Global modeling of the effect of strong lateral viscosity variations on dynamic geoid and mantle flow velocities

thesi

Maakoore vertikaalliikumised Eestis täppisnivelleerimiste andmetel

The aim of this study was to detect vertical crustal movements in Estonia and find out possible changes of vertical crustal movements in time. Vertical velocities of the benchmarks were calculated from the precise levellings between 1933 and 2011 of the Estonian levelling network using the joint weighted kinematic least squares adjustment of the levelling campaigns. Two different mathematical models, the “heights included” and the “heights eliminated” model, were used in the adjustment. Different options of the computer software Surfer were used for modelling of the vertical crustal movements. Accuracy of the models was estimated by finding differences between the velocities interpolated from the models and adjusted vertical velocities of the benchmarks, using the cross validation technique, and by comparing models with the results from other geodetic measurements (continuously operating GNSS stations, tide gauges, other land uplift models). From the variance component estimation, it appeared that levelling errors of the First levelling campaign were ~3 times larger than estimated a priori. Final adjustment was performed with the re-scaled weights according to the results of the variance component estimation. Models of the vertical crustal movements EST2013LU and EST2015LU were created based on the vertical velocities of the benchmarks. According to the models, rates of the land uplift in Estonia range from −0.7 mm/yr in SE Estonia to +2.8 mm/yr in the island of Hiiumaa in NW Estonia. Accuracy of the models was estimated to be ±0.4 mm/yr on average. The comparison of the models with the velocities from the independent measurement methods revealed best fit with the velocities of the GNSS permanent stations where residual differences remained within ±0.3 mm/yr on average. The discrepancies between the velocities of the coastal tide gauges and the velocities from the models were ±0.7…±1.0 mm/yr on average. Obtained differences implied to the systematic biases in tide gauge velocities. Comparison with the historical vertical crustal movement maps of Estonia showed that differences remained within ±0.7 mm/yr on average. The fit between the most recent Fennoscandian LU map NKG2005LU and the models obtained in the present study was very good. Differences remained within ±0.3 mm/yr on average. It appeared also that vertical velocity of the benchmarks has been significantly changed between the levelling periods. Results of the study can be used to estimate the risks to the coastal areas coming from the global warming related rise of the sea level.Doktoritöö eesmärk oli leida maakoore vertikaalliikumiste kiirused Eestis nelja kordusnivelleerimise (1933-2011) andmete põhjal ja välja selgitada reeperite kiiruste muutumine ajas. Reeperite vertikaalliikumise kiirused leiti nivelleerimiste ühisest kaalutud kinemaatilisest tasandusest vähimruutude meetodil. Tasandusel kasutati kahte matemaatilist mudelit: nn „kõrgustega“ ja „kõrgusteta“ mudelit. Maakoore vertikaalliikumiste modelleerimiseks kasutati tarkvara Surfer erinevaid võimalusi. Mudelite täpsust hinnati mudelist interpoleeritud ja reeperite tasandatud kiiruste vaheliste erinevuste leidmise, ristvalideerimise ja sõltumatute mõõtmistulemustega (GNSS-püsijaamad, veemõõdujaamad, teised mudelid) võrdlemise teel. Dispersioonikomponentide hindamisest selgus, et esimese nivelleerimiskampaania vead on ~3 korda suuremad kui a priori eeldati. Lõplik tasandus teostati dispersioonikomponentide hindamise tulemuste põhjal ümberskaleeritud kaaludega. Reeperite kiiruste põhjal loodi Eesti maakoore vertikaalliikumiste mudelid EST2013LU ja EST2015LU. Mudelite põhjal ulatuvad maatõusu kiirused Eestis alates −0.7 mm/a Kagu-Eestis kuni +2.8 mm/a Hiiumaal. Mudelite täpsuseks hinnati keskmiselt ±0.4 mm/a. Võrdluses sõltumatutest meetoditest määratud maatõusu kiirustega selgus, et parim oli sobivus GNSS-püsijaamade kiirustega, keskmiselt ±0.3 mm/a. Halvim oli sobivus ranniku veemõõdujaamade kiirustega: ±0.7….±1.0 mm/a. Saadud erinevused viitasid süstemaatilistele nihetele veemõõdujaamade kiirustes. Võrdlus Eesti varasemate maakoore vertikaalliikumiste kaartidega näitas, et erinevused jäid keskmiselt ±0.7 mm/a piiridesse. Mudelite sobivus viimase Fennoskandia maatõusu mudeliga NKG2005LU oli aga väga hea, erinevused jäid keskmiselt ±0.3 mm/a piiridesse. Samuti selgus, et nivelleerimisperioodide vahel on reeperite kiirus statistiliselt oluliselt muutunud. Uurimistöö tulemusi saab kasutada kliimasoojenemisest tulenevate meretõusu riskide hindamiseks rannikualadel

