The Earth's crust and upper mantle are made up of several large, thin, quite rigid plates (tectonic plates) that move. Fault lines develop where the plates meet and are likely sources of earthquakes. This map shows active fault lines along the length of New Zealand, where the Pacific and Australian plates meet.
East of the North Island, the Pacific Plate is dipping below the Australian Plate. This movement is causing the land to compress, squeeze upwards and move sideways. This map shows the direction and relative speed of the Pacific and Australian Plates in millimetres per year.