What Causes Earthquakes
An earthquake is a sudden release of energy stored in the Earth's crust that produces seismic waves. The vast majority of earthquakes occur at or near the boundaries of tectonic plates, the massive slabs of lithosphere that make up the Earth's outer shell.
The Earth's surface is divided into approximately 15 major tectonic plates and numerous smaller ones. These plates are in constant motion, driven by heat convection in the underlying mantle. They move at rates of a few centimeters per year, roughly the speed at which fingernails grow.
Where plates meet, they interact in three primary ways:
Convergent Boundaries
Two plates move toward each other. One plate is typically forced beneath the other in a process called subduction. These boundaries produce the world's largest earthquakes, including the events that generate devastating tsunamis. The Pacific Ring of Fire, which encircles the Pacific Ocean, is defined by convergent plate boundaries.
Divergent Boundaries
Two plates move apart, allowing magma to rise from below and create new crust. Mid-ocean ridges are the most common example. Earthquakes at divergent boundaries tend to be smaller and shallower than those at convergent boundaries.
Transform Boundaries
Two plates slide horizontally past each other. The San Andreas Fault in California is the most well-known transform boundary. These faults produce significant earthquakes, though generally not as large as the most powerful subduction zone events.
Intraplate Earthquakes
A small percentage of earthquakes occur far from plate boundaries, within the interior of tectonic plates. These intraplate earthquakes are less well understood but can be significant. The New Madrid earthquakes of 1811-1812 in the central United States were among the largest ever recorded in North America, yet they occurred hundreds of kilometers from the nearest plate boundary.
Seismic Waves
When rock along a fault suddenly breaks and slips, the stored elastic energy radiates outward as seismic waves. There are several types:
P-Waves (Primary Waves)
P-waves are compressional waves that travel fastest and arrive first at seismic stations. They propagate through both solid rock and liquid (including the Earth's liquid outer core). P-waves cause particles to move back and forth in the direction the wave is traveling, similar to sound waves in air.
S-Waves (Secondary Waves)
S-waves are shear waves that arrive after P-waves. They move particles perpendicular to the direction of wave travel and can only propagate through solid material. S-waves typically cause more damage than P-waves because they produce larger ground displacements.
Surface Waves
When seismic waves reach the Earth's surface, they generate surface waves that travel along the ground. Love waves cause horizontal shearing, while Rayleigh waves produce an elliptical rolling motion. Surface waves travel slower than body waves but often produce the strongest ground motion and cause the most damage.
The difference in arrival times between P-waves and S-waves at a given station tells seismologists how far away the earthquake occurred. Combining readings from multiple stations allows them to triangulate the earthquake's location.
Measuring Earthquakes
The Richter Scale
Developed in 1935 by Charles Richter, the original Richter scale measured the amplitude of seismic waves on a specific type of seismograph at a standard distance from the earthquake. It was designed for Southern California earthquakes and does not scale well for very large or very distant events. Despite this, the term remains widely used in popular media.
Moment Magnitude Scale (Mw)
The moment magnitude scale, introduced in the late 1970s, has largely replaced the Richter scale in scientific practice. It measures the seismic moment, which accounts for the area of the fault that ruptured, the amount of slip along the fault, and the rigidity of the surrounding rock.
The moment magnitude scale is logarithmic: each whole number increase represents approximately 31.6 times more energy released. A magnitude 7.0 earthquake releases about 31.6 times more energy than a magnitude 6.0 earthquake, and about 1,000 times more than a magnitude 5.0.
For moderate earthquakes (magnitude 3 to 7), the Richter and moment magnitude scales produce similar numbers. For very large earthquakes (above magnitude 7), the moment magnitude scale provides a more accurate measure of the total energy released.
Intensity Scales
While magnitude measures the earthquake's total energy at its source, intensity measures the effects at a specific location. The Modified Mercalli Intensity (MMI) scale, ranging from I (not felt) to XII (total destruction), describes how strongly an earthquake is experienced at a particular point. A single earthquake has one magnitude but can produce different intensities at different locations.
Earthquake Depth
The depth at which an earthquake occurs significantly affects its impact. Shallow earthquakes (less than 20 km deep) concentrate their energy near the surface and tend to cause the most damage. Intermediate-depth earthquakes (20-300 km) can affect larger areas but with less intense shaking. Deep earthquakes (300-700 km) are felt over enormous areas but rarely cause significant damage.
The world's deepest recorded earthquakes occur in subducting slabs of oceanic crust that have been pushed deep into the mantle. Below about 700 km, the rock behaves plastically rather than brittly, and earthquakes cease.
Earthquake Safety
Disclaimer: The following safety guidance is provided for general educational purposes only. Always follow the official guidance of your local emergency management agency, civil protection authority, or national geological survey. Earthquake preparedness recommendations may vary by region, building type, and local conditions.
Before an Earthquake
Preparedness significantly reduces earthquake risk. Key steps include:
- Secure heavy objects: Bookshelves, water heaters, and large appliances should be anchored to walls to prevent them from toppling during shaking.
- Know your building: Understand whether your home or workplace was built to modern seismic codes. Older unreinforced masonry buildings are significantly more vulnerable than modern engineered structures.
- Prepare an emergency kit: Water, non-perishable food, flashlight, first aid supplies, and important documents should be readily accessible.
- Identify safe spots: In each room, identify locations away from windows, heavy furniture, and exterior walls.
During an Earthquake
The widely recommended protective action during earthquake shaking is "Drop, Cover, and Hold On":
- Drop to your hands and knees to prevent being knocked down.
- Cover your head and neck under a sturdy desk or table. If no shelter is available, cover your head and neck with your arms against an interior wall.
- Hold on to your shelter until the shaking stops. Be prepared for aftershocks.
After an Earthquake
- Expect aftershocks. These secondary earthquakes can occur minutes, hours, or days after the main event.
- Check for injuries and provide first aid if trained to do so.
- Inspect your surroundings for structural damage, gas leaks, and fire hazards.
- If near a coast and you felt strong, prolonged shaking, move immediately to higher ground. A tsunami may follow within minutes.
- Listen to official emergency broadcasts for guidance and information.
Monitoring Earthquakes Today
Modern seismological networks detect thousands of earthquakes every day. The vast majority are too small to be felt by humans. Globally, there are roughly 500,000 detectable earthquakes per year, of which about 100,000 can be felt and approximately 100 cause damage.
Real-time monitoring platforms aggregate earthquake data from national and international seismological networks, providing near-instant notification of significant events. Automated systems can estimate magnitude, location, depth, and population exposure within minutes of an earthquake's occurrence, supporting rapid emergency response.