06 Earthquakes and Seismic Hazards
Learn how fault movement generates earthquakes, how scientists measure them, which hazards they create, and how communities and individuals can reduce risk.
How movement starts earthquakes
Earthquakes begin when rock suddenly slips along a , a fracture or zone of fractures where blocks of rock can move relative to each other. Faults are classified by how the blocks move:
Normal faults form as rocks are pulled apart; the block above the inclined plane moves downward relative to the block below.
Reverse faults form under compression; the upper block moves upward. A low-angle reverse is a thrust .
Strike-slip faults form where blocks slide horizontally past one another.
Tectonic forces gradually deform rocks around a . Friction can keep the locked while stress builds. Once stress overcomes friction, the slips and surrounding rock partly rebounds. This process, called , releases energy. The rupture begins at the (or hypocenter), and the point directly above it at the surface is the .
Takeaway: Earthquakes result from sudden slip after stress builds up over time.
How reveal an earthquake's location
The released energy travels through Earth as . The main wave types behave differently:
(primary waves) compress and expand material. Particles move back and forth in the same direction as the wave travels. These are the fastest and travel through solids and liquids.
(secondary waves) move material perpendicular to the direction of travel. They travel through solids, but not liquids.
Surface waves move along Earth's surface after body waves have traveled through Earth. They often produce strong, prolonged shaking.
A seismometer records ground motion in a seismogram. Because arrive before , the time between their arrivals helps estimate how far a station is from an earthquake. Comparing distance estimates from several stations helps locate the .
Takeaway: Differences in wave behavior and arrival times help scientists detect earthquakes and locate their source.
and measure different things
Earthquake describes the size of an earthquake at its source. Modern reports commonly use moment , written as , which is calculated from physical properties of the rupture, including the area, amount of slip, and rock rigidity. The scale is logarithmic: an increase of one whole unit corresponds to ten times the measured wave amplitude and roughly thirty-two times the energy release.
describes the shaking and effects at a particular place. It varies across the affected area with distance from the rupture, local ground conditions, and building design. In the United States, the Modified Mercalli scale describes observed effects, from how people feel shaking to damage to structures. One earthquake has a single but can produce different intensities in different places. For example, soft sediment may shake more strongly than nearby bedrock.
Takeaway: describes source size; describes local shaking and effects.
From shaking to cascading hazards
Ground shaking is the main earthquake hazard. It can damage buildings, bridges, roads, and utilities. Other hazards include:
Surface rupture: permanent displacement where a breaks through the ground surface.
: loose, water-saturated sediment temporarily loses strength during shaking. Buildings may settle or tilt, and the ground may spread or slump.
Earthquake-triggered landslides: shaking can destabilize steep slopes.
Tsunamis: undersea earthquakes can generate tsunamis if they suddenly displace a large area of the seafloor.
Secondary impacts: fires, damaged water or gas lines, disrupted transportation and communications, and aftershocks.
Local conditions influence which hazards are most likely. Loose, saturated sand can be susceptible to , while steep slopes may be vulnerable to landslides.
Takeaway: Earthquake effects depend on both the shaking and local conditions.
Reducing earthquake before, during, and after shaking
A hazard is a potentially damaging earthquake effect. also depends on who and what are exposed and how vulnerable they are. Earthquakes cannot currently be prevented or reliably predicted, but their risks can be reduced.
Before an earthquake, communities can use hazard maps and land-use planning, enforce seismic building codes, and retrofit vulnerable buildings and infrastructure. Individuals can secure heavy furniture and breakable objects, prepare emergency supplies, and practice protective actions.
During shaking indoors, follow Drop, Cover, and Hold On: drop to your hands and knees, protect your head and neck under sturdy cover if possible, and hold on until shaking stops. Avoid running outside while the building is shaking.
detect an earthquake that has already begun and may deliver an alert before strong shaking reaches some locations. They do not predict earthquakes, and warning time may be very short or unavailable near the source. After shaking, expect aftershocks, avoid damaged structures and downed utility lines, and follow local emergency instructions.
Takeaway: Preparedness, safer construction, and prompt protective actions reduce earthquake even when earthquakes themselves cannot be prevented.