Causes of Earthquakes
Earthquakes are primarily caused by the sudden release of energy in the Earth's crust, resulting from the movement of tectonic plates. These plates, which make up the Earth's lithosphere, float on the semi-fluid asthenosphere beneath them and constantly shift due to convection currents in the mantle. When plates collide, pull apart, or slide past each other along faults—fractures in the crust—stress builds up and eventually releases as seismic waves, shaking the ground. Other causes include volcanic activity and human-induced events like mining or reservoir filling, though tectonic movements account for the majority.
Key Principles of Earthquake Generation
The elastic rebound theory explains how earthquakes occur: rocks deform elastically under stress until they exceed the fault's strength, snapping back to their original shape and releasing energy. Earthquakes are classified by depth—shallow (less than 70 km), intermediate (70-300 km), and deep (over 300 km)—and by type, such as strike-slip (horizontal movement), thrust (upward), or normal (downward). Common misconceptions include the idea that earthquakes only happen in specific 'earthquake zones'; while plate boundaries like the Ring of Fire are hotspots, intraplate quakes can occur anywhere due to ancient faults.
Practical Example: The 1906 San Francisco Earthquake
A classic example is the 1906 San Francisco earthquake, caused by a rupture along the San Andreas Fault, a strike-slip boundary between the Pacific and North American plates. The plates slipped about 6 meters, releasing energy equivalent to a magnitude 7.9 event. This quake destroyed much of the city through shaking and subsequent fires, illustrating how surface rupture and ground acceleration can amplify damage. Seismographs recorded the event, highlighting early measurement challenges before modern instruments.
Measuring Earthquakes and Their Importance
Earthquakes are measured using two main scales: the Richter scale, which quantifies magnitude based on the amplitude of seismic waves recorded by seismographs (logarithmic, so a magnitude 7 is 10 times stronger than 6), and the Moment Magnitude Scale (Mw), a more accurate modern alternative that considers fault area, slip, and rock rigidity. Intensity, how strongly the quake affects an area, is assessed via the Modified Mercalli Intensity (MMI) scale, from I (not felt) to XII (total destruction). Accurate measurement is crucial for hazard assessment, building codes, and emergency response, enabling predictions of potential damage and informing urban planning in seismic regions.