Earthquakes and Seismic Waves

Introduction

Earthquakes and Seismic Waves is a Grades 9–10 aiMOOC about how stress in Earth's crust can produce sudden fault movement, how that movement releases energy as seismic waves, and how scientists use those waves to investigate earthquakes and Earth's interior. You will connect plate tectonics, faults, earthquakes, seismology, and earthquake engineering.

An earthquake is not simply "the ground shaking." It begins when stored elastic strain is released by sudden slip along a fault. The released energy travels away from the source as seismic waves. Different kinds of waves move in different ways and at different speeds, so their arrival times and paths become evidence that scientists can analyze.

By the end of the course, you should be able to explain the earthquake process, distinguish major seismic-wave types, read basic information from a seismogram, explain how an epicenter can be located, compare magnitude with intensity, use seismic evidence to infer properties of Earth's interior, and connect seismic science with hazard reduction.

Learning Goals

Earthquake process: Explain how stress, friction, elastic deformation, and fault slip are connected.
Seismic wave: Compare P waves, S waves, Love waves, and Rayleigh waves.
Seismogram: Interpret the order and timing of major wave arrivals.
Epicenter: Explain how distances from several seismic stations can locate an earthquake.
Earth structure: Use wave behavior to infer that Earth's interior has layers with different physical properties.
Earthquake hazard: Distinguish earthquake size from local shaking effects and apply the science to safer design and preparedness.

From Plate Motion to Earthquake Rupture

Tectonic Plates, Stress, and Faults

Earth's rigid outer shell is broken into moving tectonic plates. Plate motion is usually slow, but rocks near plate boundaries and faults can become locked by friction. While the blocks remain locked, forces continue to act and the rocks can deform elastically. This stores energy.

A fault is a fracture or zone of fractures along which blocks of rock have moved. Three broad fault-motion patterns are useful at this level. In a normal fault, extension causes one block to move downward relative to the other. In a reverse or thrust fault, compression pushes one block upward. In a strike-slip fault, blocks move mainly horizontally past one another.

The San Andreas Fault in California is a famous strike-slip fault associated with the boundary between the Pacific Plate and North American Plate. The visible landscape displacement helps you connect slow plate motion with long-term fault movement.

Elastic Rebound and Sudden Slip

The elastic rebound model explains why a fault can stay quiet while strain builds and then move suddenly. Rocks near a locked fault deform as stress increases. When the stress becomes large enough to overcome frictional resistance, the fault slips. Some stored elastic energy is released, the surrounding rocks partly rebound toward a less deformed state, and seismic waves carry energy away.

This IRIS Earthquake Science demonstration models the difference between steady loading and sudden slip. A model is useful because it isolates a process, but you should also identify its limitations: real faults are three-dimensional, rock properties vary, and earthquake ruptures can spread across large fault areas.

Hypocenter, Epicenter, and Rupture Area

The hypocenter, also called the focus, is the point inside Earth where rupture begins. The epicenter is the point on Earth's surface directly above the hypocenter. For a sizable earthquake, however, slip is not confined to one point. Rupture can propagate across a substantial area of a fault, so the epicenter marks the start location in map view rather than the entire source.

These terms help separate source geometry from what people experience at the surface. Strong shaking is not determined by epicentral distance alone; it also depends on magnitude, depth, wave propagation, local geology, and the response of structures.

Seismic Waves

Body Waves and Surface Waves

Seismic waves are elastic disturbances that transport energy through or along Earth. Body waves travel through Earth's interior. The two main body-wave types are P waves and S waves. Surface waves travel mainly near Earth's surface and include Love and Rayleigh waves.

As you watch the animation, focus on two questions: In what direction do particles move compared with the direction the wave travels, and where can each wave type propagate?

P Waves

A P wave is a compressional body wave. Particles of material move back and forth approximately parallel to the direction the wave travels, producing alternating compression and expansion. P waves are the fastest major seismic waves and therefore usually arrive first at a seismic station. They can travel through solids and liquids.

A useful analogy is a stretched spring pushed and pulled along its length. The coils move back and forth, but the disturbance travels along the spring.

S Waves

An S wave is a shear body wave. Particle motion is perpendicular to the direction in which the wave travels. S waves are slower than P waves, so they normally arrive later at a station. S waves require a material that can resist shear deformation, so they travel through solids but not through liquids.

A rope shaken sideways provides a simple model: the rope segments move across the direction in which the disturbance travels. The analogy is not perfect, but it makes the perpendicular motion easier to visualize.

