Understanding Earthquake Waves: How Seismic Energy Travels Through Earth

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What looks stable beneath your feet is actually in constant motion. Over millions of years, tectonic plates have been grinding past each other at speeds slower than your fingernails grow, yet when that accumulated stress suddenly releases, the energy radiating outward can flatten cities in seconds. The evidence preserved in fractured rock layers and offset fence lines tells us exactly how the ground moved during past earthquakes, and seismologists find the same wave patterns recorded in seismograph traces worldwide.

Every earthquake sends a complex symphony of waves rippling through Earth's interior. If you could watch this in time-lapse, you would see compression waves racing ahead, shear waves following behind, and slow-rolling surface waves bringing up the rear with devastating power. The last time a major subduction zone ruptured (the 2011 Tohoku earthquake), these waves circled the globe multiple times, detectable on seismographs as far away as Antarctica.

Our simulation lets you trigger earthquakes at different depths and watch how P-waves, S-waves, and surface waves propagate through Earth's layered structure. You will observe firsthand why S-waves vanish when they encounter the liquid outer core, providing the same evidence that led seismologists to map Earth's hidden interior.

How to Use This Simulation

If you could watch this in time-lapse, you would see waves rippling outward from the epicenter like ripples in a pond - but traveling through solid rock at kilometers per second. Here is how to explore seismic wave behavior.

Controls Overview

Getting Started

1. Click "Trigger Earthquake" to release seismic energy
2. Watch the wave fronts expand - P-waves (blue) arrive first, S-waves (red) follow, surface waves (green) bring up the rear
3. Adjust depth - Compare shallow vs deep earthquakes
4. Toggle wave types - Isolate each type to understand its behavior

What to Watch For

The simulation reveals how seismologists map Earth's hidden interior:

• The P-S time gap: The evidence preserved in seismograph traces shows P-waves arriving first, then S-waves. This gap widens with distance from the epicenter because P-waves travel faster.

• S-wave shadow zone: S-waves cannot travel through liquids. If you could watch this in time-lapse over decades of earthquake data, you would see S-waves consistently vanish when their paths cross Earth's outer core. This is how we know the outer core is liquid.

• Surface wave destruction: Surface waves travel slower but carry more energy. Over millions of years, this energy has shaped landscapes, liquefied soils, and toppled buildings along fault zones.

Exploration Tips

1. Map the shadow zone: Trigger a deep earthquake and observe where S-waves fail to penetrate. The evidence preserved in global seismograph networks reveals a shadow zone starting about 105 degrees from the epicenter. Our simulation shows you why.

2. Compare arrival times: Place virtual seismographs at different distances and record wave arrival times. The time gap between P and S arrivals increases linearly with distance - this is how seismologists calculate earthquake locations.

3. Experiment with depth: Set depth to 0 km (surface) and trigger. Now set it to 200 km (deep). If you could watch this in time-lapse, you would notice deep earthquakes produce different wave patterns because energy travels through different rock layers.

4. Watch wave refraction: Seismic waves bend when they cross layer boundaries (crust to mantle, mantle to core). The evidence preserved in wave paths reveals Earth's internal structure like a planetary CAT scan.

5. Observe amplitude decay: Wave energy spreads over larger areas as it travels outward. Surface waves maintain higher amplitudes longer because they are confined to Earth's surface rather than spreading in three dimensions.

What Causes Earthquakes?

Over millions of years, stress accumulates along geological faults as tectonic plates slowly grind against one another. When that stress exceeds the frictional strength holding rocks together, the fault ruptures, and all that stored energy releases in a violent burst. This happens at tectonic plate boundaries where plates:

• Collide (convergent boundaries): the Himalayas rise from India plowing into Asia
• Separate (divergent boundaries): the Mid-Atlantic Ridge spreads apart Iceland
• Slide past each other (transform boundaries): the San Andreas Fault creeps and lurches through California

The point where the earthquake originates underground is called the focus (or hypocenter), while the point directly above on Earth's surface is the epicenter. Seismologists find that most earthquakes occur within the top 100 km of Earth's crust, though some subduction zones produce deep-focus earthquakes down to 700 km.

Types of Seismic Waves

Seismic waves are categorized into two main groups: body waves that travel through Earth's interior, and surface waves that travel along Earth's surface.

Body Waves

P-Waves (Primary Waves)

P-waves are the fastest seismic waves and the first to arrive at seismograph stations. They are compressional waves, meaning they cause particles to move back and forth in the same direction the wave travels, like pushing and pulling a slinky.

Key characteristics:

• Travel at approximately 6 km/s in the crust
• Speed increases to 13 km/s in the inner core
• Can travel through all states of matter
• Create the initial "jolt" felt during an earthquake
• Lower amplitude than S-waves (less shaking)

S-Waves (Secondary Waves)

S-waves are slower than P-waves and arrive second. They are shear waves that cause particles to move perpendicular to the direction of wave travel, like shaking a rope up and down.

