Not to scale: the block is about 120 km wide and 40 km deep, but the slip is drawn hundreds of times bigger and the waves are slowed down so you can see them. A real fault slips centimetres to metres (up to about 50 m in the very biggest earthquakes).
Seismic waves carry the earthquake’s energy through the Earth. Body waves (P and S) travel through the inside of the Earth; surface waves (Love and Rayleigh) travel along the surface and usually do the most damage. Choose a wave and follow the red bit of rock.
Typical speeds in the crust. Because S waves cannot pass through liquid, they stop at Earth’s liquid outer core: see the shadow zones in Layers of the Earth.
Three seismometers recorded the same earthquake. P waves arrive first, S waves later; the farther away the station, the bigger the gap. 1. On each seismogram tap where the P wave starts (first small wiggles), then where the S wave starts (first big wiggles). 2. Each S − P time gives a distance, drawn as a circle. 3. Tap the map where the three circles meet.
Tap P, then S, on each trace (tap again to start over).
Tap where the circles cross.
With P waves at 6 km/s and S waves at 3.5 km/s, each second between them means about 8.4 km of distance. Real seismologists use travel-time tables for the whole Earth and dozens of stations.
Magnitude measures the size of the earthquake itself: one number for the whole earthquake. Intensity (the Modified Mercalli scale, I–XII) describes how strongly the ground shakes at one place, which depends on the distance and the ground. Change them and watch the buildings.
The intensity here comes from a simple rule of thumb for a focus 10 km deep; real shaking also depends on the direction the fault broke, the length of the rupture and local geology. Damage levels are typical, not exact.
A big earthquake under the sea can suddenly lift or drop the sea floor by metres. That moves the whole column of water above it and starts a tsunami: a series of very long waves that cross oceans at the speed of a jet plane. This model solves the shallow-water wave equation across a 380 km slice of ocean.
Warning signs: strong or long shaking near the coast, the sea suddenly going out, or a roar from the sea. Go to high ground or far inland at once and stay there for hours: the first wave is often not the biggest. The 2004 Indian Ocean tsunami killed about 230,000 people; the 2011 tsunami in Japan ran up to about 40 m above sea level in places.
Each circle is a real earthquake recorded by the US Geological Survey. Turn on the plate boundaries: earthquakes draw the edges of the plates. Colour them by depth and look at the subduction zones, where the earthquakes get deeper as the sinking plate goes down into the mantle.
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Drag to move, scroll or pinch to zoom, tap a circle or a star. Data: US Geological Survey (all M5+ earthquakes of 2018; M7+ earthquakes 1965–2016 from the USGS/NEIC catalogue of M5.5+ earthquakes; M7+ 2017 to October 2026 from USGS ComCat); plate boundaries: Bird (2003); Earth image: NASA.
The earthquake diagram you draw and label in a test: the fault, the hanging wall and footwall, the focus and epicenter, seismic waves and a seismometer. Switch between the labeled diagram, a blank one to test your memory, and a quiz where you place each label yourself. You can also download both versions or print a label worksheet.
Tap a word, then tap the numbered box it belongs to (on the diagram or in the list below). Tap a filled box to take the word back. On a phone, swipe the diagram sideways to see every number.
Tap a card, then tap the box it belongs in. Press Check when all 12 are placed.
A printable worksheet: label the earthquake diagram, sort P and S waves, work out distances from S − P times, multiple choice and explain questions, with an answer key.
A fault is a crack in the crust where the rocks on the two sides have moved. When the crust is pulled apart, a normal fault forms; when it is squeezed, a reverse (thrust) fault; when the blocks slide sideways past each other, a strike-slip fault.
Far from the fault the plates keep moving, but the fault itself is stuck. The rock near it bends like a ruler being pushed, slowly storing energy. When the stress becomes larger than the friction, the fault suddenly slips and the bent rock springs back: this is elastic rebound. A road or fence across the fault is broken and offset.