Love and Rayleigh Waves

Love waves move the ground mainly side to side in a horizontal direction. Rayleigh waves produce a rolling motion involving both vertical and horizontal displacement. Surface waves generally arrive after the faster body waves and can have large amplitudes and long durations.

It is too simple to say that one wave type is always "the most damaging." Damage depends on amplitude, frequency content, duration, distance, local soil and rock conditions, building properties, and other factors. Surface waves can contribute strongly to destructive ground motion, while shear motion from S waves can also be especially important for structures.

Arrival Order and the S–P Time Interval

Because P waves travel faster than S waves, a seismogram commonly shows a P-wave arrival before an S-wave arrival. The time between them is called the S–P interval. For earthquakes farther from a station, the difference in travel times generally becomes larger.

This is one of the key ideas in earthquake location: a single station can estimate its distance from an earthquake source from the difference in wave-arrival times, but distance alone does not give a unique direction.

Recording Earthquakes

Seismometers and Seismograms

A seismometer is an instrument that detects and measures ground motion. A seismogram is the recorded time series produced by a seismic instrument. Modern systems are electronic, but traditional mechanical designs help illustrate the principle of inertia: when the ground and instrument frame move, a suspended mass tends to resist rapid changes in motion, allowing relative movement to be measured.

Seismic stations often record motion in three components, commonly two horizontal directions and one vertical direction. Comparing components helps scientists reconstruct how the ground moved.

Reading a Basic Seismogram

To read a simple seismogram, begin with time. Identify the first clear P-wave arrival, then the later S-wave arrival. Measure the time difference. Next examine amplitude and duration, but do not assume that the largest trace automatically gives magnitude without calibration. Real magnitude calculations account for instrument response, distance, and the chosen magnitude scale.

A seismogram is evidence, not a picture of a fault. Scientists interpret waveforms by combining recordings with models of wave speed, source processes, and Earth's structure.

Locating an Earthquake

Distance from One Station

The P–S arrival-time difference can be compared with travel-time information to estimate the distance between a seismic station and an earthquake. The station must lie on a circle of possible source locations at that distance.

The exact conversion between S–P time and distance is not universal because seismic-wave speeds vary with rock type and depth. Professional location methods use travel-time models and many observations rather than a single fixed speed.

Combining Several Stations

If you repeat the distance estimate for at least three stations and draw a circle around each station, the common intersection gives an estimate of the epicenter. Classroom materials often call this triangulation. More precisely, because the method uses distances rather than angles, it is a form of trilateration.

With real data, circles may not intersect at exactly one point because measurements and Earth models have uncertainty. Scientists therefore use many stations and computational methods to find the location that best fits the observations.

What Seismic Waves Reveal About Earth's Interior

Refraction, Reflection, and Changing Speed

Seismic-wave speed changes when waves enter materials with different elastic properties and density. At boundaries, wave energy can be reflected, transmitted, refracted, or converted between wave types. Curved ray paths through Earth result because wave speed generally changes with depth.

These path changes allow scientists to investigate regions that cannot be sampled directly. Seismology is therefore similar to medical imaging in one important sense: internal structure is inferred from how waves travel through a body.

The Liquid Outer Core and Shadow Zones

One of the most important deductions from global seismology is that Earth's outer core is liquid. S waves do not travel through liquids, so direct S waves disappear beyond certain angular distances from an earthquake. P waves can pass through the liquid outer core, but they change speed and refract strongly at major boundaries, creating a P-wave shadow zone.

The shadow zones are not empty because "waves stop everywhere." Instead, they are regions where particular direct seismic phases are absent or much weaker because of the paths the waves take through Earth's layered interior.

Magnitude, Intensity, and Hazard

Magnitude Is Not the Same as Intensity

Magnitude describes the size of an earthquake at its source. Modern seismology often uses moment magnitude for moderate to very large earthquakes because it is related to fault area, slip, and rock rigidity through seismic moment. The older local magnitude scale associated with Charles Richter remains historically important and is still useful in some local settings.

Intensity describes the severity of shaking and effects at a particular location. The Modified Mercalli Intensity scale is based on observed effects on people, structures, and the environment. One earthquake has one reported magnitude value for a selected magnitude scale, but many intensity values because shaking varies from place to place.

Why Shaking Differs from Place to Place

Shaking depends on more than magnitude. Important controls include distance from the ruptured fault, earthquake depth, rupture direction, wave frequencies, duration, and local ground conditions. Soft sediments can amplify some shaking compared with nearby hard rock.

