Greg Houseman - Profile on Academia.edu (original) (raw)
Papers by Greg Houseman
Combining faulting and ductile deformation in long-term models of continental deformation
The spatial variation of strain rate in broad regions of continental collision, extension, or she... more The spatial variation of strain rate in broad regions of continental collision, extension, or shear can often be well represented by the deformation of a thin viscous shell representing the lithosphere. The simplest explanation of this observation is that the deformation of the lithosphere is to first order a ductile process, even though shallow focus earthquakes imply slip on faults and release of elastic strain. In the thin-viscous-shell concept the strain of the upper brittle layer is assumed to simply follow the ductile strain of the stronger layers beneath, at least in the inter-seismic period. If the faults extend only to depths of 10 or 20 km, the brittle upper layer is not sufficiently thick or strong to do otherwise, and the concept of the brittle upper layer controlled by the ductile substrate is consistent with ductile models of the displacement-rate field constrained by GNSS observations. However, some large-scale faults do not comply with this concept and, rather than f...
Tellus A, 1977
A numerical technique is presented for calculating where, on average, tracer particles at a parti... more A numerical technique is presented for calculating where, on average, tracer particles at a particular place in a turbulent medium were created, and where they will be destroyed. This technique requires knowledge of creation, destruction, and transport rates, and gives as results probability distributions for creation and destruction. The technique is demonstrated on a model of stratospheric ozone, and a rough quantification is made of the statement that high latitude, low altitude ozone tends to be created in low latitude, high altitude regions.
Determining the distribution of seismic hazard in the continents requires an understanding of how... more Determining the distribution of seismic hazard in the continents requires an understanding of how much deformation is accommodated by major faults. Quantifying the role of major faults in continental deformation has been hampered by a lack of high-resolution observations in the deforming interiors of continents. By combining surface movement data derived from 22,000 satellite radar images with data from sparse, ground-based GNSS stations we produce the first high-resolution present-day surface velocity field for the Tibetan Plateau and surrounding region, where the collision of rigid Indian lithosphere with Eurasia has created Earth’s largest and highest deforming region. We show that continental deformation is best characterized by a combination of continuous distributed deformation and focused strain on a few major fault systems.
The Dynamics of the India-Eurasia Collision: Faulted Viscous Continuum Models Constrained by High-Resolution Sentinel-1 InSAR and GNSS Velocities
The dynamics of lithospheric deformation in the India-Eurasia collision zone has been debated ove... more The dynamics of lithospheric deformation in the India-Eurasia collision zone has been debated over many decades. Here we test a two-dimensional (2-D) Thin Viscous Shell (TVS) approach that has been adapted to explicitly account for displacement on major faults and investigate the impact of lateral variations in depth-averaged lithospheric strength. We present a suite of dynamic models to explain the key features from new high-resolution Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) as well as Global Navigation Satellite System (GNSS) velocities. Comparisons between calculated and geodetically observed velocity and strain rate fields indicate: (a) internal buoyancy forces from Gravitational Potential Energy (GPE) acting on a relatively weak region of highest topography (>2,000 m) contribute to dilatation of the high plateau and contraction on the margins; (b) a weak central Tibetan Plateau ~10^21 Pa s compared to far-field depth-averaged effective viscosity of 10^22 ...
European Review, 2017
Istanbul and Bucharest are major European cities that face a continuing threat of large earthquak... more Istanbul and Bucharest are major European cities that face a continuing threat of large earthquakes. The geological contexts for these two case studies enable us to understand the nature of the threat and to predict more precisely the consequences of future earthquakes, although we remain unable to predict the time of those events with any precision better than multi-decadal. These two cities face contrasting threats: Istanbul is located on a major geological boundary, the North Anatolian Fault, which separates a westward moving Anatolia from the stable European landmass. Bucharest is located within the stable European continent, but large-scale mass movements in the upper mantle beneath the lithosphere cause relatively frequent large earthquakes that represent a serious threat to the city and surrounding regions.
Admittance Functions for Axisymmetric Loading of a Viscous Layered Lithosphere
AGU Spring Meeting …, 2004
ABSTRACT
Geophysical Research Letters, 2016
Seismic images of active fault zones can be used to examine the structure of faults throughout th... more Seismic images of active fault zones can be used to examine the structure of faults throughout the crust and upper mantle and give clues as to whether the associated deformation occurs within a narrow shear zone or is broadly distributed through the lower crust. Limitations on seismic resolution within the crust and difficulties imaging shallow structures such as the crust-mantle boundary (Moho) place constraints on the interpretation of seismic images. In this study we retrieve body wave reflections from autocorrelations of ambient seismic noise. The instantaneous phase coherence autocorrelations allow unprecedented ambient noise images of the North Anatolian Fault Zone (NAFZ). Our reflection profiles show a Moho reflected P wave and additional structure within the crust and upper mantle. We image a distinct vertical offset of the Moho associated with the northern branch of the NAFZ indicating that deformation related to the fault remains narrow in the upper mantle.