Key characteristics:

• Travel at approximately 3.5-4.5 km/s in the crust
• Cannot travel through liquids (this is how we know Earth's outer core is liquid!)
• Create the rolling, side-to-side motion during earthquakes
• Higher amplitude than P-waves (more damaging)

Surface Waves

Surface waves travel along Earth's surface and cause the most damage during earthquakes. They travel slower than body waves but have larger amplitudes.

Love Waves

Named after mathematician A.E.H. Love, these waves cause the ground to move horizontally perpendicular to the direction of wave propagation. They're particularly damaging to building foundations.

Rayleigh Waves

Named after Lord Rayleigh, these waves cause elliptical motion in the ground, similar to ocean waves. Particles move in circles as the wave passes, creating a rolling motion that can be very destructive.

Wave Speeds in Earth's Layers

Shadow Zones: Evidence of Earth's Structure

The evidence preserved in seismograph records from the early 20th century revealed something remarkable: certain regions on Earth's surface receive no seismic waves after major earthquakes. Seismologists observe these shadow zones with every large event, and the pattern remains stubbornly consistent. What looks like missing data is actually a window into Earth's hidden interior.

In 1906, Richard Dixon Oldham noticed that S-waves simply vanished beyond a certain distance from earthquake epicenters. The last time anyone doubted the liquid outer core was before 1936, when Inge Lehmann's careful analysis of seismograms proved the inner core exists as a solid sphere. If you could watch seismic waves travel through a transparent Earth, you would see them bend, reflect, and (in the case of S-waves) stop dead at the core-mantle boundary.

P-Wave Shadow Zone

• Located between 103° and 142° from the epicenter
• Caused by refraction as P-waves bend sharply when entering the liquid outer core
• P-waves re-emerge beyond 142°, but the shadow region receives no direct arrivals
• This bending pattern allowed seismologists to calculate the core's exact depth

S-Wave Shadow Zone

• Located beyond 103° from the epicenter, a vast region covering roughly 40% of Earth's surface
• S-waves require a shear-resistant medium; liquids cannot support shear stress
• The evidence is unambiguous: S-waves enter the outer core and simply cease to exist
• Over decades of observation, this pattern confirmed Earth's outer core is molten iron-nickel alloy

Reading a Seismograph

A seismograph records ground motion over time. Key features include:

1. P-wave arrival: First small amplitude waves
2. S-wave arrival: Larger amplitude waves arriving later
3. Surface wave arrival: Largest amplitude, longest duration

Calculating Distance: The time difference between P and S wave arrivals indicates distance: $\text{Distance} = \Delta t \times \frac{v_P \times v_S}{v_P - v_S}$

Where Δt is the time between P and S arrivals.

Earthquake Magnitude and Intensity

Learning Objectives

After using this simulation, you should be able to:

1. Identify the four main types of seismic waves and their characteristics
2. Explain why P-waves travel faster than S-waves
3. Describe how shadow zones reveal Earth's internal structure
4. Interpret seismograph readings to determine wave arrivals
5. Understand the relationship between wave speed and Earth's layers
6. Calculate approximate earthquake distance using P-S arrival times

Exploration Activities

Activity 1: Compare Wave Speeds

1. Trigger an earthquake at shallow depth
2. Watch P-waves (red) race ahead of S-waves (blue)
3. Note the time when each reaches the seismograph
4. Calculate the speed ratio (P should be ~1.7x faster)

Activity 2: Observe Shadow Zones

1. Enable the "Show Shadow Zones" option
2. Trigger an earthquake
3. Notice the regions where S-waves cannot reach
4. Understand why this proves the outer core is liquid

Activity 3: Depth Effects

1. Set earthquake depth to shallow (10 km)
2. Observe the wave patterns and surface damage
3. Change to deep focus (300 km)
4. Compare how depth affects surface wave intensity

Activity 4: Read the Seismograph

1. Watch the seismograph panel during an earthquake
2. Identify P-wave, S-wave, and surface wave arrivals
3. Note the amplitude differences between wave types
4. Use arrival times to estimate distance

Real-World Applications

Historical Earthquake Examples

Challenge Questions

1. Basic: Why do P-waves always arrive at seismograph stations before S-waves?

2. Basic: What type of particle motion do S-waves cause, and why can't they travel through liquids?

3. Intermediate: An earthquake occurs and the P-wave arrives at a station at 10:05:00, while the S-wave arrives at 10:07:30. If P-waves travel at 6 km/s and S-waves at 3.5 km/s, approximately how far away was the earthquake?