Water-saturated, loose sediment can sometimes undergo liquefaction, in which shaking raises pore-water pressure and reduces the sediment's strength. Earthquakes can also trigger landslides, surface faulting, fires, and tsunamis depending on the setting.

Engineering and Preparedness

Earthquake engineering uses knowledge of ground motion, structural dynamics, materials, and site conditions to reduce risk. A flexible building that can deform safely may perform better than a brittle structure that cannot accommodate motion. Engineers also consider resonance, because a structure can respond strongly when earthquake shaking contains frequencies near the structure's natural frequencies.

Preparedness turns science into action. During strong shaking, guidance commonly emphasizes Drop, Cover, and Hold On where that advice is used by local authorities. Outside the event itself, communities reduce risk through building codes, retrofits, secure utilities, emergency planning, public education, and hazard-aware land use. Always follow official local emergency guidance.

Working Like a Seismologist

Evidence, Models, and Uncertainty

Seismologists do not observe most fault rupture directly. They infer what happened from measurements. A useful scientific explanation links a claim to evidence and reasoning. For example, the claim that P waves travel faster than S waves is supported by repeated arrival patterns on seismograms. The claim that the outer core is liquid is supported by the global behavior of S waves and the refraction of P waves.

Models are essential, but every model simplifies reality. When you use a spring, rope, block, or computer animation to represent seismic processes, identify what the model captures and what it leaves out. This habit is part of scientific literacy.

A Simple Quantitative Example

Imagine that a station records a P wave at 10:14:20 and an S wave at 10:15:00. The S–P interval is 40 seconds. You would use an appropriate travel-time graph or Earth model to convert that interval into an estimated distance. A second and third station provide additional distances. The intersection of the resulting distance constraints gives an estimated epicenter.

The important reasoning is the sequence: observe arrivals → calculate time differences → infer distances → combine stations → estimate location → evaluate uncertainty.

Reliable Sources and Further Reading

USGS: The Science of Earthquakes: Clear explanations of faults, P waves, S waves, and earthquake location.
USGS: Seismographs — Keeping Track of Earthquakes: Background on instruments, wave arrivals, and travel times.
USGS: What Are the Effects of Earthquakes?: Ground shaking, surface faulting, ground failure, and related hazards.
USGS: Classroom Shaking Simulations: Teaching material on P waves, S waves, surface waves, and shaking.
Wikimedia Commons: Seismic waves: Openly licensed diagrams and animations for further study.
IRIS Earthquake Science on YouTube: Educational videos and animations about seismology.

Interactive Tasks

Quiz: Test Your Knowledge

Which major seismic wave normally arrives first at a station? (P wave) (!S wave) (!Love wave) (!Rayleigh wave)

What is the point inside Earth where earthquake rupture begins called? (Hypocenter) (!Epicenter) (!Seismogram) (!Plate boundary)

Which seismic wave cannot travel through the liquid outer core? (S wave) (!P wave) (!Rayleigh wave) (!Love wave)

What does a larger S–P arrival-time interval usually indicate? (Greater distance from the earthquake) (!Smaller earthquake magnitude) (!Lower instrument sensitivity) (!Shallower ocean water)

Which instrument detects and measures ground motion? (Seismometer) (!Barometer) (!Thermometer) (!Anemometer)

What does earthquake magnitude describe? (Size of the earthquake source) (!Shaking at one street) (!Building code quality) (!Distance to the coast)

What does earthquake intensity describe? (Shaking effects at a location) (!One fixed value for all locations) (!Speed of plate motion) (!Depth of the inner core)

How do particles move in a P wave? (Parallel to wave travel) (!Only upward) (!Perpendicular to wave travel) (!In permanent circles)

Why are recordings from several seismic stations useful for locating an epicenter? (Their distance constraints can intersect) (!They make P waves travel faster) (!They prevent aftershocks) (!They change fault direction)

What is liquefaction? (Loss of sediment strength during shaking) (!Melting of the mantle) (!Formation of a new tectonic plate) (!Conversion of S waves into light)

Memory Game

Hypocenter	Point inside Earth where rupture begins
Epicenter	Surface point directly above the starting point of rupture
P wave	Fast compressional body wave
S wave	Shear body wave that does not travel through liquids
Seismometer	Instrument that measures ground motion
Seismogram	Recorded trace of ground motion through time
Magnitude	Measure describing earthquake source size

Drag and Drop

Match the correct terms.	Topic
Compressional motion	P wave
Shear motion	S wave
Rolling surface motion	Rayleigh wave
Horizontal surface shear	Love wave
Recorded ground-motion trace	Seismogram

...