Hydrocode modeling of oblique impacts into terrestrial planets

The abundance of moderately siderophile elements (“iron-loving”; e.g., Co, Ni) in the Earth’s mantle is 10 to 100 times larger than predicted by chemical equilibrium between silicate melt and iron at low pressure, but it does match expectation for equilibrium at high pressure and temperature. Recent studies of differentiated planetesimal impacts assume that planetesimal cores survive the impact intact as concentrated masses that passively settle from a zero initial velocity and undergo turbulent entrainment in a global magma ocean; under these conditions, cores greater than 10 km in diameter do not fully mix without a sufficiently deep magma ocean. I have performed hydrocode simulations that revise this assumption and yield a clearer picture of the impact process for differentiated planetesimals possessing iron cores with radius = 100 km that impact into magma oceans. The impact process strips away the silicate mantle of the planetesimal and then stretches the iron core, dispersing the liquid iron into a much larger volume of the underlying liquid silicate mantle. Lagrangian tracer particles track the initially intact iron core as the impact stretches and disperses the core. The final displacement distance of initially closest tracer pairs gives a metric of core stretching. The statistics of stretching imply mixing that separates the iron core into sheets, ligaments, and smaller fragments, on a scale of 10 km or less. The impact dispersed core fragments undergo further mixing through turbulent entrainment as the molten iron fragments sink through the magma ocean and settle deeper into the planet. My results thus support the idea that iron in the cores of even large differentiated planetesimals can chemically equilibrate deep in a terrestrial magma ocean. The largest known impact on the Moon formed the South Pole-Aitken (SP-A) basin and excavated material as deep as the mantle. Here I suggest that large impacts eject enough material to cover the farside of the Moon. During the impact process, ejecta leave the crater and travel well beyond the transient crater. Ejecta blankets depend on impactor size and angle. I use iSALE, an impact hydrocode, to determine the ejecta distribution, volume, and thickness. I calculate the trajectory of ejecta that leave the crater and return to the lunar surface. In these simulations, an ejecta blanket forms, with a thickness of kilometers, over the lunar farside. The ejecta blanket thicknesses are comparable to the difference between nearside and farside crustal thickness. Previous studies suggest other possible mechanisms for the lunar farside-nearside dichotomy. However, the impact that formed SP-A basin was large enough to eject material onto the farside. I also suggest a differentiated impactor’s core would disperse downrange of the impact point underneath the basin. Doublet craters form within crater rays on terrestrial bodies. The near simultaneous impact of two projectiles results in overlapping craters. This process results in modified crater morphologies and ejecta morphologies. I modeled the impact of two identical projectiles and vary the angle, timing, and initial separation distance. In this work, I identified projectiles with a separation distance of four times their initial diameter will form distinct craters, but the ejecta from the uprange crater will overfill the downrange crater and result in a smaller crater depth. This result implies the direction of the impactor may be inferred from the crater depths. Also, I found impacts that form closer together result in elliptical or dumbbell craters depending upon the impact parameters. The ejecta curtains interact in each simulation and result in structures similar to the V-shaped ridges or “herringbone” patterns traversing clusters of secondary craters in observations. The ejecta that lands within the ridges comes from a depth that is 100 to 125 m for a 500 m impactor traveling at 1 km/s. This is less deep than the maximum excavation depth of 125 to 150 m, depending upon the impact angle. This work represents a first step towards a more comprehensive method for not only determining how doublet craters form and how aberrant craters form, such as Messier A on the Moon, but also determining how the regolith changes and the ejecta blanket forms for such impacts