The break starts at the focus (hypocenter) underground and spreads along the fault. The epicenter is the point on the surface directly above the focus. Fast P waves arrive first, then S waves, then the large, slower surface waves. Seismometers record them, and the gap between P and S tells how far away the earthquake was.
Magnitude is the size of the earthquake: each step up releases about 32 times more energy. Intensity is how strongly the ground shakes at a particular place: it is highest near the epicenter, on soft ground and in badly built buildings.
Choose Strike-slip and press Move the plates: watch the road bend, then break. Then choose Reverse and compare where the epicenter is: it is not on the fault line at the surface, because the fault slopes. Finally, find the epicenter yourself in the seismogram game.
| Fault | Forces | Movement | Plate boundary | Examples |
|---|---|---|---|---|
| Normal | Tension (pulled apart) | Hanging wall slides down | Divergent, rifts | East African Rift; Basin and Range (USA); L’Aquila, Italy 2009 |
| Reverse (thrust) | Compression (squeezed) | Hanging wall pushed up | Convergent (including subduction megathrusts) | Himalayas (Nepal 2015); Japan Trench (Tōhoku 2011); Chile (1960) |
| Strike-slip | Shear (sideways) | Blocks slide past each other | Transform | San Andreas (1906); East Anatolian Fault (Türkiye 2023); Sagaing Fault (Myanmar 2025) |
| Magnitude | About how many a year worldwide | Energy (TNT equivalent) | Typical effects |
|---|---|---|---|
| 2–2.9 | 1,300,000 | about 15 kg at M2 | Usually not felt |
| 3–3.9 | 130,000 | about half a tonne at M3 | Often felt; little damage |
| 4–4.9 | 13,000 | about 15 tonnes at M4 | Felt by most nearby; slight damage |
| 5–5.9 | 1,300 | about 500 tonnes at M5 | Damage to weak buildings near the epicenter |
| 6–6.9 | 134 | about 15,000 tonnes at M6 (like the Hiroshima bomb) | Can be destructive in populated areas |
| 7–7.9 | 15 | about 500,000 tonnes at M7 | Serious damage over large areas |
| 8 and more | about 1 | about 15 million tonnes at M8 | Great damage hundreds of km away; possible tsunami |
Yearly numbers: USGS estimates from observations since 1900. On a seismogram each step of magnitude means about 10 times bigger wiggles, and about 32 times more energy.
| Intensity | What people notice |
|---|---|
| I–III | Not felt, or felt only by a few people at rest indoors |
| IV–V | Felt by most people; dishes and windows rattle; some objects fall |
| VI–VII | Felt by everyone; hard to stand; plaster falls; damage to badly built buildings |
| VIII–IX | Walls and chimneys fall; heavy damage even to good buildings |
| X–XII | Most buildings destroyed; ground cracks; rails bend |
Bangladesh lies near the meeting point of the Indian, Eurasian and Burma plates. To the north, India pushes under the Himalayas; to the east, the Indian Plate sinks below the Burma Plate along the Indo-Burman ranges. Strong historical earthquakes include the 1897 Great Assam (Shillong) earthquake (about M8), which damaged buildings as far away as Dhaka; the 1918 Srimangal earthquake; and the 1950 Assam–Tibet earthquake (M8.6). The 2015 Nepal and 2025 Myanmar earthquakes were felt across the country, and on 21 November 2025 a M5.4–5.5 earthquake near Tongi and Narsingdi shook Dhaka and killed about 10 people. Dhaka’s soft, river-delta soils and crowded, often poorly built buildings make it one of the cities most at risk of earthquake damage.
During the shaking: Drop to your hands and knees, take Cover under a sturdy table (or protect your head and neck against an inside wall), and Hold On until the shaking stops. Stay away from windows, glass and heavy furniture; do not run outside while the ground is shaking (falling glass and bricks are the main danger) and do not use lifts. Near the coast: after strong shaking, go to high ground at once. Before: fix heavy furniture to walls, know safe spots and an emergency plan, and support earthquake-resistant building codes.
