hypothesis: evolution of the overlying plate, specifically thickening of the arc root, causes arc front migration. We investigate the effects of varying crustal morphology and viscous decoupling of the shallow slab-mantle interface on melt production using 2D numerical models involving a stationary overriding plate, a subducting plate with prescribed motion, and a dynamic mantle wedge. Melt production is quantified using a hydrous melting parameterization. We conclude: 1) Localized lithospheric thickening shifts the locus of melt production trenchward while thinning shifts melting landward. 2) Inclined LAB topography modulates the asthenospheric flow field, producing a narrow, well-defined arc front. 3) Thickening of the overriding plate exerts increased torque on the slab, favoring shallowing of the dip angle. 4) Viscous decoupling produces a cold, stagnant forearc mantle but promotes arc front melting due to reduction in the radius of corner flow, leading to higher temperatures at the coupling/decoupling transition.

The Nicoya Peninsula of Costa Rica is unusually close to the Middle America Trench (MAT), such that interface locking along the megathrust is observable under land. Here, rapid convergence between the downgoing Cocos and the over-riding Caribbean plates at ~85mm/yr allows for observable high strain rates, frequent large earthquakes and ongoing micro-seismicity. By taking advantage of this ideal location, a network of 20 on-land broadband seismometers was established in cooperation between UC Santa Cruz, Georgia Tech, and OVSICORI, with most stations operating since 2008. To evaluate what seismicity tells us about the ongoing state of coupling along the interface, we must consistently evaluate the location and magnitude of ongoing micro- seismicity. Because of large levels of anthropogenic, biologic, and coastal noise, automatic detection of earthquakes remains problematic in this region. Thus, we resorted to detailed manual investigation of earthquake phases. So far, we have detected nearly 7,000 earthquakes below or near Nicoya between February and August 2009. From these events we evaluate the fine-scale frequency-magnitude distribution (FMD) along the subduction megathrust. The results from this b-value mapping‟ are compared with an earlier study of the seismicity 9 years prior. In addition, we evaluate them relative to the latest geodetically derived locking. Preliminary comparisons of spatial and temporal variations of the b-values will be reported here. Because ongoing manual detection of earthquakes is extremely laborious and some events might be easily neglected, we are implementing a match-filter detection algorithm to search for new events from the continuous seismic data. This new approach has been previously successful in identifying aftershocks of the 2004 Parkfield earthquake. To do so, we use the waveforms of 858 analyst-detected events as templates to search for similarly repeating events during the same periods that have been manually detected. Preliminary results on the effectiveness of this technique are reported. The overall goal of this research is to evaluate the evolution of stress along the megathrust that may indicate the location and magnitude of potentially large future earthquakes. To do so, I make the comparison between the FMD and the interface locking. Only positive correlations are observed in the Nicoya region. The result is different from the one derived from the seismic data set that was recorded 9 years before our data. Therefore, to substantiate the causes for the different relationships between the b-value and the coupling degree, we need additional data with more reliable magnitudes.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (p. 103-112).

Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2015. / Cataloged from PDF version of thesis. / Includes bibliographical references. / This thesis investigates lithospheric dynamics of Earth's subduction zones and Martian tectonic provinces on multiple time scales ranging from short-term earthquake deformation to long-term tectonic loading. In Chapter 2, I use geodetic observations to constrain the postseismic viscoelastic deformation following the 1960 M9.5 Valdivia, Chile earthquake and quantify its stress loading on the rupture zone of the 2010 M8.8 Maule, Chile earthquake. Results of analysis reveal that the post-1960 viscoelastic process might have contributed to the triggering of the 2010 earthquake. Chapter 3 presents numerical experiments to investigate elastoplastic deformation and faulting in the overriding plates of subduction zones caused by the movement of subducted seamounts. Numerical simulations show that a group of normal faults would first appear on the seaward side of a subducted seamount, followed by a group of thrust faults on the landward side of the seamount. In Chapter 4, I use the most recent Martian gravity and topography data to constrain spatial variations in lithospheric flexural deformation for various tectonic regions on Mars. The effective lithospheric thickness is estimated to be relatively small for the plain regions in the southern highland, but relatively large for the impact basins in the northern lowland as well as for volcanic montes in the Tharis province. The regional variations in the estimated effective lithospheric thickness might reflect both spatial and temporal changes in the thermal state of Mars. / by Min Ding. / Ph. D.

