An earthquake (also known as a quake, tremor, or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves.
Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude (or the related and mostly obsolete Richter magnitude) of an earthquake is conventionally reported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by shaking and sometimes displacing the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
That's the case with an earthquake in Lorca, Spain. The quake measured 5.1 on the Richter scale and killed nine people. According to an analysis published in Nature Geoscience, the Lorca quake was caused by the extraction of groundwater from an aquifer near the fault that slipped.
In fact, it doesn't take much to trigger an earthquake. Oil and gas wells, rock quarries, even the added pressure of a reservoir lake behind a new dam can cause the ground to rumble. But we don't know what pressure levels are safe, nor is it clear whether man-made quakes are unique or just the early arrival of temblors that would have occurred naturally.