МОДЕЛИРОВАНИЕ ПОСТСЕЙСМИЧЕСКИХ ПРОЦЕССОВ В СУБДУКЦИОННЫХ ЗОНАХ

Large intraplate subduction earthquakes are generally accompanied by prolonged and intense postseismic anomalies. In the present work, viscoelastic relaxation in the upper mantle and the asthenosphere is considered as a main mechanism responsible for the occurrence of such postseismic effects. The study of transient processes is performed on the basis of data on postseismic processes accompanying the first Simushir earthquake on 15 November 2006 and Maule earthquake on 27 February 2010.The methodology of modelling a viscoelastic relaxation process after a large intraplate subduction earthquake is presented. A priori parameters of the selected model describing observed postseismic effects are adjusted by minimizing deviations between modeled surface displacements and actual surface displacements recorded by geodetic methods through solving corresponding inverse problems.The presented methodology yielded estimations of Maxwell’s viscosity of the asthenosphere of the central Kuril Arc and also of the central Chile. Besides, postseismic slip distribution patterns were obtained for the focus of the Simushir earthquake of 15 November 2006 (Mw=8.3) (Figure 3), and distribution patterns of seismic and postseismic slip were determined for the focus of the Maule earthquake of 27 February 2010 (Mw=8.8) (Figure 6). These estimations and patterns can provide for prediction of the intensity of viscoelastic stress attenuation in the asthenosphere; anomalous values should be taken into account as adjustment factors when analyzing inter-seismic deformation in order to ensure correct estimation of the accumulated elastic seismogenic potential.Крупные межплитовые субдукционные землетрясения, как правило, сопровождаются длительными и интенсивными постсейсмическими аномалиями. В настоящей работе в качестве основного механизма, ответственного за возникновение подобных постсейсмических эффектов, рассматривается процесс вязкоупругой релаксации в верхней мантии и астеносфере. Исследование переходных процессов проводится на примере постсейсмических явлений, сопровождающих первое Симуширское землетрясение 15 ноября 2006 г., а также землетрясение Мауле 27 февраля 2010 г.Описана методология моделирования процесса вязкоупругой релаксации после крупных межплитовых субдукционных землетрясений. Уточнение априорных параметров выбранной модели, описывающей наблюдаемые постсейсмические эффекты, осуществляется за счет уменьшения невязки между моделируемыми и наблюдаемыми геодезическими методами смещениями земной поверхности при решении соответствующей обратной задачи.В соответствии с представленной методологией получены оценки Максвелловской вязкости астеносферы в срединной части Курильской островной дуги, а также в регионе Центрального Чили. Кроме того, получены распределения постсейсмической подвижки в очаге Симуширского землетрясения, Mw=8.3 (рис. 3), а также распределения сейсмической и постсейсмической подвижек в очаге землетрясения Мауле, Mw=8.8 (рис. 6). Результат таких оценок и построений позволяет прогнозировать интенсивность затухания вязкоупругих напряжений в астеносфере. Учет соответствующих аномалий в качестве поправок необходим при анализе межсейсмических деформаций для корректного оценивания накапливающегося упругого сейсмогенного потенциала

Coseismic Deformation Detection and Quantification for Great Earthquakes Using Spaceborne Gravimetry