Abstract not available

The interaction of magma with continental crust at convergent margins is fundamental to understanding if and how continents grow. Isotopic and elemental data constrain the progressive stages of development of the magmatic underpinnings of the long-lived Aucanquilcha Volcanic Cluster (AVC), situated atop the thick continental crust of the central Andes in northern Chile. Whole rock data are used in conjunction with mineral compositions to infer processes that gave rise to eleven million years of intermediate, dominantly dacite, arc volcanism. A pulse of volcanic activity at the AVC between ~5 and 2 Ma is bracketed by more sluggish rates. We document chemical changes in the lavas that accompany this eruptive evolution. Trace element data suggest that crystal fractionation and magma mixing were the dominant mechanisms generating the diversity observed in the AVC whole rock data. Fractionation was dominant during early and waning stages of magmatism, and magma mixing was an important process during the high flux period. Peak thermal maturity of the AVC underpinnings coincided with the high magma flux and likely promoted open system processes during this time. Mineral compositions from zircon, amphibole, pyroxene, and Fe-Ti oxides confirm the importance of material recycling in the production of evolved AVC rocks. Various geothermometers were employed to calculate the pre-eruptive conditions of AVC magma using mineral compositions. Pressure estimates from amphibole and two-pyroxene barometry indicate crystallization depths of 1 ��� 5 kb and 4 ��� 6 kb, respectively. Temperature estimates from zircon, Fe-Ti oxides, amphiboles, and pyroxenes indicate temperatures ranging from ~700��C to 1100��C. Zircon temperatures are always the lowest (700��C - 950��C), and pyroxene temperatures are always the highest (1000��C - 1100��C), with Fe-Ti oxide and amphiboles temperatures falling in between. U-Pb ages from zircons and thermometry from individual samples evidence the thermal maturation and consolidation of the underpinnings below the AVC, presumably culminating in a large, crystal-rich mush zone where magmas were trapped and processed. It is in these middle to upper crustal zones where magmatic diversity is attenuated and giant, relatively homogeneous batholiths are formed. Isotopes of AVC lavas are similar to values observed from other central Andes volcanic centers. Lead isotopes are consistent with the AVC's location within a Pb isotope transition zone between the Antofalla and Arequipa basement terranes. Oxygen and Sr isotopic ratios are high and Nd isotopic ratios low with respect to a depleted mantle. Through time, ������Sr/������Sr values of AVC lavas progressively increase from lows of ~0.70507 to ~0.70579 (upper values of 0.70526 to 0.70680), and ��Nd values decrease from highs of -1.0 to -4.6 (lows of -1.6 to -7.3). Similarly, O isotopes (�������O) show a slight increase in base level through time from lows of 6.5��� to 7.0��� (highs of 6.75��� ��� 7.5���). Dy/Yb and Sm/Yb ratios also increased systematically from highs of 2.11 to 3.45, and 2.76 to 6.67, respectively. Despite the temporal isotopic variation, there is little isotopic variation with indices of fractionation, suggesting this signal is the consequence of deep magmatic processing, here attributed to an expanding zone of melting, assimilation, storage, and homogenization (MASH) of mantle-derived magma in the deep crust. Upward expansion brought the MASH zone into contact with rocks that were increasingly evolved with respect to Sr and Nd isotopes, explaining the isotopic shifts. Downward expansion of the MASH zone enhanced garnet stability during basalt fractionation, explaining the increased Dy/Yb and Sm/Yb ratios. Mass balance calculations involving Sr, Nd, and O isotope modeling are consistent with a crustal component making up 10 - 30% of AVC lavas, implying that although the history of central Andean magmatism is replete with large scale crustal recycling, the current phase is largely a crust formation event. / Graduation date: 2012

We develop a two dimensional model that simulates the response of the crust to prolonged mantle-derived intrusions in arc environments. The domain includes the entire crustal section and upper mantle and focuses on the evolving thermal structure due to intrusions and external tectonic forcing. We monitor the thermal response, melt fraction and volume for different environments after a definite time by considering geologically relevant melt flux and extensional tectonic rates. The amount of crustal melt versus fractionated primary mantle melts present in the crustal column helps determine crustal structure and growth through time. We observe that with a geophysically estimated flux and tectonic rate, the mantle-derived magma bodies can melt the surrounding volume of crust. We express the amount of crustal melting in terms of an efficiency; therefore we define the melting efficiency as the ratio of the melted volume of crustal material to the volume of melt expected from a strict enthalpy balance as explained by Dufek and Bergantz (2005). Melting efficiencies are less than 1.0 in real systems because heat diffuses to sections of the crust that never melt. The maximum calculated efficiency is 0.05 in our model while most of our simulations show zero efficiency. Additionally, maximum total melt amount is observed in relatively greater extensional environments (0.02 m/yr) and high intrusion rates (10⁻² m³/m²/yr) and in long time periods (2 x 10⁶ years). However, maximum crustal melting in the same environment is reached in 1.2 x 10⁶ years. The relative amounts of mantle-derived and crustal melts in the total volume of magma suggest that the majority of magma composition in crustal column is derived from the mantle material.

