A Fine Velocity and Strain Rate Field of Present-Day Crustal Motion of the Northeastern Tibetan Plateau Inverted Jointly by InSAR and GPS

Interferometric synthetic aperture radar (InSAR) data from 6 Envisat ASAR descending tracks; spanning the 2003⁻2010 period; was used to measure interseismic strain accumulation across the Northeastern Tibetan Plateau. Mean line-of-sight (LOS) ratemaps are computed by stacking atmospheric-corrected and orbital-corrected interferograms. The ratemaps from one track with different atmospheric-corrected results or two parallel; partially overlapping tracks; show a consistent pattern of left-lateral motion across the fault; which demonstrates the MERIS and ECMWF atmospheric correction works satisfactorily for small stain measurement of this region; even with a limited number of interferograms. By combining the measurements of InSAR and GPS; a fine crustal deformation velocity and strain rate field was estimated on discrete points with irregular density depending on the fault location; which revealed that the present-day slip rate on the Haiyuan fault system varies little from west to east. A change (2⁻3 mm/year) in line-of-sight (LOS) deformation rate across the fault is observed from the Jinqianghe segment to its eastern end. Inversion from the cross-fault InSAR profiles gave a shallow locking depth of 3⁻6 km on the main rupture of the 1920 earthquake. We therefore infer that the middle-lower part of the seismogenic layer on the 1920 rupture is not yet fully locked since the 1920 large earthquake. Benefit from high spatial resolution InSAR data; a low strain accumulation zone with high strain rates on its two ends was detected; which corresponds to the creeping segment; i.e., the Laohushan fault segment. Contrary to the previous knowledge of squeezing structure; an abnormal tension zone is disclosed from the direction map of principal stress; which is consistent with the recent geological study. The distribution of principal stress also showed that the expanding frontier of the northeastern plateau has crossed the Liupan Shan fault zone; even arrived at the northeast area of the Xiaoguan Shan. This result agrees with the deep seismic reflection profile

Present-day activity and seismic potential of the north Qinling fault, southern ordos block, central China, as revealed from GPS data and seismicity

The North Qinling Fault, located at the boundary of the North China Block and the South China Block, represents an important tectonic structure between the Weihe Basin and the Qinling Mountains, and controls the subsidence and expansion of the Weihe Basin. This fault has been highly active and has caused strong earthquakes since the Holocene and in a pre-seismic stage currently, as indicated by the many paleoearthquake traces found along it. To determine the present-day activity and seismic potential of the North Qinling Fault, by inverting GPS data, we produced fault locking depth, slip rate, and regional strain fields maps; moreover, based on seismicity, we produced a seismic b-value map. Combining this information with modern seismicity, we were able to comprehensively analyze the seismic potential of different fault segments. Our inversion of GPS data showed that the slip rate of the western segment of the fault (Qingjiangkou–Xitangyu) and the correspondent locking depth are 1.33 mm/a and 13.54 km, respectively, while the slip rate of the middle segment (Xitangyu–Fengyukou) and the correspondent locking depth are 0.45 mm/a and 8.58 km, respectively; finally, the slip rate of the eastern segment (Xitangyu–Daiyu) and the correspondent locking depth are 0.36 mm/a and 21.46 km, respectively. The locking depths of the western and middle segments of the fault are shallower than 90% of the seismic cutoff depth, while the locking depth of the eastern segment of the fault is similar to 90% of the seismic cutoff depth, indicating that “deep creep” occurs in the western and middle segments, while the eastern segment is locked. Modern small earthquakes have involved the western and middle segments of the fault, while the eastern segment has acted as a seismic gap with weak seismicity, characterized by a higher shear strain value and a lower b-value. These characteristics reflect the relationship between the locking depth and seismicity distribution. The results of our comprehensive analysis, combined with field geological surveys, show that the eastern segment of the North Qinling Fault has a strong seismic potential and is presently locked

Mechanisms of strain localisation in the lithosphere

This thesis examines the development of shear-zone localisation in the continental lithosphere. I use non-Newtonian, viscous models to examine the controls on strain localisation with depth and on the development of horizontal shear-zones in regions away from strength contrasts. I show how the vertical extent of strain localisation is principally controlled by lithology and geothermal gradient, and how the horizontal extent of localisation is a consequence of strain-weakening and the geometry of strength contrasts. I explore how strain localisation develops from an initial isolated weak inclusion. The progress of strain localisation follows a power-law growth that is strongly non-linear. When applied to the rheological laws for common lithospheric minerals, the temperature and stress-dependence provide a direct means of predicting the depth below which localisation does not occur. I apply the calculations to four major continental strike-slip zones and find observations from seismic data agree with the calculations. Localisation to the base of the lithosphere is not supported by the calculations or the geophysical data. I use a model configured to resemble the India-Asia convergence that includes an isolated weak region within the Tibetan Plateau area and, in selected experiments,strong regions representing the Tarim and Sichuan Basins. I rotate a strong India region into a weaker Asia and observe the evolving strain. Shear zones develop adjacent and propagate outwards from the weak region. Where the Basins are present then high strain- rate zones develop adjacent to them and the overall distribution of strain within the model is altered. A high strain-weakening component enables shear-zones to localise. Micro-plate models assume the pre-existence of such high strain regions but I show how a continuum model can initiate and grow localised deformation within a region of generally diffuse deformation

