Measuring continental deformation

Zones of continental deformation commonly are wider and more diffuse than zones of deformation affecting oceanic lithosphere. This characteristic results from the thickness, composition, and pressure-temperature profile of continental crust, which makes ductile flow in its lower parts more likely than it is in oceanic regions. The width and diffusivity of these zones make some of the concepts of plate tectonics, such as the rigid motion of plates along narrow boundaries, difficult to apply to the continents. Consequently, the analysis of continental deformation commonly requires a framework that is different to that used to study deformation in oceanic lithosphere (e.g. Section 8.5).

At the scale of large tectonic features such as wide intracontinental rifts (Section 7.3), continental transforms (Section 8.5), and orogenic belts (Section 10.4.3), deformation may be described by a regional horizontal velocity field rather than by the relative motion of rigid blocks (e.g. Fig. 8.18b). Methods of estimating the regional velocity field of deforming regions usually involves combining information from Global Positioning System (GPS) satellite measurements (Clarke et al., 1998), fault slip rates (England & Molnar, 1997), and seismicity (Jackson et al., 1992). One of the challenges of this approach is the short, decade-scale time intervals over which GPS data are collected. These short intervals typically include relatively few major earthquakes. Consequently, the measured surface motions mostly reflect nonpermanent, elastic strains that accumulate between major seismic events (i.e. interseismic) rather than the permanent strains that occur during ruptures (Bos & Spakman, 2005; Meade & Hager, 2005). This characteristic results in a regional velocity field that rarely shows the discontinuities associated with slip on major faults. Instead the displacements on faults are described as continuous functions and the velocity field is taken to represent the average deformation over a given region (Jackson, 2004). Nevertheless, regional velocity fields have proven to be a remarkably useful way of describing continental deformation. The methods commonly used to process and interpret them are discussed further in Sections 5.3 and 8.5.

Synthetic Aperture Radar (SAR) also is used to measure ground displacements, including those associated with volcanic and earthquake activity (Massonnet & Feigl, 1998). The technique involves using SAR data to measure small changes in surface elevations from satellites that fly over the same area at least twice, called repeat-pass Interferometric SAR, or InSAR. GPS data and strain meters provide more accurate and frequent observations of deformation in specific areas, but InSAR is especially good at revealing the spatial complexity of displacements that occur in tectonically active areas. In

Moho

_Crust_ Mantle

Oceanic strength profile

Moho

Moho

Continental strength profile

E 20

Differential stress (MPa) 1000 2000 3000

E 20

-600

-800

-600

Differential stress (MPa) 1000 2000 3000

-800

Figure 2.26 Schematic strength profiles through (a) oceanic and (b) continental lithosphere (after Ranalli, 1995, fig. 12.2. Copyright © 1995, with kind permission of Springer Science and Business Media). Profile in (a) shows a 10-km-thick mafic crust and a 75-km-thick lithosphere. Profile in (b) shows a 30-km-thick unlayered crust and a thin, 50-km-thick lithosphere. Profiles in (c) and (d) incorporate a wet middle crust and show a dry lower crust and a wet upper mantle, and a wet lower crust and dry upper mantle, respectively (modified from Mackwell et al., 1998, by permission of the American Geophysical Union. Copyright © 1998 American Geophysical Union) See text for explanation.

the central Andes and Kamchatka InSAR measurements have been used to evaluate volcanic hazards and the movement of magma in volcanic arcs (Pritchard & Simons, 2004). In southeast Iran, InSAR data have been used to determine the deformation field and source parameters of a magnitude Mw = 6.5 earthquake that affected the city of Bam in 2003 (Wang et al., 2004). The combined use of GPS and InSAR data have revealed the vertical displacements associated with a part of the San Andreas Fault system near San Francisco (Fig. 8.7b).

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