Understanding Extension Within a Convergent Orogen: Initial Results From the Carpathian Basins Seismic Project
The Carpathian Basins Project (CBP) aims to understand the origin of Miocene-age extensional basi... more The Carpathian Basins Project (CBP) aims to understand the origin of Miocene-age extensional basins, of which the Pannonian Basin is the largest, within the arc of the Alpine-Carpathian Mountain Ranges - a compressional structure. Analysis of the subsidence history of the Pannonian Basin shows that its mantle lithosphere has undergone a much greater degree of extension than the overlying crust.
Geophysical Journal International, 1990
A mantle plume is probably a complex 3-D thermal structure that possesses approximate axisymmetry... more A mantle plume is probably a complex 3-D thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below, but followed down towards the base of the layer, probably consists of a triple-junction or quadruple-junction of connected hot sheets. A relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes. These conclusions are suggested by numerical experiments on a 3-D constant-viscosity, plane layer with stress-free boundaries, which detail the gradational change in the planform of a convecting layer from the top of the layer to its base. The planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer. These anomalies are the main sources of positive (for hot fluid) or negative (for cold fluid) buoyancy, and therefore they drive the convective flow. They may appear in cross-section as structures with either axial symmetry (columns), planar symmetry (sheets) or some complex asymmetric form. When convection is driven at least partially by basal heating, the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns, while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns. The hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface, and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface. Near the upper surface, the apparent planform of this experiment is analogous to that of mantle convection, the cold sheets compared to subduction zones and the hot columns compared to mantle plumes. The hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies, surface uplift and extensional stress fields. However, the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume, formed by the junction of three or four hot sheets on the base of the layer. It seems likely that the commonly occurring triple-junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath.
Coronae Formation on Venus by Rayleigh-Taylor Instability
ABSTRACT
Journal of Geophysical Research, 2011
The two-dimensional thin viscous sheet approximation is widely used to describe large-scale conti... more The two-dimensional thin viscous sheet approximation is widely used to describe large-scale continental deformation. It treats the lithosphere as a fluid layer in which deformation results from a balance between buoyancy forces and tectonic boundary conditions. Comparisons between two-dimensional thin sheet and full three-dimensional solutions of a simple indenter model show that appreciable differences exist, especially when the indenter half width, D, is of the same order as the thickness of the deforming layer, L (i.e., D/L ≈ 1). These differences are amplified by increasing the power law exponent of the viscous constitutive law (n) but decrease as the Argand number (Ar) is increased. The greatest differences between two-dimensional and three-dimensional solutions are found at the indenter corner, where the thin sheet consistently overestimates vertical strain rates. Differences between strain rates at the corner may be 50% or greater for small Argand numbers. Other differences arise because a lithospheric root zone is formed in the three-dimensional solutions and vertically averaged strain rate is decreased in regions close to the indenter. This effect is absent from thin sheet calculations since the thickness of the load-bearing layer is assumed constant. In general, the thin viscous sheet approximation provides a reasonably accurate estimate of long wavelength deformation for D/L as low as 1 if n is less than ∼3. However, even at large D/L the solutions may be inaccurate close to strain rate concentrations at the indenter corners where horizontal gradients of deformation are large.
Journal of Geophysical Research: Solid Earth, 2016
• Physical model that treats lithosphere of Anatolia and the Aegean as a fluid sheet fits GPS vel... more • Physical model that treats lithosphere of Anatolia and the Aegean as a fluid sheet fits GPS velocities with RMS misfit of 5 mm/yr. • Distributions of strain, earthquakes, and active faulting determined by lateral variations in gravitational potential energy of lithosphere. • Variation in strength of lithosphere not required to explain large-scale deformation though moderate weakening likely near N Anatolian fault.
Geophysical Journal International, 2001
We present a method for the determination of crustal structure by simultaneous inversion of seism... more We present a method for the determination of crustal structure by simultaneous inversion of seismic refraction and wide-angle reflection traveltimes for 3-D interface geometry and layer velocity. Crustal structure is represented by layers in which velocity varies linearly with depth, separated by smooth interfaces with a cubic B-spline parametrization. Lateral variations in structure are therefore represented by variations in interface depth only. The model parametrization we have chosen means that ray paths consist of piecewise circular arc segments for which analytic expressions of trajectory and traveltime are calculated. The two-point problem of finding the first-arrival ray path and traveltime of a specified phase between a given source and receiver is solved using a shooting technique. A subspace inversion method is used to solve the inverse problem, which is formulated as a non-linear optimization problem in which we seek to minimize an objective function that consists of a data residual term and a regularization term. Before performing the inversion, each data pick must be assigned as a refraction or reflection from a particular layer or interface. Since our method represents structure in terms of interfaces, fewer parameters would generally be used in a reconstruction compared to an equivalent 3-D variable-velocity inversion. The method is well suited to wide-angle surveys that consist of many sources and relatively few receivers (or vice versa), such as marine shot lines used in conjunction with land-based receivers. Data coverage in this kind of survey is often sparse and, especially if near-offset ray paths are unavailable, highly variable. A 3-D synthetic test with an array consisting of eight sources lying within a three-sided square of 79 receivers is described. The test model consists of a three-interface structure that includes a layer pinch-out, and the synthetic data set comprises 987 refraction and 930 reflection travel times contaminated with 75 ms of data noise. Six iterations of an 18-D subspace method demonstrate that the method can produce an accurate reconstruction that satisfies the data from a 1-D starting model. We also find that estimates of a posteriori model covariance and resolution obtained from linear theory are useful in analysing solution reliability despite the nonlinear nature of the problem. Application of the method to data collected as part of the 1995 TASGO project in Tasmania shows that the method can satisfy 1345 refraction and reflection traveltime picks with a geologically reasonable and robust 254-parameter three-interface model. The inversion results indicate that the Moho beneath NW Tasmania varies in depth from 27 km near the coast to 37 km near central Tasmania, with the major increase in depth occurring across the Arthur Lineament.