Investigating the postseismic deformation of strike-slip earthquakes on the Tibetan Plateau

InSAR is a useful technique to detect large-scale surface deformation from space. To place constraints on the rheological structure of the lithosphere in the Tibetan Plateau, two strike-slip earthquakes have been investigated. One is the Mw 7.6 Manyi earthquake, which occurred in the north-central Tibetan Plateau. The other is the Mw 6.5 Jiuzhaigou earthquake, which happened that on the eastern part of the Tibetan Plateau. My InSAR data cover 12 years following the Manyi earthquake, much longer than previous researchers’ dataset. I test three viscoelastic models (Maxwell, Standard linear solids, and Burgers body) and one afterslip model. The viscoelastic models cannot match the observed temporal–spatial deformation patterns. The distributions of deformation in the viscoelastic models extend into the far field and the residuals tend to increase, which are inconsistent with the data. The afterslip model has the lowest misfit and explains the temporal and spatial pattern of the observed deformation with decent result. A combined model that considers the effects of both afterslip and viscoelastic relaxation has also been tested. In this combined model, the viscoelastic relaxation that occurs with an elastic layer of thickness of 30 km over a half-space place, produce an estimate for viscosity of 5 × 1019 Pa s for this area. Therefore, either the afterslip model or the combined model can be used to explain the 12 years postseismic deformation of Manyi earthquake. The long time series of the Manyi earthquake enable us to distinguish between afterslip and viscoelastic relaxation. The seismogenic fault of the Jiuzhaigou earthquake was previously unidentified and no surface rupture is found after the earthquake. I first determined the fault geometry and calculated coseismic slip model. The slip model indicates a left-lateral strike-slip pattern, which is consistent with focal mechanisms were determined by different agencies. There is no visible postseismic deformation signal of the fault, which means the surface deformation generated by fault creeping is smaller than the noise of our observation method over that period. Therefore, I try to find the lower bound of the viscosity for this area. My preferred minimum possible viscosity of the underlying half-space is ∼6 × 1017 Pa s. Together with previous geodetic studies, the viscosities obtained from central Tibet show at least one order of magnitude difference with the viscosities obtained from the eastern Tibet. The heterogeneity indicates the rheology has a relatively large spatial change through the whole Plateau. The viscoelastic model always been proposed to explain long-term postseismic deformation and afterslip is used to explain the short-term deformation or localised deformation. Sometimes, the viscoelastic deformation signal is invisible in the moderate earthquakes as the stress is not large enough to generate observable deformation

The Tectonic Evolution of the Tibetan Plateau: Insights from the Deformation and Erosion History of Northern Tibet and the Surrounding Region.

The Tibetan Plateau-Himalaya system is the archetype of a continental collision thus, defining its tectonic and topographic evolution is of great importance to our understanding of continental tectonics. Ongoing India-Eurasia convergence since ~55 Ma is the widely accepted cause of great crustal thickness (>60 km) and high elevation (>5 km) within this region; however, the manner in which this convergence is accommodated is controversial. In this dissertation, I employ a multidisciplinary approach to constrain the structural and topographic evolution of the Tibetan Plateau at two spatial scales: a detailed study along the northeastern plateau margin and a broader-scope study across the width of the plateau interior. Results from geologic mapping, low-temperature thermochronometry, and 40Ar/39Ar geochronology indicate that pre-Miocene deformation within northern Tibet occurred primarily along contractional structures oriented favorably to accommodate NNE-SSW Indo-Eurasian plate convergence. Deformation patterns appear to have changed beginning in the post middle-Miocene, as both thrusting and strike-slip fault motion of variable orientation is observed in a zone of transpression bounded by the Kunlun and Haiyuan left-lateral faults. Results from the broad-scale, detrital low-temperature thermochronometry erosion study suggest that erosion rates increase by at least an order of magnitude between 11-4 Ma following a period of slow erosion across the entire east-central Tibetan Plateau. Taken together, the finding of an early to mid-Cenozoic deformation history in northern Tibet and the synchroneity of accelerated erosion across the entire eastern plateau challenges the widely accepted view that the orogen grew northward through time. Instead, widespread accelerated river incision during the mid-to-late Miocene is consistent with regional scale uplift that occurred in concert with eastern expansion of the orogen by lower crustal flow. Timing of proposed eastward crustal flow overlaps with the shift to a predominantly left-lateral strike-slip fault regime documented in northern Tibet and with the timing of onset of other major intracontinental strike-slip faults in southern and central Tibet. Here I propose that these crustal processes occur as a consequence of long-term plate convergence rather than resulting from a specific event, such as mantle delamination, or abrupt change in plate kinematics.Ph.D.GeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/89636/1/duvall_1.pd

