A. John Haines

Dynamics of the India‐Eurasia collision zone

We present simple new dynamic calculations of a vertically averaged deviatoric stress field (over a depth average of 100 km) for Asia from geodetic, geologic, topographic, and seismic data. A first estimate of the minimum absolute magnitudes and directions of vertically averaged deviatoric stress is obtained by solving force balance equations for deviatoric stresses associated with gravitational potential energy differences within the lithosphere plus a first-order contribution of deviatoric stresses associated with stress boundary conditions. This initial estimate of the vertically averaged deviatoric stress field is obtained independent of assumptions about the rheology of the lithosphere. Absolute magnitudes of vertically averaged deviatoric stresses vary between 5 and 40 MPa. Assuming bulk viscous behavior for the lithosphere, the magnitudes of deviatoric stresses, together with the magnitudes of strain rates inferred from Quaternary fault slip rate and GPS data, yield vertically averaged effective viscosities for Tibet of 0.5–5×1022 Pa s, compared with 1–2.5×1023 Pa s in more rigid areas elsewhere in the region. A forward modeling method that solves force balance equations using velocity boundary conditions allows us to refine our estimates of the vertically averaged effective viscosity distribution and deviatoric stress field. The total vertically averaged deviatoric stress and effective viscosity field are consistent with a weak lower crust in Tibet; they are consistent with some eastward motion of Tibet and south China lithosphere relative to Eurasia; and they confirm that gravitational potential energy differences have a profound effect on the spatially varying style and magnitude of strain rate around the Tibetan Plateau. Our results for the vertically averaged deviatoric stress argue for a large portion of the strength of the lithosphere to reside within the seismogenic upper crust to get deviatoric stress magnitudes there to be as high as 100–300 MPa (in accord with laboratory and theoretical friction experiments indicating that stress drops in earthquakes are small fractions of the total deviatoric stress).

Improvements To The Method Of Fourier Shape Analysis As Applied In Morphometric Studies

Fourier outline shape analysis is a powerful tool for the morphometric study of two-dimensional form in organisms lacking many biologically homologous landmarks. Several improvements to the method are described herein; these modifications are incorporated into the new computer programs Hangle, Hmatch and Hcurve. First, automated tracing of outlines using image capture software, although desirable, results in high frequency pixel ‘noise’ that can corrupt the Fourier analysis. Program Hangle eliminates this noise using optional and variable levels of outline smoothing. Secondly, a widely used Fourier technique, elliptic Fourier analysis (EFA, Kuhl and Giardina 1982), yields coefficients that are not computationally independent of each other, a condition that hampers and compromises statistical analysis. In addition, EFA increasingly downweights successively more detailed features of the outline. Program Hangle solves both of these problems. Lastly, Fourier methods in general are sensitive to the placement of the starting position of the digitized trace. This problem is acute when the organisms under study have no unambiguously defined, homologous point on the outline from which to start the trace. Program Hangle allows the user to normalize for starting position using various properties of individual outlines. Alternatively, Hmatch takes a new approach and can be used to normalize using properties of the entire population under study.key words: Fourier shape analysis, morphometric studies, new computer programs, foraminiferal outlines.

Gravitational potential energy of the Tibetan Plateau and the forces driving the Indian plate

We present a study of the vertically integrated deviatoric stress field for the Indian plate and the Tibetan Plateau associated with gravitational potential energy (GPE) differences. Although the driving forces for the Indian plate have been attributed solely to the mid-oceanic ridges that surround the entire southern boundary of the plate, previous estimates of vertically integrated stress magnitudes of ~6–7 x 1012 N/m in Tibet far exceed those of ~3 x 1012 N/m associated with GPE at mid-oceanic ridges, calling for an additional force to satisfy the stress magnitudes in Tibet. We use the Crust 2.0 data set to infer gravitational potential energy differences in the lithosphere. We then apply the thin sheet approach in order to obtain a global solution of vertically integrated deviatoric stresses associated only with GPE differences. Our results show large N-S extensional deviatoric stresses in Tibet that the ridge-push force fails to cancel. Our results calibrate the magnitude of the basal tractions, associated with density buoyancy driven mantle flow, that are applied at the base of the lithosphere in order to drive India into Tibet and cancel the N-S extensional stresses within Tibet. Moreover, our deviatoric stress field solution indicates that both the ridge-push influence (~1 x 1012 N/m) and the vertically integrated deviatoric stresses associated with GPE differences around the Tibetan Plateau (~3 x 1012 N/m) have previously been overestimated by a factor of two or more. These overestimates have resulted from either simplified two-dimensional approximations of the thin sheet equations, or from an assumption about the mean stress that is unlikely to be correct.

Project helps constrain continental dynamics and seismic hazards

The Global Strain Rate Map project II-8, initiated in 1998 by the International Lithosphere Program (ILP), provides constraints for understanding continental dynamics and for quantifying seismic hazards in general. To date, the Global Strain Rate Map (GSRM) model is a numerical velocity gradient tensor fi eld solution (i.e., spatial variations of horizontal strain rate tensor components and rotation rates) for the entire Earth surface

Enhanced Surface Imaging of Crustal Deformation

In the last few decades increased GPS monitoring has provided detailed insight into distributed deformation. As the density of GPS stations increases, it becomes even more important to develop advanced analytic methods to use the data to image the deformation of the continental crust and upper mantle and in particular the subsurface deformation sources. Capturing high resolution measurements of crustal deformation and strain rate measurements is fundamental in understanding the underlying mechanisms and dynamics of continental deformation and allows better recognition and assessment of seismic hazards. Vertical derivatives of horizontal stress (VDoHS) rates are the horizontal-component surface manifestation of all subsurface deformation. In this book, we outline how VDoHS rates can be obtained from GPS velocity data without regard to the precise nature and location of the subsurface source or the rheology of the underlying medium, and show examples of how VDoHS rates can be used in practice.