Geophysical Journal International, 1998
An inversion method is presented for the reconstruction of interface geometry between two or more... more An inversion method is presented for the reconstruction of interface geometry between two or more crustal layers from teleseismic traveltime residuals. The method is applied to 2-D models consisting of continuous interfaces separating constant-velocity layers. The forward problem of determining ray paths and traveltimes between incident wave fronts below the structure and receivers located on the Earth's surface is solved by an efficient and robust shooting method. A conjugate gradient method is employed to solve the inverse problem of minimizing a least-squares type objective function based on the difference between observed and calculated traveltimes. Teleseismic data do not accurately constrain average vertical structure, so a priori information in the form of layer velocities and average layer thicknesses is required. Synthetic tests show that the method can be used to reconstruct interface geometry accurately, even in the presence of data noise. Tests also show that, if layer velocities and initial interface positions are poorly chosen, lateral structure is still recoverable. The inversion method was applied to previously published teleseismic data recorded by an in-line array of portable seismographs that traversed the northern margin of the Musgrave Block, central Australia. The solution based on interface parametrization is consistent with models given by other studies that used the same data but different methods, most notably the standard tomographic approach that inverts for velocity rather than interface structure.
Geophysical Journal International, 2013
We use ambient noise tomography to investigate the crust and uppermost mantle structure beneath t... more We use ambient noise tomography to investigate the crust and uppermost mantle structure beneath the Carpathian-Pannonian region of Central Europe. Over 7500 Rayleigh wave empirical Green's functions are derived from interstation cross-correlations of vertical component ambient seismic noise recordings (2005-2011) using a temporary network of 54 stations deployed during the South Carpathian Project (2009-2011), 56 temporary stations deployed in the Carpathian Basins Project (2005-2007) and 100 permanent and regional broad-band stations. Rayleigh wave group velocity dispersion curves (4-40 s) are determined using the multiple-filter analysis technique. Group velocity maps are computed on a grid of 0.2 • × 0.2 • from a non-linear 2-D tomographic inversion using the subspace method. We then inverted the group velocity maps for the 3-D shear wave velocity structure of the crust and uppermost mantle beneath the region. Our shear wave velocity model provides a uniquely complete and relatively high-resolution view of the crustal structure in the Carpathian-Pannonian region, which in general is validated by comparison with previous studies using other methods to probe the crustal structure. At shallow depths (<10 km) we find relatively high velocities below where basement is exposed (e.g. Bohemian Massif, Eastern Alps, most of Carpathians, Apuseni Mountains and Trans-Danubian Ranges) whereas sedimentary areas (e.g. Vienna, Pannonian, Transylvanian and Foçsani Basins) are associated with low velocities of well defined depth extent. The mid to lower crust (16-34 km) below the Mid-Hungarian Line is associated with a broad NE-SW trending relatively fast anomaly, flanked to the NW by an elongated low-velocity region beneath the Trans-Danubian Ranges. In the lowermost crust and uppermost mantle (between 30 and 40 km), relatively low velocities are observed beneath the Bohemian Massif and Eastern Alps but the most striking features are the broad low velocity regions beneath the Apuseni Mountains and most of the Carpathian chain, which likely is explained by relatively thick crust. Finally, most of the Pannonian and Vienna Basin regions at depths >30 km are relatively fast, presumably related to shallowing of the Moho consequent on the extensional history of the Pannonian region.
Solid Earth Discussions, 2018
We use observations of surface waves in the ambient noise field recorded at a dense seismic array... more We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike slip fault system extending ∼ 1200 km across northern Turkey and poses a high level of seismic hazard, particularly to the city of Istanbul. Assuming isotropy, we obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high resolution images of S-wave velocity in the upper 10 km of a 70 km by 30 km region around Lake Sapanca. We observe low S-wave velocities (< 2.5 km s −1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we detect higher velocities (> 3.2 km s −1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the direction of maximum extension for the region (∼ 45 •). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results suggest that the development of the NAFZ has exploited this pre-existing contrast in physical properties. Copyright statement. 1 Introduction The formation of fault zones appears to be a balance between the accommodation of the tectonic strain field, and the exploitation of pre-existing weak zones such as tectonic suture zones or lithological boundaries (e.g. Bercovici and Ricard (2014), Dayem et al. (2009), Gerbi et al. (2016), Tapponier et al. (1982)). Studying how structural changes affect strain localisation in the upper crust is critical to understanding the earthquake cycle (Bürgmann and Dresen, 2008). Imaging the seismic velocity structure of fault zones provides information essential to understanding the long-term behaviour of faults and the earthquakes that occur on them.