4. Advanced: Explain how the S-wave shadow zone provides evidence that Earth's outer core is liquid.

5. Advanced: Why are surface waves typically more damaging than body waves, even though they travel slower?

Common Misconceptions

Key Formulas

Wave Speed: $v = \sqrt{\frac{K + \frac{4}{3}G}{\rho}}$ (P-waves) $v = \sqrt{\frac{G}{\rho}}$ (S-waves)

Where K = bulk modulus, G = shear modulus, ρ = density

Travel Time: $t = \frac{d}{v}$

Richter Magnitude: $M_L = \log_{10}(A) + 3\log_{10}(8\Delta t) - 2.92$

Tips for Using This Simulation

1. Start with body waves: Focus on P and S waves first
2. Use slow motion: Reduce speed to see wave propagation clearly
3. Compare wave types: Toggle individual waves on/off
4. Watch the seismograph: Correlate visual waves with the trace
5. Try different depths: See how focus depth affects the pattern
6. Enable shadow zones: Understand Earth's core structure

Frequently Asked Questions

Why are P-waves called "Primary" waves?

P-waves are called primary waves because they are the first to arrive at seismograph stations after an earthquake [1]. Their faster speed (approximately 6-13 km/s depending on the medium) means they always outrace other seismic waves. The "P" can also be thought of as "pressure" waves since they compress and expand material as they travel.

How do seismologists locate an earthquake's epicenter?

Seismologists use a technique called triangulation [2]. By measuring the P-S arrival time difference at three or more seismograph stations, they can calculate the distance from each station to the epicenter. The intersection of circles drawn from each station pinpoints the earthquake's location.

What is the difference between magnitude and intensity?

Magnitude (like the Richter or moment magnitude scale) measures the total energy released by an earthquake; it's a single value for each event [3]. Intensity (like the Modified Mercalli scale) measures the effects of shaking at specific locations and varies depending on distance, soil conditions, and building types.

Can we predict earthquakes?

Currently, reliable short-term earthquake prediction is not possible [4]. While scientists can identify high-risk zones and estimate long-term probabilities, predicting the exact time, location, and magnitude of future earthquakes remains beyond our current capabilities. Research continues into potential precursor signals.

Why do some earthquakes cause tsunamis while others don't?

Tsunamis are generated when earthquakes cause vertical displacement of the seafloor [5]. This typically occurs with thrust faults at subduction zones. Strike-slip earthquakes, which cause horizontal motion, rarely generate significant tsunamis. The 2004 Sumatra and 2011 Japan earthquakes were both thrust events that displaced massive amounts of water.

References

1. USGS Earthquake Hazards Program: Comprehensive information on earthquake science, monitoring, and hazards. Available at: https://earthquake.usgs.gov (Public Domain)

2. IRIS Education Resources: Seismology educational materials and data access from the Incorporated Research Institutions for Seismology. Available at: https://www.iris.edu/hq/programs/education_and_outreach (Creative Commons)

3. MIT OpenCourseWare 12.001: Introduction to Geology course materials covering Earth's internal structure. Available at: https://ocw.mit.edu/courses/12-001-introduction-to-geology-fall-2013/ (CC BY-NC-SA)

4. NOAA Tsunami Warning Centers: Information on tsunami generation, propagation, and warning systems. Available at: https://tsunami.gov (Public Domain)

5. USGS Earthquake Glossary: Definitions of seismological terms and concepts. Available at: https://earthquake.usgs.gov/learn/glossary/ (Public Domain)

6. British Geological Survey Education: Educational resources on seismology and Earth structure. Available at: https://www.bgs.ac.uk/discovering-geology/earthquakes/ (Open Government Licence)

7. Khan Academy Earth Science: Free educational videos on plate tectonics and earthquakes. Available at: https://www.khanacademy.org/science/earth-science (CC BY-NC-SA)

8. Pacific Northwest Seismic Network: Regional seismic monitoring and education resources. Available at: https://pnsn.org (Public Domain)

9. Southern California Earthquake Center: Research and education on earthquake science. Available at: https://www.scec.org (Public Domain)

About the Data

Wave speed values used in this simulation are based on the Preliminary Reference Earth Model (PREM) [1], which provides average seismic velocities for Earth's layers. Actual speeds vary with local composition, temperature, and pressure. The layer depths shown are simplified for visualization; real boundaries are gradational. Earthquake magnitude examples are from USGS historical records.

How to Cite

Simulations4All. (2025). Earthquake Wave Propagation Simulator. Retrieved from https://simulations4all.com/simulations/earthquake-wave-propagation

For academic use:

@misc{s4a_earthquake_waves,
  author = {Simulations4All},
  title = {Earthquake Wave Propagation Simulator},
  year = {2025},
  url = {https://simulations4all.com/simulations/earthquake-wave-propagation}
}

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