62 In situ stresswould expect the highest ratios to occur very close to, or at, the surface
because the vertical stress is zero at the surface.
With reference to Fig. 4.14, we saw that the calculation for the vertical
stress component gave a reasonable prediction of the overall trend. This
cannot be said for the data presented in Fig. 4.15: the horizontal stress
components do not follow the trends predicted by simple elasticity theory,
except asymptotically at depths of several kilometres. We should consider
the reasons for this large variation in what are universally higher k-ratios
than predicted. Furthermore, it is likely to be of interest in different regions
of the world to plot the orientations of the maximum horizontal principal
stress as has been done in Fig. 4.16 for North West Europe. From this map,
it can be seen that there is a general trend of north west-south east for the
maximum principal stress in the region. This leads us naturally into a
discussion of the reasons for high horizontal stresses.
4.8 Reasons for high horizontal stresses
High horizontal stresses are caused by factors which fall into the categories
of erosion, tectonics, rock anisotropy, local effects near discontinuities and
consequential scale effects.LEGEND
\ Direction of maximum horizontal stress from in situ measurement.
Horizontal stress equal in all directions as found
from in sifu measurement.
\ Direction of maximum horizontal stress inferred
from earthquake focal plane solution.
7 Alpine fold belt.
R Direction of maximum
horizontal stress inferred
from breakout analysis
\ Direction of maximum
horizontal stress as
measured in BRE
directed in sifu stress
programme.Figure 4.16 Orientation of the maximum horizontal principal stress in North West
Europe (after Hudson and Cooling, 1988).