Geophysical Journal International, 2012
Geodetic observations of post-seismic ground motion reflect the integrated effects of several rel... more Geodetic observations of post-seismic ground motion reflect the integrated effects of several relaxation mechanisms. To evaluate the viscous relaxation component accurately the crustal viscosity structure beneath a region must be estimated. In this study, using a 3-D finite element model, we describe the viscoelastic relaxation that follows an instantaneous strike-slip faulting event in a crustal layer whose viscosity decreases exponentially with depth. At any surface observation point the depth-dependent viscosity (DDV) model displacement history closely fits the history predicted for a uniform viscosity (UNV) model. The difference can be minimized to obtain the UNV viscosity (η u ) that best fits the DDV model displacement at that location. The differences in displacement histories between DDV and best-fit UNV models are minor but depend on distance from fault, the viscosity gradient and the fault configuration. In the near field, the DDV model prediction can be well approximated by the UNV model, regardless of the viscosity gradient and fault configuration. On the other hand, in the far field, small differences between DDV model and best-fit UNV model become apparent: the displacements are controlled by greater viscosities in later phases, and the differences are greater for DDV models with greater viscosity gradient. The more important result we obtain is that: apparent viscosity η u decreases with distance from the fault, and its rate of decrease is directly diagnostic of the vertical viscosity gradient. This result points to a practical method of analysing postseismic ground-displacement data for the variation of viscosity in the ductile crust; the apparent crustal viscosity should be determined at a series of points at increasing distance from the fault. The variation of apparent viscosity with distance can then be used to directly infer the vertical gradient of viscosity in the crust.
Geophysical Journal International, 2020
SUMMARY Since the Mesozoic, central and eastern European tectonics have been dominated by the clo... more SUMMARY Since the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological bound...
Geophysical Research Letters, 2001
In March 1995, 44 land-based recorders were deployed throughout Tasmania, SE Australia, to record... more In March 1995, 44 land-based recorders were deployed throughout Tasmania, SE Australia, to record seismic energy from an encircling array of marine normal-incidence reflection shot lines. We invert refraction and wide-angle reflection traveltimes for crustal structure, with the principal outcome being a map of the Tasmanian Moho. Key tectonic inferences from this map include: (1) the Arthur Lineament metamorphic belt in NW Tasmania overlies a major change in crustal thickness (over 5 km) and probably represents the NW limit of deformation in Tasmania during the Mid-Late Cambrian Tyennan Orogeny, (2) thickening of the crust beneath central northern Tasmania may be associated with the juxtaposition of the Eastern and Western Tasmania Terranes during the Mid-Devonian Tabberabberan Orogeny, and (3) the difference in crustal thickness between the east and west coasts reflects the presence of differing strain regimes during the Cretaceous break-up of Gondwana.
Journal of Geophysical Research: Solid Earth, 2014
GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Tur... more GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Turkey) reveal a preseismic strain localization within about 25 km of the fault and a rapid postseismic transient. Using 3-D finite element calculations of the earthquake cycle in an idealized model of the crust, comprising elastic above Maxwell viscoelastic layers, we show that spatially varying viscosity in the crust can explain these observations. Depth-dependent viscosity without lateral variations can reproduce some of the observations but cannot explain the proximity to the fault of maximum postseismic velocities. A localized weak zone beneath the faulted elastic lid satisfactorily explains the observations if the weak zone extends down to midcrustal depths, and the ratio of relaxation time to earthquake repeat time ranges from~0.005 tõ 0.01 (for weak-zone widths of~24 and 40 km, respectively) in the weakened domain and greater than~1.0 elsewhere, corresponding to viscosities of~10 18 ± 0.3 Pa s and greater than~10 20 Pa s. Models with sharp weak-zone boundaries fit the data better than those with a smooth viscosity increase away from the fault, implying that the weak zone may be bounded by a relatively abrupt change in material properties. Such a change might result from lithological contrast, grain size reduction, fabric development, or water content, in addition to any effects from shear heating. Our models also imply that viscosities inferred from postseismic studies primarily reflect the rheology of the weak zone and should not be used to infer the mechanical properties of normal crust. Many authors have attempted to estimate viscosity structure of crust and upper mantle from geodetic observations of postseismic [e.g.,
Combining faulting and ductile deformation in long-term models of continental deformation
The spatial variation of strain rate in broad regions of continental collision, extension, or she... more The spatial variation of strain rate in broad regions of continental collision, extension, or shear can often be well represented by the deformation of a thin viscous shell representing the lithosphere. The simplest explanation of this observation is that the deformation of the lithosphere is to first order a ductile process, even though shallow focus earthquakes imply slip on faults and release of elastic strain. In the thin-viscous-shell concept the strain of the upper brittle layer is assumed to simply follow the ductile strain of the stronger layers beneath, at least in the inter-seismic period. If the faults extend only to depths of 10 or 20 km, the brittle upper layer is not sufficiently thick or strong to do otherwise, and the concept of the brittle upper layer controlled by the ductile substrate is consistent with ductile models of the displacement-rate field constrained by GNSS observations. However, some large-scale faults do not comply with this concept and, rather than f...
