According to classical plate tectonic theory, plates are essentially stable internally, and tectonic effects are concentrated at their boundaries. However it has always been recognized that this was only true to a first approximation, and that all regions of the Earth’s surface experience tectonic effects to some degree. The most common type of intraplate (within-plate) tectonic activity is undoubtedly vertical movements. Accurate geodetic measurements involving precise levelling and other techniques have shown that most parts of the crust are undergoing slow uplift or depression. The frequency and distribution of intraplate earthquakes is also an indication of widespread tectonic activity, albeit to a notably lesser degree than at plate boundaries.