Foreshock
Definition
An earthquake that occurs before the mainshock in the same region. Foreshocks can only be identified in retrospect — there is no reliable way to distinguish them from ordinary earthquakes beforehand.
Example
A M6.4 foreshock occurred two days before the 2023 Turkey M7.8 mainshock.
Related Terms
Related Guides
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The 1976 Tangshan Earthquake: China's Deadliest Disaster
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Related Case Studies
The 2016 Kumamoto Earthquakes: A Double Mainshock That Rewrote Seismic Hazard Rules
The earthquake doublet that overturned the assumption that the first large shock is always the mainshock, forcing Japan to revise its public messaging and early warning protocols.
The 1960 Valdivia Earthquake: The Most Powerful Earthquake Ever Recorded (M9.5)
The most powerful earthquake ever instrumentally recorded, releasing energy equivalent to 25% of all global seismic energy over a century and generating a tsunami that crossed the entire Pacific Ocean.
Frequently Asked Questions
The epicenter is the point on the Earth's surface directly above the hypocenter (focus) where the earthquake rupture begins. It is typically reported as latitude and longitude coordinates. The strongest shaking usually occurs near the epicenter, though local soil conditions and fault geometry can shift the zone of maximum damage.
A seismograph (or seismometer) is an instrument that detects and records ground motion caused by seismic waves. Modern broadband seismometers can detect movements smaller than the width of an atom. Networks of seismographs around the world enable scientists to locate earthquakes and determine their magnitude within minutes.
P-waves (primary waves) are compressional waves that travel fastest through rock, arriving first at seismic stations. S-waves (secondary waves) are shear waves that arrive later but cause more ground shaking. P-waves travel through solids, liquids, and gases; S-waves only travel through solids. The time difference between them helps determine earthquake distance.
The hypocenter (or focus) is the point within the Earth where an earthquake rupture initiates. It is described by latitude, longitude, and depth. The vertical distance between the hypocenter and the surface directly above is the earthquake's depth, which strongly influences how the earthquake is felt at the surface.
Seismology is the scientific study of earthquakes and the propagation of seismic waves through the Earth. It encompasses earthquake detection, location, and characterization; Earth's internal structure; seismic hazard assessment; and earthquake engineering. Seismologists use data from global seismograph networks to study these phenomena.