Tellus A, 1977
A numerical technique is presented for calculating where, on average, tracer particles at a parti... more A numerical technique is presented for calculating where, on average, tracer particles at a particular place in a turbulent medium were created, and where they will be destroyed. This technique requires knowledge of creation, destruction, and transport rates, and gives as results probability distributions for creation and destruction. The technique is demonstrated on a model of stratospheric ozone, and a rough quantification is made of the statement that high latitude, low altitude ozone tends to be created in low latitude, high altitude regions.
Determining the distribution of seismic hazard in the continents requires an understanding of how... more Determining the distribution of seismic hazard in the continents requires an understanding of how much deformation is accommodated by major faults. Quantifying the role of major faults in continental deformation has been hampered by a lack of high-resolution observations in the deforming interiors of continents. By combining surface movement data derived from 22,000 satellite radar images with data from sparse, ground-based GNSS stations we produce the first high-resolution present-day surface velocity field for the Tibetan Plateau and surrounding region, where the collision of rigid Indian lithosphere with Eurasia has created Earth’s largest and highest deforming region. We show that continental deformation is best characterized by a combination of continuous distributed deformation and focused strain on a few major fault systems.
The Dynamics of the India-Eurasia Collision: Faulted Viscous Continuum Models Constrained by High-Resolution Sentinel-1 InSAR and GNSS Velocities
The dynamics of lithospheric deformation in the India-Eurasia collision zone has been debated ove... more The dynamics of lithospheric deformation in the India-Eurasia collision zone has been debated over many decades. Here we test a two-dimensional (2-D) Thin Viscous Shell (TVS) approach that has been adapted to explicitly account for displacement on major faults and investigate the impact of lateral variations in depth-averaged lithospheric strength. We present a suite of dynamic models to explain the key features from new high-resolution Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) as well as Global Navigation Satellite System (GNSS) velocities. Comparisons between calculated and geodetically observed velocity and strain rate fields indicate: (a) internal buoyancy forces from Gravitational Potential Energy (GPE) acting on a relatively weak region of highest topography (>2,000 m) contribute to dilatation of the high plateau and contraction on the margins; (b) a weak central Tibetan Plateau ~10^21 Pa s compared to far-field depth-averaged effective viscosity of 10^22 ...
European Review, 2017
Istanbul and Bucharest are major European cities that face a continuing threat of large earthquak... more Istanbul and Bucharest are major European cities that face a continuing threat of large earthquakes. The geological contexts for these two case studies enable us to understand the nature of the threat and to predict more precisely the consequences of future earthquakes, although we remain unable to predict the time of those events with any precision better than multi-decadal. These two cities face contrasting threats: Istanbul is located on a major geological boundary, the North Anatolian Fault, which separates a westward moving Anatolia from the stable European landmass. Bucharest is located within the stable European continent, but large-scale mass movements in the upper mantle beneath the lithosphere cause relatively frequent large earthquakes that represent a serious threat to the city and surrounding regions.
Admittance Functions for Axisymmetric Loading of a Viscous Layered Lithosphere
AGU Spring Meeting …, 2004
ABSTRACT
Geophysical Research Letters, 2016
Seismic images of active fault zones can be used to examine the structure of faults throughout th... more Seismic images of active fault zones can be used to examine the structure of faults throughout the crust and upper mantle and give clues as to whether the associated deformation occurs within a narrow shear zone or is broadly distributed through the lower crust. Limitations on seismic resolution within the crust and difficulties imaging shallow structures such as the crust-mantle boundary (Moho) place constraints on the interpretation of seismic images. In this study we retrieve body wave reflections from autocorrelations of ambient seismic noise. The instantaneous phase coherence autocorrelations allow unprecedented ambient noise images of the North Anatolian Fault Zone (NAFZ). Our reflection profiles show a Moho reflected P wave and additional structure within the crust and upper mantle. We image a distinct vertical offset of the Moho associated with the northern branch of the NAFZ indicating that deformation related to the fault remains narrow in the upper mantle.
Understanding Extension Within a Convergent Orogen: Initial Results From the Carpathian Basins Seismic Project
The Carpathian Basins Project (CBP) aims to understand the origin of Miocene-age extensional basi... more The Carpathian Basins Project (CBP) aims to understand the origin of Miocene-age extensional basins, of which the Pannonian Basin is the largest, within the arc of the Alpine-Carpathian Mountain Ranges - a compressional structure. Analysis of the subsidence history of the Pannonian Basin shows that its mantle lithosphere has undergone a much greater degree of extension than the overlying crust.
