5 Earthquakes and Seismic Hazards

How earthquakes begin

Most earthquakes happen when rocks suddenly slip along a fault. Tectonic plates move slowly, but friction can lock sections of their boundaries together. The surrounding rock deforms and stress builds until it overcomes friction, allowing the fault to slip and release energy.

The rupture begins underground at the focus, also called the hypocenter. The epicenter is the point on Earth's surface directly above the focus. These locations help describe where an earthquake begins and where it lies relative to the surface.

Seismic waves and ground motion

The energy released by an earthquake travels through Earth as seismic waves. Their different motions and speeds help explain how shaking spreads:

• P waves (primary waves) compress and expand material in the direction they travel. They are the fastest seismic waves and arrive first.

• S waves (secondary waves) move material perpendicular to their direction of travel. They arrive after P waves and travel through solids, not liquids.

• Surface waves move along or near Earth's surface and often produce strong, damaging ground motion.

The shaking at a particular location depends on the waves, the distance from the rupture, and local ground conditions. Thus, places at similar distances can experience different levels of shaking.

Measuring earthquakes

A seismometer detects ground motion, and its recording is called a seismogram. Scientists compare records from multiple stations to locate an earthquake. Because P waves arrive before S waves, the difference between their arrival times helps estimate how far each station is from the source. Combining estimates from several stations narrows down the location.

Magnitude describes an earthquake's size, and each earthquake has one magnitude. Modern reports commonly use moment magnitude (Mw), based on the earthquake's physical size and fault slip. Magnitude scales are logarithmic: an increase of one magnitude unit means ten times the measured wave amplitude and roughly $32$ times the energy released.

Intensity, by contrast, describes the shaking and effects at a particular place. It can vary from location to location because of distance, ground conditions, and building construction. The Modified Mercalli Intensity scale describes observed effects.

Earthquake hazards and risk

Earthquake hazards include several kinds of damage and related events:

• Ground shaking can damage buildings, bridges, roads, and utilities.

• Surface fault rupture occurs when fault movement reaches the ground surface.

• Liquefaction can make some loose, water-saturated sediments temporarily lose strength during strong shaking, causing settlement or ground movement.

• Landslides can occur on susceptible slopes.

• Tsunamis can be generated when an undersea earthquake suddenly displaces the seafloor.

• Aftershocks are smaller earthquakes that follow a mainshock and may further damage weakened structures.

Hazard is the potential for damaging shaking or related effects; risk concerns the expected harm to people and property. Risk rises when many people or vulnerable buildings are exposed. For example, similar shaking may cause much greater losses where buildings are poorly engineered or soils amplify ground motion.

Takeaway: Earthquake effects depend on both the hazard and the exposure and vulnerability of people and structures.

Reducing earthquake risk

Earthquakes cannot currently be predicted precisely enough to provide reliable advance warnings of the event itself. Risk can still be reduced through planning, safer construction, and preparedness.

Communities can use hazard maps and land-use planning to guide development. Building codes and seismic retrofits help structures resist shaking and reduce the chance of collapse. Individuals can secure heavy furniture and water heaters, prepare emergency supplies, and learn local tsunami evacuation routes where relevant.

During shaking, follow Drop, Cover, and Hold On: drop to your hands and knees, cover your head and neck under sturdy shelter if available, and hold on until the shaking stops.

Takeaway: Although earthquakes cannot be precisely predicted, thoughtful planning, stronger buildings, and practical preparedness can reduce harm.