| The gravitational force acting on lateral density heterogeneities within the lithospheric plate is another major source of stress. |
| The picture on the left describes the present day spatial distribution of tectonic regime in the western part of the US. White arrows represent a tectonic regime in extension with the orientation of the arrows parallel to the direction of σ3. Thin black arrows indicate a tectonic regime in contraction with the arrows parallel to the direction of σ1. The map on the right is a computer generated model showing the predicted distribution of tectonic regime resulting from plate boundary forces only. The model predict that the displacement towards the northwest of the Pacific Plate relatively to the North American Plate should result in a tectonic regime in extension oriented WNW-ESE throughout the region. The discrepancy between the observation (map on the left) and the model (map on the right) suggests that another source of stress is interfering. Gravitational forces related to lateral variations in density can explain the discrepancy.
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