Volatile elements released from the subducting slab play a fundamental role during the formation of arc magmas in the mantle wedge. Advances of melt inclusion studies enlarged the data on volatile abundance in arc magmas, and it is now possible to characterize some volatile contents in arc primary magmas, in particular F and Cl. A recent study of Mt Shasta melt inclusions (LeVoyer et al., 2010) shows that fractionation of F and Cl potentially contains information about arc magma genesis. In order to trace the source of arc magmas, fluorine and chlorine partitioning was investigated. Here, I present new experimental determinations of Cl and F partition coefficients between dry and hydrous silicate melts and mantle minerals: olivine, orthopyroxene, clinopyroxene, plagioclase, garnet and also pargasite and phlogopite. The values were compiled from more than 300 measurements in 24 melting experiments, conducted between 8 and 25 kbars and between 1180 and 1430˚C. The low abundance F, Cl measurements in minerals were done by Cameca IMF 1280 at WHOI using the negative secondary ion mode. The results show that DOpx/meltF ranges from 0.123 to 0.021 and DCpx/meltF ranges from 0.153 to 0.083, while Cl partition coefficient varies from DOpx/meltCl from 0.002 to 0.069 and DCpx/meltCfrom 0.008 to 0.015, as well. Furthermore, DOl/meltF ranges from 0.116 to 0.005 and DOl/meltCl from 0.001 to 0.004; DGrt/meltF ranges from 0.012 to 0.166 and DGrt/meltCl from 0.003 to 0.087 with the increasing water amount and decreasing temperature. I also show that F is compatible in phlogopite DPhl/meltF > 1.2) while DAmp/meltF is incompatible in pargasite DAmp/meltF from 0.36 to 0.63). On the contrary, Cl is more incompatible in phlogopite (DPhl/meltCl > 1.2 on average 0.09 ± 0.02), than in pargasite (DPhl/meltCl from 0.12 to 0.38). This study demonstrates that F and Cl are substituted in specific oxygen site in minerals that lead then to be more sensitive than trace elements to crystal chemistry and water amount variations thus melting conditions. Using those new partition coefficients, I modelled melting of potential sub-arc lithologies with variable quantity aqueous-fluid. This model is able to decipher 1) amount of aqueous-fluid involved in melting, 2) melting induced by fluid or melting of an hydrous mineral-bearing source and 3) melting of either pargasite-bearing lithology or phlogopite-bearing lithology and shows that sources of some primitive melts, for instance from Italy, bear pargasite and phlogopite, while some primitve melts seem to be the results of fluid-induced melts.

The thermal structure of subduction zones depends on the age-controlled thermal state of the subducting slab and mantle wedge flow. Observations indicate that the shallow part of the forearc mantle wedge is stagnant and the slab-mantle interface is weakened. In this dissertation, the role of the interface strength in controlling mantle wedge flow, thermal structure, and a wide range of subduction zone processes is investigated through two-dimensional finite-element modelling and a global synthesis of geological and geophysical observations. The model reveals that the strong temperature-dependence of the mantle strength always results in full slab-mantle decoupling along the weakened part of the interface and hence complete stagnation of the overlying mantle. The interface immediately downdip of the zone of decoupling is fully coupled, and the overlying mantle is driven to flow at a rate compatible with the subduction rate. The sharpness of the transition from decoupling to coupling depends on the rheology assumed and increases with the nonlinearity of the flow system. This bimodal behaviour of the wedge flow gives rise to a strong thermal contrast between the cold stagnant and hot flowing parts of the mantle wedge. The maximum depth of decoupling (MDD) thus dictates the thermal regime of the forearc. Observed surface heat flow patterns and petrologically and geochemically estimated mantle wedge temperatures beneath the volcanic arc require an MDD of 70-80 km in most, if not all, subduction zones regardless of their thermal regime of the slab. The common MDD of 70-80 km explains the observed systematic variations of the petrologic, seismological, and volcanic processes with the thermal state of the slab and thus explains the rich diversity of subduction zones in a unified fashion. Models for warm-slab subduction zones such as Cascadia and Nankai predict shallow dehydration of the slab beneath the cold stagnant part of the mantle wedge, which provides ample fluid for mantle wedge serpentinization in the forearc but little fluid for melt generation beneath the arc. In contrast, models for colder-slab subduction zones such as NE Japan and Kamchatka predict deeper dehydration, which provides greater fluid supply for melt generation beneath the arc and allows deeper occurrence of intraslab earthquakes but less fluid for forearc mantle wedge serpentinization. The common MDD also explains the intriguing uniform configuration of subduction zones, that is, the volcanic arc always tends to be situated where the slab is at about 100 km depth. The sudden onset of mantle wedge flow downdip of the common MDD overshadows the thermal effect of the slab, and the resultant thermal field and slab dehydration control the location of the volcanic arc. The recognition of the fundamental importance of the MDD has important implications to the study of geodynamics and earthquake hazard in subduction zones.