Earthquakes At the CEA's military applications Division (Direction des Applications Militaires, DAM), the Detection and Geophysics Laboratory teams take the Earth's pulse on an ongoing basis. Their aim is to detect and identify the slightest seismic event in real time and also to learn more about the Earth's movements with a view to better assessing the related hazards.     What is a seismic event?   A seismic event is any sudden, sometimes violent, shaking of the ground. Its origin can be natural or artificial. Natural earthquakes are produced by tectonic or volcanic movement, and artificial earthquakes are those caused by human activities such as dam priming, oilfield mining, mine or quarry blasts and underground nuclear tests.     How do you detect an earthquake?   When a seismic event occurs, movement in the ground produces seismic waves that propagate from the focus or hypocenter in all directions, through the ground and water. Therefore, detecting a seismic event involves detecting the waves it generates. Two types of detection station are used and the adopted technology depends on the environment:   Seismic stations equipped with seismographs capable (at near and far distances) of measuring low-amplitude ground motions. Find out more. Hydroacoustic stations detect hydroacoustic waves generated by underwater or underground explosions in the vicinity of a sea or an ocean. These stations rely on submerged receivers and coastal seismic stations. Find out more.   In order to detect the slightest event and precisely determine the geographical coordinates of the hypocenter, a network of sensors must be deployed in and around the area to be monitored. The collected data are sent by satellite to data analysis centers where seismologists study each event in order to determine its precise location, identify its origin and, if necessary, sound the alert. In France, this task falls to the Environmental Assessment and Monitoring Department (under the Military Applications Division (DAM)) of the CEA in Bruyères-le-Châtel where the National Data Center is located.     Valuable data   All collected data is compiled in a seismicity catalog in France. This catalog is a national reference guide and was used to draw up a new seismic zoning map for mainland France.     How do you determine an earthquake's location?     Data sent by detection stations dotted throughout the globe do not give the precise location of an event directly. The data need to be analyzed by a seismologist to determine the hypocenter of the phenomenon. Once the waves have been detected and the hypocenter determined, the origin of the event needs to be found. It can be due, for example, to a natural movement of the Earth's crust, mining work, or a nuclear test.     Fundamental research in seismology   Observations of the planet's movements, which are based on the processing of data received from detection stations, are not only used to produce seismicity and alert catalogs, but also numerous seismological research works. This research can be applied, as in seismic hazard assessment, or more fundamental, e.g. research regarding understanding of the earthquake source mechanism.