Mainshock
Definition
The largest earthquake in a sequence, which defines the overall magnitude of the event. Preceded by foreshocks (sometimes) and followed by aftershocks (always).
Example
The M7.8 mainshock struck at 4:17 AM local time on February 6, 2023.
Related Terms
Related Guides
Aftershocks Explained: Why Earthquakes Come in Clusters
Aftershocks can continue for months or years after a major earthquake. Learn what causes them, how they're predicted, and when they'll stop.
Foreshocks: Can They Predict the Main Event?
Foreshocks occur before some major earthquakes, but can they be used for prediction? Learn the science behind foreshocks and their limitations.
Earthquake Swarms: When Small Quakes Won't Stop
Earthquake swarms produce hundreds of small tremors without a clear mainshock. Learn what causes them and whether they signal bigger events.
Nepal and the Himalayan Collision Zone
The India-Eurasia collision builds the Himalayas and creates major earthquake risk for Nepal, Bangladesh, and northern India.
Related Case Studies
The 2023 Turkey-Syria Earthquakes: A Double Mainshock That Killed 59,000
The deadliest earthquake disaster of the 2020s, notable for its unprecedented double mainshock sequence and the subsequent criminal prosecution of contractors for building code violations.
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 2011 Christchurch Earthquake: How a M6.2 Aftershock Destroyed a City Center
One of the few cases where an aftershock was more devastating than its mainshock, with a M6.2 at just 5 km depth producing 2.2g ground acceleration that obliterated Christchurch's city center.
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.
Related Tools
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.