Geophysical Journal International, 1990
A mantle plume is probably a complex 3-D thermal structure that possesses approximate axisymmetry... more A mantle plume is probably a complex 3-D thermal structure that possesses approximate axisymmetry as it approaches the base of the lithosphere from below, but followed down towards the base of the layer, probably consists of a triple-junction or quadruple-junction of connected hot sheets. A relatively weak hot sheet rising only part way through the layer probably connects two neighbouring mantle plumes. These conclusions are suggested by numerical experiments on a 3-D constant-viscosity, plane layer with stress-free boundaries, which detail the gradational change in the planform of a convecting layer from the top of the layer to its base. The planform of a convecting layer is a map in the horizontal plane of the principal thermal anomalies in the layer. These anomalies are the main sources of positive (for hot fluid) or negative (for cold fluid) buoyancy, and therefore they drive the convective flow. They may appear in cross-section as structures with either axial symmetry (columns), planar symmetry (sheets) or some complex asymmetric form. When convection is driven at least partially by basal heating, the planform near the top of the layer may be described as a network of cold sinking sheets and isolated hot columns, while near the base of the layer it appears as a network of hot rising sheets and isolated cold columns. The hot columns near the upper surface arise from the vertices or nodes of the network of hot sheets on the lower surface, and similarly the cold columns at the base of the layer form below the vertices of the network of cold sheets near the upper surface. Near the upper surface, the apparent planform of this experiment is analogous to that of mantle convection, the cold sheets compared to subduction zones and the hot columns compared to mantle plumes. The hot plumes impinging on the upper surface produce approximately axisymmetric temperature anomalies, surface uplift and extensional stress fields. However, the relatively minor deviations from axisymmetry of surface observables reflect the deep structure of the mantle plume, formed by the junction of three or four hot sheets on the base of the layer. It seems likely that the commonly occurring triple-junction form of continental rifts may reflect an underlying structure that is implicit in the convective circulation of the mantle beneath.
Coronae Formation on Venus by Rayleigh-Taylor Instability
ABSTRACT
Journal of Geophysical Research, 2011
The two-dimensional thin viscous sheet approximation is widely used to describe large-scale conti... more The two-dimensional thin viscous sheet approximation is widely used to describe large-scale continental deformation. It treats the lithosphere as a fluid layer in which deformation results from a balance between buoyancy forces and tectonic boundary conditions. Comparisons between two-dimensional thin sheet and full three-dimensional solutions of a simple indenter model show that appreciable differences exist, especially when the indenter half width, D, is of the same order as the thickness of the deforming layer, L (i.e., D/L ≈ 1). These differences are amplified by increasing the power law exponent of the viscous constitutive law (n) but decrease as the Argand number (Ar) is increased. The greatest differences between two-dimensional and three-dimensional solutions are found at the indenter corner, where the thin sheet consistently overestimates vertical strain rates. Differences between strain rates at the corner may be 50% or greater for small Argand numbers. Other differences arise because a lithospheric root zone is formed in the three-dimensional solutions and vertically averaged strain rate is decreased in regions close to the indenter. This effect is absent from thin sheet calculations since the thickness of the load-bearing layer is assumed constant. In general, the thin viscous sheet approximation provides a reasonably accurate estimate of long wavelength deformation for D/L as low as 1 if n is less than ∼3. However, even at large D/L the solutions may be inaccurate close to strain rate concentrations at the indenter corners where horizontal gradients of deformation are large.
Journal of Geophysical Research: Solid Earth, 2016
• Physical model that treats lithosphere of Anatolia and the Aegean as a fluid sheet fits GPS vel... more • Physical model that treats lithosphere of Anatolia and the Aegean as a fluid sheet fits GPS velocities with RMS misfit of 5 mm/yr. • Distributions of strain, earthquakes, and active faulting determined by lateral variations in gravitational potential energy of lithosphere. • Variation in strength of lithosphere not required to explain large-scale deformation though moderate weakening likely near N Anatolian fault.
Geophysical Journal International, 2001
We present a method for the determination of crustal structure by simultaneous inversion of seism... more We present a method for the determination of crustal structure by simultaneous inversion of seismic refraction and wide-angle reflection traveltimes for 3-D interface geometry and layer velocity. Crustal structure is represented by layers in which velocity varies linearly with depth, separated by smooth interfaces with a cubic B-spline parametrization. Lateral variations in structure are therefore represented by variations in interface depth only. The model parametrization we have chosen means that ray paths consist of piecewise circular arc segments for which analytic expressions of trajectory and traveltime are calculated. The two-point problem of finding the first-arrival ray path and traveltime of a specified phase between a given source and receiver is solved using a shooting technique. A subspace inversion method is used to solve the inverse problem, which is formulated as a non-linear optimization problem in which we seek to minimize an objective function that consists of a data residual term and a regularization term. Before performing the inversion, each data pick must be assigned as a refraction or reflection from a particular layer or interface. Since our method represents structure in terms of interfaces, fewer parameters would generally be used in a reconstruction compared to an equivalent 3-D variable-velocity inversion. The method is well suited to wide-angle surveys that consist of many sources and relatively few receivers (or vice versa), such as marine shot lines used in conjunction with land-based receivers. Data coverage in this kind of survey is often sparse and, especially if near-offset ray paths are unavailable, highly variable. A 3-D synthetic test with an array consisting of eight sources lying within a three-sided square of 79 receivers is described. The test model consists of a three-interface structure that includes a layer pinch-out, and the synthetic data set comprises 987 refraction and 930 reflection travel times contaminated with 75 ms of data noise. Six iterations of an 18-D subspace method demonstrate that the method can produce an accurate reconstruction that satisfies the data from a 1-D starting model. We also find that estimates of a posteriori model covariance and resolution obtained from linear theory are useful in analysing solution reliability despite the nonlinear nature of the problem. Application of the method to data collected as part of the 1995 TASGO project in Tasmania shows that the method can satisfy 1345 refraction and reflection traveltime picks with a geologically reasonable and robust 254-parameter three-interface model. The inversion results indicate that the Moho beneath NW Tasmania varies in depth from 27 km near the coast to 37 km near central Tasmania, with the major increase in depth occurring across the Arthur Lineament.
