Earthquakes, tropical storms, and floods are the three main global natural disasters causing the biggest losses in the world. As natural disasters have become major threats to human life and the world economy, governments and international organizations are cooperating to promote global and regional risk management, and to improve the capability to mitigate the effects of disasters. In December 2004, nearly a quarter of a million people lost their lives and over 1.6 million were displaced from their homes in the devastating Indian Ocean tsunami. An earthquake is a sudden movement of the Earth's lithosphere, which is caused by the release of built-up stresses within rocks along geological faults, or by the movement of magma in volcanic areas. Smaller earthquakes occur frequently, but annually, only as many as 18–20 reach a magnitude above Ms 7. Approximately 40 disastrous earthquakes have occurred since the end of the twentieth century, and the total death toll is nearly 1.7 million. Floods and droughts have devastating consequences. According to the Asian Disaster Reduction Centre, half the population worldwide who suffered natural disasters was affected by floods and one third by drought during the period from 1975 to 2005.