This Ohio State University Geodetic Science Report was prepared for, in part, and submitted to the Graduate School of the Ohio State University as a Dissertation in partial fulfillment of the requirements of the Doctor of Philosophy (PhD) degree.This research is conducted under the supervision of Professor C.K. Shum, Division of Geodetic Science, School of Earth Sciences, The Ohio State University. The research results documented in this report resulted in a PhD Dissertation by Lei Wang (2012), Division of Geodetic Science, School of Earth Sciences, The Ohio State University. This research is partially funded by grants from NASA’s Interdisciplinary Science Program (NNG04GN19G), NASA’s Ocean Surface Topography Mission (OSTM) and Physical Oceanography Program (JPL1283230), the Air Force Materiel Command (FA8718-07-C-0021), and NSF’s Division of Earth Sciences (EAR-1013333). We would like to acknowledge Professor Frederik J. Simons, Department of Geosciences, Princeton University, for his hosting of Dr. Lei Wang for the summer visits.Because of Earth’s elasticity and its viscoelasticity, earthquakes induce mass redistributions in the crust and upper mantle, and consequently change Earth’s external gravitational field. Data from Gravity Recovery And Climate Experiment (GRACE) spaceborne gravimetry mission is able to detect the permanent gravitational and its gradient changes caused by great earthquakes, and provides an independent and thus valuable data type for earthquake studies. This study uses a spatiospectral localization analysis employing the Slepian basis functions and shows that the method is novel and efficient to represent and analyze regional signals, and particularly suitable for extracting coseismic deformation signals from GRACE. For the first time, this study uses the Monte Carlo optimization method (Simulated Annealing) for geophysical inversion to quantify earthquake faulting parameters using GRACE detected gravitational changes. GRACE monthly gravity field solutions have been analyzed for recent great earthquakes. For the 2004 Mw 9.2 Sumatra-Andaman and 2005 Nias earthquakes (Mw 8.6), it is shown for the first time that refined deformation signals are detectable by processing the GRACE data in terms of the full gravitational gradient tensor. The GRACE-inferred gravitational gradients agree well with coseismic model predictions. Due to the characteristics of gradient measurements, which have enhanced high-frequency contents, the GRACE observations provide a more clear delineation of the fault lines, locate significant slips, and better define the extent of the coseismic deformation; For the 2010 Mw 8.8 Maule (Chile) earthquake and the 2011 Mw 9.0 Tohoku-Oki earthquake, by inverting the GRACE detected gravity change signals, it is demonstrated that, complimentary to classic teleseismic records and geodetic measurements, the coseismic gravitational change observed by spaceborne gravimetry can be used to quantify large scale deformations induced by great earthquakes

Coseismic Deformation Detection and Quantification for Great Earthquakes Using Spaceborne Gravimetry

This Ohio State University Geodetic Science Report was prepared for, in part, and submitted to the Graduate School of the Ohio State University as a Dissertation in partial fulfillment of the requirements of the Doctor of Philosophy (PhD) degree.This research is conducted under the supervision of Professor C.K. Shum, Division of Geodetic Science, School of Earth Sciences, The Ohio State University. The research results documented in this report resulted in a PhD Dissertation by Lei Wang (2012), Division of Geodetic Science, School of Earth Sciences, The Ohio State University. This research is partially funded by grants from NASA’s Interdisciplinary Science Program (NNG04GN19G), NASA’s Ocean Surface Topography Mission (OSTM) and Physical Oceanography Program (JPL1283230), the Air Force Materiel Command (FA8718-07-C-0021), and NSF’s Division of Earth Sciences (EAR-1013333). We would like to acknowledge Professor Frederik J. Simons, Department of Geosciences, Princeton University, for his hosting of Dr. Lei Wang for the summer visits.Because of Earth’s elasticity and its viscoelasticity, earthquakes induce mass redistributions in the crust and upper mantle, and consequently change Earth’s external gravitational field. Data from Gravity Recovery And Climate Experiment (GRACE) spaceborne gravimetry mission is able to detect the permanent gravitational and its gradient changes caused by great earthquakes, and provides an independent and thus valuable data type for earthquake studies. This study uses a spatiospectral localization analysis employing the Slepian basis functions and shows that the method is novel and efficient to represent and analyze regional signals, and particularly suitable for extracting coseismic deformation signals from GRACE. For the first time, this study uses the Monte Carlo optimization method (Simulated Annealing) for geophysical inversion to quantify earthquake faulting parameters using GRACE detected gravitational changes. GRACE monthly gravity field solutions have been analyzed for recent great earthquakes. For the 2004 Mw 9.2 Sumatra-Andaman and 2005 Nias earthquakes (Mw 8.6), it is shown for the first time that refined deformation signals are detectable by processing the GRACE data in terms of the full gravitational gradient tensor. The GRACE-inferred gravitational gradients agree well with coseismic model predictions. Due to the characteristics of gradient measurements, which have enhanced high-frequency contents, the GRACE observations provide a more clear delineation of the fault lines, locate significant slips, and better define the extent of the coseismic deformation; For the 2010 Mw 8.8 Maule (Chile) earthquake and the 2011 Mw 9.0 Tohoku-Oki earthquake, by inverting the GRACE detected gravity change signals, it is demonstrated that, complimentary to classic teleseismic records and geodetic measurements, the coseismic gravitational change observed by spaceborne gravimetry can be used to quantify large scale deformations induced by great earthquakes

