Distribution of inferred maximum stress differences. The numbers refer to: 1)Ridge push: Zoback and Magee (1991); 2) Upper crustal amplification of ridge push: Kusznir and Bott (1977); Hasegawa et al. (1985); 3) Tensile fracturing : Etheridge (1983); 4) Earthquake stress drops in intraplate environments: Hasegawa et al. (1985); 5) Estimate along the San Andreas fault: Lachenbruch and Sass (1980); 6) Hydrofracturing-strain relief: McGarr and Gay (1978); 7) Composite estimates from the summation of eastern Canadian stress sources: Hasegawa et al. (1985); 8) Isostatic considerations: Molnar and Lyon-Caen (1988); 9) Mylonitization: Christie and Ord (1980); 10) Paleopiezometers: Kohlstedt and Weathers, 1980); 11) KTB borehole value measured at 6 km: Zoback et al.(1993); 12) Estimates along the Alpine fault of New Zealand: Scholz et al. (1979); 13) Mechanical twinning: Tullis (1980). Except for mechanical twinning, a 200 MPa is a conservative upper bound estimate of the maximum crustal stress difference.