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. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time.
Plate tectonics is the outcome of thinking that began with an idea posed by German meteorologist Alfred Wegener in 1915. Wegener saw that the continents of the globe looked like a puzle that had broken apart, and he believed this was because they once been one large continent had split up.
Most seismic action arises not from diverging plates, but from plates of the earth striking each other. These collisions create land features like mountains and volcanoes, and unleash the forces of earthquakes and tsunamis.
Ninety percent of all earthquakes are found at crustal plate boundaries such as the Pacific Plate. Earthquakes can also occur within plates, such as the New Madrid earthquakes of 1811–1812 and the 1886 Charleston earthquake which occurred within the North American plate.
The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs.
Great earthquakes, such as the 1964 Good Friday earthquake in Alaska, have magnitudes of 8.0 or higher. On the average, one earthquake of such size occurs somewhere in the world each year. The Richter Scale has no upper limit.
Although success in predicting observations is usually cause for celebration, aftershock sequences continue to present something of a conundrum. We know that several factors must conspire to control aftershock triggering, but we don't yet have a single unified model that includes all of these factors and shows how their confluence yields the characteristics of aftershock sequences that had been established by the 1960s.
The tradition of symbiosis between military operations and academic geophysics continues today. Seismology's contribution to the critical issue of nuclear-test-ban-treaty verification has resulted in substantial support for seismological research and monitoring that would otherwise not have been available.
Although ground shaking can occur from many causes (such as volcanic eruptions), clearly the great majority of earthquakes, including the very largest, are related to movement along faults. So in order to understand earthquakes we must also understand faults and faulting.
Earthquakes occur along faults because of a buildup of stress in the rock. Stress builds as a result of friction on the fault surface, which resists any movement along it. Friction occurs because of the weight of rock against the fault surface,
Q: Can you predict earthquakes?
A: No. Neither the USGS nor Caltech nor any other scientists have ever predicted a major earthquake. They do not know how, and they do not expect to know how any time in the foreseeable future. However based on scientific data, probabilities can be calculated for potential future earthquakes.