Earthquakes and sea-level change in Hokkaido, north-east Japan

This thesis details the results of an investigation into the pattern of relative sea-level (RSL) changes in north-east Hokkaido, Japan. The aim of the research is to better understand the importance of seismic and non-seismic processes in controlling spatial patterns of vertical land motions over a range of timescales. The main focus is on using salt-marsh sediments as a source of data to reconstruct RSL change during the current interseismic period, since c. 300 calibrated years before present (cal. yr BP). Previous research on the Pacific coast of Hokkaido suggests that this period is characterised by subsidence caused by strain accumulation on the locked part of the Pacific/North American plates. I apply foraminiferal-based methods of palaeoenvironmental reconstruction to develop, using transfer functions, quantitative reconstructions of RSL change at five sites in north-east Hokkaido. Contemporary foraminifera are zoned with respect to elevation and tidal inundation, and my preferred transfer function (a model that contains 87 samples and 24 taxa) has a prediction r2 of 0.75 and a root mean squared error of prediction of ± 0.32 m. I apply this transfer function to shallow fossil sediment sequences at five salt marshes and use a combination of 210Pb, 137Cs and tephra chronology to establish age models for the sequences. The reconstructions are consistent in demonstrating little net RSL change during the last 300-100 cal. yrs, with the exception of data from one site, Sarfutsu-toh, located on the northern tip of Hokkaido. Chronologies from two profiles developed on the Pacific coast record strong evidence for recent RSL rise since the mid-1980s, but during earlier periods of the 20th century reconstructed RSL was stable or falling. I compare my reconstructions with other direct and proxy records of land and sea-level motions. Previously published GPS and repeat levelling data indicates subsidence in south-east Hokkaido during the 20th century, although the spatial patterns and rates of change have varied. An unknown amount of this subsidence at the Kushiro tide gauge likely reflects anthropogenic activities associated with sediment compaction as well as mining-induced subsidence. An analysis of the tide-gauge records from Hokkaido show a more varied pattern of land motions, although they also confirm subsidence on the Pacific coast, close to the Kuril trench. A database of Holocene sea-level index points provides insights into longer-term millennial-scale trends in RSL. Data from six regions of Hokkaido demonstrate stable RSL close to present during the mid- and late Holocene; only the northern tip of Hokkaido (around Sarubetsu) is there evidence for a small mid-Holocene highstand of 1-3 m above present. Finally, a review of Pleistocene raised marine terrace data shows net uplift over the last c. 330 k yr, with two areas of particularly high uplift at Abashiri and on the Pacific coast near Kushiro. The evidence presented in this research demonstrates that it is incorrect to infer that the current interseismic period is characterised by subsidence. Overall, RSL has changed little in the last 300-100 cal. yrs. The subsidence recorded in the mid- and late 20th century on the Pacific coast of Hokkaido is not typical of the full interseismic period, nor can it have been sustained over Holocene or Pleistocene timescales. Limited data from previous earthquake cycles indicate that RSL was stable, rising or falling during previous interseismic intervals. These observations suggest that a representative ‘Hokkaido earthquake deformation cycle’ may not exist. Future research should better understand the controls of Quaternary volcanic activity on regional deformation patterns, and apply microfossil-based techniques to multiple earthquake cycles at sites to help define the spatial extent of land motions associated with different events