MODELLING THE EARTH: COMPRESSIBLE VISCOELASTODYNAMICS, GRAVITATIONAL SEISMOLOGY AND TRUE POLAR WANDER

This thesis reviews and sheds new light on compressible Earth models and theories for the modelling of megathrust earthquakes and rotational instabilities caused by glacial isostatic adjustments and mantle convection. The basic theory is outlined in the first chapter, where we discuss the response of a self-gravitating Earth to external forces and loads seated at its surface or interior and we focus on elastic static perturbations and the transition between the elastic and fluid behaviours of the Earth that occurs on thousand and million years time scales. In the first part of this thesis, we derive the analytical solution of the momentum and Poisson equations for a spherically symmetric viscoelastic Earth model that accounts for compressibility both at the initial state of hydrostatic equilibrium and during the perturbations. This constitutes a step ahead with respect to all previous analytical solutions, which actually neglect compressibility in some aspects, and allows to gain deep insight into the relaxation spectrum of compressible viscoelastic Earth models. In the second part, we discuss long-wavelength gravity anomalies caused by the 2004 Sumatra earthquake and detected by the Gravity Recovery And Climate Experiment (GRACE) space mission. We extend the classic theory in order to interpret gravity anomalies in terms of volume changes within the solid Earth, advection of the initial density field and ocean water redistribution caused by perturbations of the ocean floor and surface topographies. This physics is then exploited in order to develop a novel procedure for the inversion of the principal seismic source parameters (hypocentre and moment tensor) of large earthquakes relying solely on space gravity data. This procedure, which complements traditional seismology and which we shall name Gravitational Centroid Moment Tensor (GCMT) analysis, is applied for the first time to the 2011 Tohoku earthquake. In the third part of the thesis, we discuss issues related to long time scale instabilities of the Earth's rotation. The slow motion of the rotation axis with respect to the mantle, called True Polar Wander (TPW), has continuously been debated after the pioneering works in the sixties by Munk, MacDonald and Gold. We thus discuss TPW due to variations of surface loading from ice ages on hundreds of thousand year time scales, its sensitivity to the elastic or viscoelastic rheologies of the lithosphere and the stabilizing role of mantle density heterogeneities. Also, we face the problem of TPW driven by mantle convection on the million years time scale. Most studies have assumed that on this long time scale the planet readjusts without delay and that the Earth's rotation axis and the maximum inertia direction of mantle convection coincide. We herein overcome this approximation and we provide a novel treatment of the Earth's rotation, which clearly explains the interaction between mantle convection and rotational bulge readjustments, and the physical laws for the characteristic times controlling the polar motion in the directions of the intermediate and minimum principal axes of the mantle convection inertia tensor. We thus clarify a fundamental issue related to mantle mass heterogeneities and TPW dynamics