Improved modeling of segmented earthquake rupture informed by enhanced signal analysis of seismic and geodetic observations

Earthquake source modeling has emerged from the need to be able to describe and quantifythe mechanism and physical properties of earthquakes. Investigations of earthquake ruptureand fault geometry requires the testing of a large number of such potential sets of earthquakesources models. Earthquakes often rupture across more than one fault segment. If such rupturesegmentation occurs on a significant scale, a simple model may not represent the rupture processwell. This thesis focuses on the data-driven inclusion of earthquake rupture segmentation intoearthquake source modeling. The developed tools and the modeling are based on the jointuse of seismological waveform far-field and geodetic Interferometric Synthetic Aperture Radarnear-field surface displacement maps to characterise earthquake sources robustly with rigorousconsideration of data and modeling errors.A strategy based on information theory is developed to determine the appropriate modelcomplexity to represent the available observations in a data-driven way. This is done inconsideration of the uncertainties in the determined source mechanisms by investigating theinferences of the full Bayesian model ensemble. Application on the datasets of four earthquakesindicated that the inferred source parameters are systematically biased by the choice of modelcomplexity. This might have effects on follow-up analyses, e. g. regional stress field inversionsand seismic hazard assessments.Further, two methods were developed to provide data-driven model-independent constraints toinform a kinematic earthquake source optimization about earthquake source parameter priorestimates. The first method is a time-domain multi-array backprojection of teleseismic datawith empirical traveltime corrections to infer the spatio-temporal evolution of the rupture. Thisenables detection of potential rupture segmentation based on the occurrence of coherent high-frequency sources during the rupture process. The second developed method uses image analysismethods on satellite radar measured surface displacement maps to infer modeling constraints onrupture characteristics (e.g. strike and length) and the number of potential segments. These twomethods provide model-independent constraints on fault location, dimension, orientation andrupture timing. The inferred source parameter constraints are used to constrain an inversion forthe source mechanism of the 2016 Muji Mw 6.6 earthquake, a segmented and bilateral strike-slipearthquake.As a case study to further investigate a depth-segmented fault system and occurrence of co-seismic rupture segmentation in such a system the 2008-2009 Qaidam sequence with co-seismicand post-seismic displacements is investigated. The Qaidam 2008-2009 earthquake sequence innortheast Tibet involved two reverse-thrust earthquakes and a postseismic signal of the 2008earthquake. The 2008 Qaidam earthquake is modeled as a deep shallow dipping earthquakewith no indication of rupture segmentation. The 2009 Qaidam earthquake is modeled on threedistinct south-dipping high-angle thrusts, with a bilateral and segmented rupture process. Agood agreement between co-seismic surface displacement measurements and coherent seismicenergy emission in the backprojection results is determined.Finally, a combined framework is proposed which applies all the developed methods and tools inan informed parallel modeling of several earthquake source model complexities. This frameworkallows for improved routine determination of earthquake source modeling under considerationof rupture segmentation. This thesis provides overall an improvement for earthquake sourceanalyses and the development of modeling standards for robust determination of second-orderearthquake source parameters

Kinematic models of interseismic deformation from inversion of GPS and InSAR measurements to estimate fault parameters and coupling degree

We have used kinematic models in two Italian regions to reproduce surface interseismic velocities obtained from InSAR and GPS measurements. We have considered a Block modeling, BM, approach to evaluate which fault system is actively accommodating the occurring deformation in both considered areas. We have performed a study for the Umbria-Marche Apennines, obtaining that the tectonic extension observed by GPS measurements is explained by the active contribution of at least two fault systems, one of which is the Alto Tiberina fault, ATF. We have estimated also the interseismic coupling distribution for the ATF using a 3D surface and the result shows an interesting correlation between the microseismicity and the uncoupled fault portions. The second area analyzed concerns the Gargano promontory for which we have used jointly the available InSAR and GPS velocities. Firstly we have attached the two datasets to the same terrestrial reference frame and then using a simple dislocation approach, we have estimated the best fault parameters reproducing the available data, providing a solution corresponding to the Mattinata fault. Subsequently we have considered within a BM analysis both GPS and InSAR datasets in order to evaluate if the Mattinata fault may accommodate the deformation occurring in the central Adriatic due to the relative motion between the North-Adriatic and South-Adriatic plates. We obtain that the deformation occurring in that region should be accommodated by more that one fault system, that is however difficult to detect since the poor coverage of geodetic measurement offshore of the Gargano promontory. Finally we have performed also the estimate of the interseismic coupling distribution for the Mattinata fault, obtaining a shallow coupling pattern. Both of coupling distributions found using the BM approach have been tested by means of resolution checkerboard tests and they demonstrate that the coupling patterns depend on the geodetic data positions