Geophysical Journal International, 1998
An inversion method is presented for the reconstruction of interface geometry between two or more... more An inversion method is presented for the reconstruction of interface geometry between two or more crustal layers from teleseismic traveltime residuals. The method is applied to 2-D models consisting of continuous interfaces separating constant-velocity layers. The forward problem of determining ray paths and traveltimes between incident wave fronts below the structure and receivers located on the Earth's surface is solved by an efficient and robust shooting method. A conjugate gradient method is employed to solve the inverse problem of minimizing a least-squares type objective function based on the difference between observed and calculated traveltimes. Teleseismic data do not accurately constrain average vertical structure, so a priori information in the form of layer velocities and average layer thicknesses is required. Synthetic tests show that the method can be used to reconstruct interface geometry accurately, even in the presence of data noise. Tests also show that, if layer velocities and initial interface positions are poorly chosen, lateral structure is still recoverable. The inversion method was applied to previously published teleseismic data recorded by an in-line array of portable seismographs that traversed the northern margin of the Musgrave Block, central Australia. The solution based on interface parametrization is consistent with models given by other studies that used the same data but different methods, most notably the standard tomographic approach that inverts for velocity rather than interface structure.
Geophysical Journal International, 2013
We use ambient noise tomography to investigate the crust and uppermost mantle structure beneath t... more We use ambient noise tomography to investigate the crust and uppermost mantle structure beneath the Carpathian-Pannonian region of Central Europe. Over 7500 Rayleigh wave empirical Green's functions are derived from interstation cross-correlations of vertical component ambient seismic noise recordings (2005-2011) using a temporary network of 54 stations deployed during the South Carpathian Project (2009-2011), 56 temporary stations deployed in the Carpathian Basins Project (2005-2007) and 100 permanent and regional broad-band stations. Rayleigh wave group velocity dispersion curves (4-40 s) are determined using the multiple-filter analysis technique. Group velocity maps are computed on a grid of 0.2 • × 0.2 • from a non-linear 2-D tomographic inversion using the subspace method. We then inverted the group velocity maps for the 3-D shear wave velocity structure of the crust and uppermost mantle beneath the region. Our shear wave velocity model provides a uniquely complete and relatively high-resolution view of the crustal structure in the Carpathian-Pannonian region, which in general is validated by comparison with previous studies using other methods to probe the crustal structure. At shallow depths (<10 km) we find relatively high velocities below where basement is exposed (e.g. Bohemian Massif, Eastern Alps, most of Carpathians, Apuseni Mountains and Trans-Danubian Ranges) whereas sedimentary areas (e.g. Vienna, Pannonian, Transylvanian and Foçsani Basins) are associated with low velocities of well defined depth extent. The mid to lower crust (16-34 km) below the Mid-Hungarian Line is associated with a broad NE-SW trending relatively fast anomaly, flanked to the NW by an elongated low-velocity region beneath the Trans-Danubian Ranges. In the lowermost crust and uppermost mantle (between 30 and 40 km), relatively low velocities are observed beneath the Bohemian Massif and Eastern Alps but the most striking features are the broad low velocity regions beneath the Apuseni Mountains and most of the Carpathian chain, which likely is explained by relatively thick crust. Finally, most of the Pannonian and Vienna Basin regions at depths >30 km are relatively fast, presumably related to shallowing of the Moho consequent on the extensional history of the Pannonian region.
Solid Earth Discussions, 2018
We use observations of surface waves in the ambient noise field recorded at a dense seismic array... more We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault Zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike slip fault system extending ∼ 1200 km across northern Turkey and poses a high level of seismic hazard, particularly to the city of Istanbul. Assuming isotropy, we obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high resolution images of S-wave velocity in the upper 10 km of a 70 km by 30 km region around Lake Sapanca. We observe low S-wave velocities (< 2.5 km s −1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we detect higher velocities (> 3.2 km s −1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the direction of maximum extension for the region (∼ 45 •). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results suggest that the development of the NAFZ has exploited this pre-existing contrast in physical properties. Copyright statement. 1 Introduction The formation of fault zones appears to be a balance between the accommodation of the tectonic strain field, and the exploitation of pre-existing weak zones such as tectonic suture zones or lithological boundaries (e.g. Bercovici and Ricard (2014), Dayem et al. (2009), Gerbi et al. (2016), Tapponier et al. (1982)). Studying how structural changes affect strain localisation in the upper crust is critical to understanding the earthquake cycle (Bürgmann and Dresen, 2008). Imaging the seismic velocity structure of fault zones provides information essential to understanding the long-term behaviour of faults and the earthquakes that occur on them.