지진학적 방법을 이용한 동북아시아 상부 맨틀 속도 구조 규명 및 맨틀 동역학과 판내 화산 활동에 대한 고찰

학위논문(박사) -- 서울대학교대학원 : 자연과학대학 지구환경과학부, 2021.8. 이준기.Volcanism and plate subduction are manifestations of vigorous mantle convection, and are important research subjects for understanding the thermodynamic state of the Earth's interior. Seismic waves propagating through the mantle reflect the physical properties of the mantle, providing detailed information on its thermal and compositional structure. Here we use various seismic methods to investigate the detailed upper mantle structure in northeast Asia where there are active intraplate volcanoes and ongoing plate subduction. Based on our results, we discuss the evolution of the upper mantle and mechanism of intraplate volcanism. Specifically, we apply teleseismic body-wave traveltime tomography to image the volcanic structure beneath Jeju Volcanic Island (Chapter 1), lithosphere and asthenosphere structures beneath the Korean Peninsula (Chapter 2), the stagnant Pacific slab and mantle transition zone structures (Chapter 3), and shallow asthenosphere structures beneath Quaternary volcanoes (Chapter 4). We measure relative arrival time residuals of the teleseismic P and S waves in high precision based on waveform similarity, and invert the datasets for constructing three-dimensional velocity structures using an iterative nonlinear tomography inversion technique. In Chapter 1, we use a unique dataset collected from the deployment of temporary broadband stations in Jeju Island and find out magmatic structures associated with a complex volcanic system that show evidence of a strong interaction between the continental lithosphere and ascending magmas. In Chapter 2, we image the detailed upper mantle structures beneath the Archean-Proterozoic massifs in the Korean Peninsula, which suggest the possible presence of a long-lasting lithospheric root that has experienced heterogeneous modification and reactivation at a craton margin in northeast Asia. In chapter 3, we image high-resolution deeper upper mantle structures including the mantle transition zone in northeast Asia by using the datasets of dense seismic arrays in the Korean Peninsula and southwest Japan. We find segmented stagnant Pacific slabs with a pronounced gap beneath the Korean Peninsula that was not revealed by previous studies due to limited resolution in this region. We detect and model wavefield variations of teleseismic body waves caused by the deeper upper mantle heterogeneities using 3-D waveform simulation. In the last chapter, we constrain thermal and compositional properties of the upper mantle based on quantitative assessment of high-resolution upper mantle seismic velocity models using thermodynamic calculations, mineralogical modeling, and seismic attenuation. We find evidence for shallow upper mantle melting zones focused beneath major Quaternary volcanoes and suggest an important role of shallow decompressional melting in developing and sustaining localized, long-lived intraplate volcanoes. Throughout all chapters, we have general implications for the origin, evolution, and relationships between the imaged upper mantle structures in the context of geology, tectonics, and geodynamics.화산 활동과 판구조 운동(예, 판의 섭입)은 지구 내부의 활발한 맨틀 대류를 입증하며, 지구 내부 열역학적 상태를 이해하는데 중요한 연구 대상이다. 