Subject-Oriented Finite Fault Inversions and their Applications

Accurate images of the growth of slip and faulting are critical in understanding the physics of earthquakes, and subsequently aid in the prediction of the ground motion that could be expected from future rupture events. Many methods, namely finite fault inversions, have been developed that use observed seismic waveforms and static deformations to constrain the spatiotemporal rupture evolutions of great earthquakes. This work explores the development of novel inversion methods to further the investigation into properties of earthquake physics. Earthquakes lead to the overall reduction of stress across the ruptured fault plane, and stress drop is a key parameter in accurately estimating the strong ground motion. Thus, we have developed a new procedure to determine if it is possible to robustly constrain an uncertainty range for the co-seismic stress drop of large earthquakes. For a given earthquake, we loop through a series of target stress drops, and for each case we conduct a modified finite fault inversion to find the solution that matches the seismic and/or geodetic data, and also matches a prescribed stress drop value. From this, we can determine a relationship between the resulting misfit between our synthetic model and the observations, and the pre-assigned target stress drop value. This new inversion technique is applied to several rupture events with varying datasets in order to test its robustness. First, we examine the 2014 Mw 7.9 Rat Islands earthquake using far-field data and discovered that only the lower bound of the average stress drop could be well constrained. To investigate whether such a conclusion also holds for near field data, the slip distribution and the stress drop of the 2015 Mw 7.8 Gorkha, Nepal earthquake was studied using GPS and InSAR data. Our results revealed similar patterns: that even a comprehensive geodetic dataset could also only constrain the lower bound of stress drop. Furthermore, one of the important uses of stress drop for earthquake physics is in the study of energy partitioning. The average stress drop is equivalent to twice the ratio of apparent available energy to the total seismic potency, and so our result has a direct impact on the study of the earthquake energy budget. As stress drop is proportional to the available seismic energy, our results imply that only the lower bound of the available energy can be constrained. The November 14th 2016 MW 7.8 Kaikoura, New Zealand earthquake occurred along the northern part of the South Island. Available information indicates that this earthquake involved multiple fault segments of the Marlborough fault system (MFS). Additionally, slip might also have occured on the subduction interface of the Pacific Plate under the Australian Plate, beneath the MFS. The exact number of involved fault segments as well as the temporal co-seismic rupture sequence has not been fully determined. We propose two inversion strategies to determine the fault geometry and spatiotemporal rupture history: first we use teleseismic and strong motion waveforms to determine point-source focal mechanisms for all of the faults that participated in the rupture; second, we use seismic and geodetic data to invert for the kinematic rupture parameters on a limited number of fault segments. The first approach allows us to determine a rupture timing sequence for different fault segments. Once we have the timing among fault segments, we invert the seismic and geodetic data for the spatial and temporal kinematic parameters. Both of these methods allow us to evaluate the potential of slip on the Hikurangi subduction interface

Global investigation of large earthquakes using InSAR and long-period seismic data

In this thesis I present the first comprehensive global archive of InSAR-determined source models (ICMT database) compiled from the literature, which I use to independently assess source parameters reported in global and regional seismic catalogues. In general there is good agreement between InSAR and seismic source models, but there are some large discrepancies, particularly in location and seismic moment. There is a large intra-event variability in source parameters in the ICMT database, which highlights the uncertainties introduced by errors in the data and by simplified assumptions used in the modelling. Large discrepancies for five earthquakes with magnitudes Mw 6.0 -Mw 8.1 are investigated in detail by comparing seismic data with theoretical seismograms calculated using two forward modelling techniques and 1-D and 3-D Earth models. For moderate magnitude events the InSAR location improves the fit to the seismic data, but this is not the case for the larger earthquakes, which is partly due to errors in the Earth models used. These findings motivated the development of a new seismo-geodesy joint source inversion technique that takes into account the effects of 3-D Earth structure when modelling the seismic data. It is tested on three synthetic events with different faulting mechanisms and for three real earthquakes in various tectonic settings (Mw 6.0 Eureka Valley, Mw 6.6 Aiquile and Mw 6.5 Zarand events). These tests clearly show the advantages of taking into account 3-D Earth structure in the modelling, and the combination of InSAR and seismic datasets reduces parameter tradeoffs and enables the robust characterisation of the earthquake source