This value of average, nett horizontal stress, at the end of a 12,500 km plate, which is specifically related to the plate moving with a velocity of 8.3 cm a-1, is shown as the end of curve A in Figure 3.14. The potential horizontal stress within the wedge changes in magnitude in a smooth curve (Figure 3.11b). The reduction in the nett horizontal stress for specific distances from the spreading-ridge can be estimated by using diagrams of the form shown in Figure 3.12. Such changes in magnitude are a consequence of the

Figure 3.14 Distribution of nett horizontal stress which can develop in a 12,500 km long oceanic lithosphere. This section is an approximate model of the Pacific plate. The vertical lines i to v represent the estimated magnitude of differential stress, based on seismic data (see Figure 2.32) at specific distances along a line approximately parallel to the direction of absolute plate motion, from the vicinity of Easter Island to the Marianas.

Figure 3.14 Distribution of nett horizontal stress which can develop in a 12,500 km long oceanic lithosphere. This section is an approximate model of the Pacific plate. The vertical lines i to v represent the estimated magnitude of differential stress, based on seismic data (see Figure 2.32) at specific distances along a line approximately parallel to the direction of absolute plate motion, from the vicinity of Easter Island to the Marianas.

reduction of the effective mass of the plate, as the spreading-ridge is approached, and also because of the decrease in thickness of the elastic layer.

Because it has been assumed that the low-velocity zone is constant in thickness, the basal viscous restraint will increase linearly from the spreading-ridge to the maximum extent of the plate. Consequently, the nett horizontal stresses at specific distances from the spreading-ridge are as listed in Table 3.3.

If one assumes the LVZ to have a thickness of 75 km or 100 km, the total stress curves are as shown by B and C respectively in Figure 3.14. The values of the available nett horizontal stress at these distances are plotted together with the values of stress inferred by Govers et al. (1992) from seismic data in the Pacific (Figure 2.32). The sites of these earthquakes lie adjacent to a line from the vicinity of Easter Island to the Mariana Trench, which is also approximately parallel to the direction of plate motion.

There are only five data points (I-V), in this section, regarding intraplate stresses, based on interpretation of seismic evidence. Stress bar I supports, and IV lends some support to, curve C. Stress bars I, II and IV are in reasonable agreement with curve B, while stress bars

Table 3.3

Distance from ridge (km) Average sustainable glide Nett resistive stress (108 Pa) Total available stress (108 Pa stress (108 Pa) s)

Table 3.3

Distance from ridge (km) Average sustainable glide Nett resistive stress (108 Pa) Total available stress (108 Pa stress (108 Pa) s)

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