지진파는 지각과 맨틀을 통과하면서 이들의 물리적인 상태를 반영하며, 이를 분석하여 지구 내부의 열적 및 성분적 특성에 대해 연구할 수 있다. 본 논문에서는 다양한 지진학적 방법을 활용하여 판 내 화산활동과 판의 섭입이 활발히 일어나고 있는 동북 아시아 상부 맨틀 구조를 연구한다. 구체적으로, 원거리 지진 주시 토모그래피를 사용하여 판 내 화산으로 알려진 제주도 하부 암석권 및 연약권의 구조(제 1장), 한반도 하부의 암석권 및 연약권 구조(제 2장), 동북아시아 지역의 상부 맨틀 전이대와 정체된 슬랩(제 3장), 한반도 주변 제 4기 화산체 하부의 얕은 연약권(제 4장)의 3차원 구조를 영상화하고 분석한다. 연구 자료로 원거리 지진의 실체파 신호(P, S파)를 이용하여 연구 지역에 조밀하게 설치된 지진계 간의 파형 유사성을 기반한 방법으로 정밀하게 측정된 상대 주시 잔차를 사용하며, 해당 잔차를 최소화할 수 있는 최적의 3차원 속도 구조 모델을 비선형 반복 역산 방법을 통해 구한다. 제 1장에서는, 서울대학교 지진학 연구실에서 제주도에 약 2년간 설치한 임시 지진 관측망 자료를 사용하였으며, 제주도 하부 상승하는 마그마와 대륙 암석권 간의 상호작용으로 발달한 복잡한 형태의 마그마 시스템을 영상화 하였다. 제 2장에서는 한반도의 시생대 및 원생대 대륙암권 하부의 상부 맨틀을 영상화하며, 그 결과 동북 아시아 대륙 연변부에서 불균질한 수정 및 재활성화를 겪은 오래되고 두꺼운 암석권 구조를 확인하였다. 제 3장에서는 한반도와 일본 남서부의 관측망 자료를 사용하여 한반도를 포함한 동북아시아 하부의 깊은 상부 맨틀 구조를 영상화 하였다. 그 결과 유라시아판 하부에 정체된 태평양 슬랩의 불균질한 분포를 확인하였고, 기존 연구에서 해상도 한계로 보이지 않았던 한반도 하부의 슬랩 간극의 존재를 새롭게 밝혀낸다. 또한 한반도에 기록된 원거리 지진의 S파를 분석하여 슬랩 간극에 의한 파면 왜곡을 관측하였고 이를 3차원 파형 시뮬레이션을 통해 모사하였다. 마지막으로 제 4장에서는 기존에 얻어진 고해상도 속도 구조 모델을 바탕으로 동북 아시아 상부 맨틀의 열적 및 성분적 특성을 열역학적 계산 및 지진파 감쇠 특성을 고려하여 정량적으로 추정한다. 그 결과 한반도 주변 제 4기 화산 지역(예, 울릉도, 독도, 백두산, 제주도, 한탄강 유역)의 국지적이고 지속적인 마그마 발달에 기여하였을 것으로 생각되는 집중된 얕은 연약권 맨틀의 부분 용융을 확인하였다. 전 장을 통틀어, 상부 맨틀 구조들의 기원, 진화, 그리고 상호 관계에 대해 지질학적, 지구조적, 그리고 지구동역학적인 관점에서 해석하고 논의한다.Introduction 1 Chapter 1. Imaging of Lithospheric Structure Beneath Jeju Volcanic Island by Teleseismic Traveltime Tomography 4 1.1 Introduction 4 1.2 Methods 10 1.3 Results and Discussions 38 1.4 Conclusions 64 Chapter 2. Heterogeneous Modification and Reactivation of a Craton Margin Beneath the Korean Peninsula by Teleseismic Traveltime Tomography 66 2.1 Introduction 66 2.2 Methods 73 2.3 Results and Discussions 95 2.4 Conclusions 109 Chapter 3. Segmented Stagnant Pacific Slab and Its Interaction with the Mantle Transition Zone Beneath Northeast Asia Continental Margin by Teleseismic Traveltime Tomography 111 3.1 Introduction 111 3.2 Methods 115 3.3 Results and Discussions 131 3.4 Conclusions 153 Chapter 4. Seismic Evidence of Persistent Intraplate Volcanism by Shallow Mantle Melting in Northeast Asia 154 4.1 Introduction 154 4.2 Methods 158 4.3 Results 188 4.4 Discussions 198 4.5 Conclusions 211 Summary and Conclusions 212 Bibliography 217 Abstract in Korean (국문 초록) 263박

Geodetic monitoring of tectonic deformation: Toward a strategy

Issues of interest and importance to society and science are presented. The problems considered are of national concern; their solutions may contribute to a better understanding of tectonic deformation and earthquake hazards. The need for additional field data, the role of geodetic measurements, the importance of both ground and space techniques, and the need for advanced instrumentation development are discussed

Structure and anisotropy of the upper mantle

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1995.Includes bibliographical references (p. 171-180).by James B. Gaherty.Ph.D

Mantle convection and the state of the Earth's interior

During 1983–1986, the four year period covered by this review, emphasis in the study of mantle convection shifted away from fluid mechanical analysis of simple systems with uniform material properties and simple geometries, toward analyzing the effects of more complicated, presumably more realistic models