No one can predict the exact time, place and size of an earthquake. Scientists can say how likely strong shaking is in a region over decades (hazard maps), which guides building codes. Early warning systems (in Japan, Mexico and the west coast of the USA, for example) detect the fast P waves and send alerts seconds before the slower, stronger S and surface waves arrive.
Seismologists now use the moment magnitude Mw = ⅔ log10(M0) − 6.07 (the IASPEI standard form), where the seismic moment M0 = μAD (rock rigidity × fault area × average slip, in N·m). Unlike the original Richter scale it does not saturate for great earthquakes. The number of earthquakes follows the Gutenberg–Richter law, log10N = a − bM with b ≈ 1: about ten times more earthquakes for each step down in magnitude. Earthquakes deeper than about 70 km happen only in subducting slabs, where they outline a dipping Wadati–Benioff zone down to about 700 km. P-wave speed is √((K + ⁴⁄₃μ)/ρ) and S-wave speed √(μ/ρ); in a liquid the shear modulus μ is zero, so S waves cannot travel.
Most earthquakes happen when rocks suddenly slip along a fault. The plates move slowly all the time, but friction locks the fault, so the rocks bend and store energy. When the stress becomes too great, the fault slips in seconds and the energy spreads out as seismic waves that shake the ground.
Key takeaway: stored stress released by sudden slip on a fault.The focus (or hypocenter) is the point underground where the rock first breaks. The epicenter is the point on the Earth’s surface directly above the focus. News reports give the epicenter because it is easy to show on a map.
Key takeaway: focus underground, epicenter above it on the surface.P (primary) waves are the fastest seismic waves; they push and pull the rock in the direction they travel and can pass through solids, liquids and gases. S (secondary) waves are slower and shake the rock at right angles to their direction; they can only travel through solids.
Key takeaway: P = fast push–pull; S = slower shear, solids only.At each seismometer they measure the time between the arrival of the P waves and the S waves. The bigger the gap, the farther away the earthquake. Each distance gives a circle around the station; with three or more stations the circles meet at the epicenter. This is called triangulation.
Key takeaway: S − P times from three stations give three circles that meet at the epicenter.Magnitude measures the size of the earthquake itself, with one number for each earthquake. Intensity, on the Modified Mercalli scale from I to XII, describes how strong the shaking and damage are at a particular place, so one earthquake has many intensities.
Key takeaway: magnitude = size of the earthquake; intensity = shaking at a place.Each step of magnitude releases about 32 times more energy, so two steps release about 32 × 32 ≈ 1,000 times more energy. A magnitude 8 earthquake releases about as much energy as a thousand magnitude 6 earthquakes.
Key takeaway: about 1,000 times more energy.Mostly along plate boundaries. About 9 in 10 large earthquakes happen within a few hundred kilometres of one, especially around the Pacific Ocean (the Ring of Fire) and along the belt from the Mediterranean through Türkiye and Iran to the Himalayas and Indonesia. Some, like the 2001 Gujarat earthquake, happen inside plates.
Key takeaway: along plate boundaries, especially around the Pacific.Not the exact time, place and size. Scientists can estimate how likely strong earthquakes are in an area over many years, and early warning systems can give a few seconds of warning after an earthquake has started, using the fast P waves.
Key takeaway: no exact predictions; hazard maps and early warnings help.Drop, Cover and Hold On: get down, shelter under a sturdy table and hold on until the shaking stops, away from windows and heavy objects. Do not run outside while the ground is shaking and do not use lifts. If you are near the coast and feel strong shaking, move to high ground at once in case of a tsunami.
Key takeaway: Drop, Cover and Hold On; then high ground near the coast.The 1960 Valdivia earthquake in Chile, magnitude 9.5. A fault about 1,000 km long slipped under the coast of Chile, and the tsunami crossed the Pacific to Hawaii and Japan.
Key takeaway: Chile 1960, magnitude 9.5.