Geophysical Journal International, 2012
Geodetic observations of post-seismic ground motion reflect the integrated effects of several rel... more Geodetic observations of post-seismic ground motion reflect the integrated effects of several relaxation mechanisms. To evaluate the viscous relaxation component accurately the crustal viscosity structure beneath a region must be estimated. In this study, using a 3-D finite element model, we describe the viscoelastic relaxation that follows an instantaneous strike-slip faulting event in a crustal layer whose viscosity decreases exponentially with depth. At any surface observation point the depth-dependent viscosity (DDV) model displacement history closely fits the history predicted for a uniform viscosity (UNV) model. The difference can be minimized to obtain the UNV viscosity (η u ) that best fits the DDV model displacement at that location. The differences in displacement histories between DDV and best-fit UNV models are minor but depend on distance from fault, the viscosity gradient and the fault configuration. In the near field, the DDV model prediction can be well approximated by the UNV model, regardless of the viscosity gradient and fault configuration. On the other hand, in the far field, small differences between DDV model and best-fit UNV model become apparent: the displacements are controlled by greater viscosities in later phases, and the differences are greater for DDV models with greater viscosity gradient. The more important result we obtain is that: apparent viscosity η u decreases with distance from the fault, and its rate of decrease is directly diagnostic of the vertical viscosity gradient. This result points to a practical method of analysing postseismic ground-displacement data for the variation of viscosity in the ductile crust; the apparent crustal viscosity should be determined at a series of points at increasing distance from the fault. The variation of apparent viscosity with distance can then be used to directly infer the vertical gradient of viscosity in the crust.
Geophysical Journal International, 2020
SUMMARY Since the Mesozoic, central and eastern European tectonics have been dominated by the clo... more SUMMARY Since the Mesozoic, central and eastern European tectonics have been dominated by the closure of the Tethyan Ocean as the African and European plates collided. In the Miocene, the edge of the East European Craton and Moesian Platform were reworked in collision during the Carpathian orogeny and lithospheric extension formed the Pannonian Basin. To investigate the mantle deformation signatures associated with this complex collisional-extensional system, we carry out SKS splitting analysis at 123 broad-band seismic stations in the region. We compare our measurements with estimates of lithospheric thickness and recent seismic tomography models to test for correlation with mantle heterogeneities. Reviewing splitting delay times in light of xenolith measurements of anisotropy yields estimates of anisotropic layer thickness. Fast polarization directions are mostly NW–SE oriented across the seismically slow West Carpathians and Pannonian Basin and are independent of geological bound...
Geophysical Research Letters, 2001
In March 1995, 44 land-based recorders were deployed throughout Tasmania, SE Australia, to record... more In March 1995, 44 land-based recorders were deployed throughout Tasmania, SE Australia, to record seismic energy from an encircling array of marine normal-incidence reflection shot lines. We invert refraction and wide-angle reflection traveltimes for crustal structure, with the principal outcome being a map of the Tasmanian Moho. Key tectonic inferences from this map include: (1) the Arthur Lineament metamorphic belt in NW Tasmania overlies a major change in crustal thickness (over 5 km) and probably represents the NW limit of deformation in Tasmania during the Mid-Late Cambrian Tyennan Orogeny, (2) thickening of the crust beneath central northern Tasmania may be associated with the juxtaposition of the Eastern and Western Tasmania Terranes during the Mid-Devonian Tabberabberan Orogeny, and (3) the difference in crustal thickness between the east and west coasts reflects the presence of differing strain regimes during the Cretaceous break-up of Gondwana.
Journal of Geophysical Research: Solid Earth, 2014
GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Tur... more GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Turkey) reveal a preseismic strain localization within about 25 km of the fault and a rapid postseismic transient. Using 3-D finite element calculations of the earthquake cycle in an idealized model of the crust, comprising elastic above Maxwell viscoelastic layers, we show that spatially varying viscosity in the crust can explain these observations. Depth-dependent viscosity without lateral variations can reproduce some of the observations but cannot explain the proximity to the fault of maximum postseismic velocities. A localized weak zone beneath the faulted elastic lid satisfactorily explains the observations if the weak zone extends down to midcrustal depths, and the ratio of relaxation time to earthquake repeat time ranges from~0.005 tõ 0.01 (for weak-zone widths of~24 and 40 km, respectively) in the weakened domain and greater than~1.0 elsewhere, corresponding to viscosities of~10 18 ± 0.3 Pa s and greater than~10 20 Pa s. Models with sharp weak-zone boundaries fit the data better than those with a smooth viscosity increase away from the fault, implying that the weak zone may be bounded by a relatively abrupt change in material properties. Such a change might result from lithological contrast, grain size reduction, fabric development, or water content, in addition to any effects from shear heating. Our models also imply that viscosities inferred from postseismic studies primarily reflect the rheology of the weak zone and should not be used to infer the mechanical properties of normal crust. Many authors have attempted to estimate viscosity structure of crust and upper mantle from geodetic observations of postseismic [e.g.,