A fault line may send out tiny shocks, called foreshocks, days or even weeks before a major earthquake. When a fault line is about to rupture and cause an earthquake, the types of waves it sends out change.Continue Reading
Rupturing faults send out two different types of waves: P-waves and S-waves. The P-waves move faster, but the S-waves are the ones that cause the heavy damage -- along with the waves that move along the Earth's surface. Faults do not send out waves continuously, but once the rupture begins, earthquake sensors can pick up on incoming waves, and alert center staff have time to provide detailed warnings about the coming earthquakes.
The sensors are embedded in the ground, and when the P-waves hit, alert signals go to the center. While these systems provide only minutes or seconds of warning time, often that is enough to allow for the evacuation of major buildings if the quake is a major one. People living in earthquake-sensitive areas often have apps on their phones designed to receive these alerts, so they can prepare for the intensity of the earthquake at their location.
An earthquake early warning system called ShakeAlert was tested in California in January of 2012. ShakeAlert is able to transmit messages almost instantaneously, and may be able to save lives.Learn more about Earthquakes
The type of stress placed on a normal fault is tensional stress. In normal faulting, tensional stress gradually weakens the Earth's crust until the rock cracks, with one crustal block moving downwards relative to its adjacent fault block.Full Answer >
Earthquakes produce two types of potentially destructive waves that move through the earth from the point of the fault: primary, or pressure waves and secondary, or shear waves. Primary waves, also called P waves, exert a force of compression and travel through rock at speeds that can exceed 225 mph. Secondary waves, also called S waves, exert a shearing force and travel only half as fast as P waves, but are capable of causing much greater damage when they reach the surface.Full Answer >
The 2008 Sichuan earthquake was caused by seismic activities that focused along the Longmenshan fault, a massive fault structure located between the Eurasian Plate and the Indo-Australian Plate. This caused a rupture along the fault, displacing land by up to 9 meters over approximately 90 seconds.Full Answer >
The Tangshan earthquake was caused by the shift of faults located under the city, such as the Tangcheng-Lijiang fault. Northern China has a high level of seismic activity and, in 1974, scientists predicted a large earthquake would hit the region within 2 years. The Tangshan earthquake occurred on July 28, 1976.Full Answer >