Crossword Puzzle

Epicenter	What is the surface point directly above the earthquake focus?
Hypocenter	What is the underground point where rupture begins?
Seismometer	What instrument measures earthquake ground motion?
Magnitude	What term describes the size of an earthquake source?
Refraction	What process bends wave paths when wave speed changes?
Liquefaction	What process can make saturated loose sediment lose strength during shaking?

LearningApps

Cloze Text

An earthquake begins when sudden slip occurs on a

. The underground starting point of rupture is the

. The surface point directly above it is the

. The fastest major seismic body wave is the

. A shear body wave that cannot travel through liquid is the

. The time difference between the first P and S arrivals is the

. An instrument that measures ground motion is a

. A recorded trace of that motion is a

. The absence of direct S waves through the outer core supports the conclusion that the outer core is

. Earthquake source size is described by

.

Open-Ended Tasks

Easy

Seismic wave model: Use a spring or slinky to model compressional motion and a rope to model transverse motion. Photograph or sketch the models and explain one strength and one limitation of each analogy.
Earthquake vocabulary: Create a one-page illustrated glossary for hypocenter, epicenter, fault, seismometer, seismogram, magnitude, and intensity using your own definitions.
Seismogram annotation: Find a teacher-provided seismogram and mark the first P-wave and S-wave arrivals. Write two sentences explaining how you identified them.
Earthquake safety: Design a clear safety poster for your school that connects one preparedness action with the earthquake hazard it is intended to reduce.

Standard

Epicenter location: Use three sample station records and a travel-time graph to estimate distances, plot distance circles on a map, and determine an epicenter with a short uncertainty statement.
Earthquake interview: Interview a civil engineer, geologist, emergency manager, teacher, or community member about earthquake risk and summarize which parts of the interview are scientific evidence, professional judgment, or personal experience.
Seismometer model: Build a safe tabletop model that records vibrations on paper or with a sensor. Change one variable at a time and compare the resulting traces.
Earthquake case study: Produce a short video or slide-free digital report about one well-documented earthquake, connecting plate setting, magnitude, local intensity, secondary hazards, and lessons for risk reduction.

Advanced

Travel-time analysis: Analyze several P- and S-wave arrival times, graph arrival-time difference against distance, identify the pattern, and discuss why a single constant wave speed is only an approximation.
Earth interior inference: Create an evidence-based diagram showing how P-wave refraction and the absence of direct S waves through the outer core support a layered-Earth model.
Structural resonance: Design a safe shake-table investigation with simple model buildings of different heights or stiffnesses. Record which designs respond most strongly and explain the role of frequency and resonance.
Local seismic risk: Visit or virtually examine a relevant museum, geological site, emergency-management resource, or engineering project. Produce a map-based report that separates hazard, exposure, vulnerability, and possible mitigation.

Learning Assessment

Fault-to-wave reasoning: Explain the full causal chain from plate motion and locked-fault stress to sudden slip, elastic energy release, and the production of seismic waves, using a labeled diagram and a paragraph.
Wave comparison assessment: Given unfamiliar wave-motion diagrams, classify each as P-like, S-like, Love-like, or Rayleigh-like and justify each classification from particle motion and propagation.
Seismogram evidence assessment: Interpret three station records to estimate relative distance from an earthquake, explain which station is closest, and identify at least two sources of uncertainty.
Earth interior assessment: Use a provided global wave-path diagram to argue why a completely solid, uniform Earth is inconsistent with observed P- and S-wave behavior.
Hazard transfer assessment: Compare two locations the same distance from a fault but with different ground materials and building types. Predict how damage could differ and support the prediction with seismic concepts.
Magnitude and intensity assessment: Evaluate the claim that every location affected by the same earthquake must have the same intensity because the earthquake has one magnitude, and correct the reasoning with a real or hypothetical example.

Evidence of Learning

Knowledge: Accurate explanations of fault slip, elastic rebound, seismic-wave types, wave arrival order, magnitude, intensity, and major earthquake hazards.
Data skills: Correct identification of P and S arrivals, calculation of S–P intervals, use of travel-time information, and interpretation of uncertainty.
Modeling skills: Careful use of physical or visual models with explicit discussion of what each model represents and what it simplifies.
Scientific reasoning: Claims supported by seismic evidence, including the use of shadow zones and wave paths to infer properties of Earth's interior.
Products: Maps, annotated seismograms, diagrams, posters, experiments, interviews, videos, or reports that communicate scientific ideas clearly.
Transfer: Application of seismic-wave knowledge to earthquake engineering, preparedness, local hazard analysis, and evaluation of new earthquake information.