Locating an earthquake – S–P triangulation, Richter magnitude, Mercalli intensity and tsunami warning
GeographyEarth's Structure & LandformsAges 15–16Virtual lab
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A seismology lab. Three stations record an earthquake: drag the P and S markers onto the arrivals on each seismogram, turn each S–P lag into a distance with the travel-time graph, draw three circles on the map and find the epicentre where they meet, above the focus at depth. Read the largest swing in millimetres and use Richter's nomogram to find the local magnitude, compare it with the moment magnitude from the size and slip of a fault and see that each whole step means about 32 times more energy. Map the felt reports on the Modified Mercalli scale with isoseismal lines, and watch a megathrust earthquake lift the sea floor and send a tsunami across the ocean at v = √(g·d) to the coast, with the warning time and the time needed to walk to high ground. The default tsunami (Mw 9.0, ocean 4,000 m deep, trench 200 km offshore) is close to the 2004 Sumatra–Andaman earthquake off western Indonesia.
Lesson: Earthquakes: locating the epicentre by triangulation, magnitude and intensity scales, tsunamis and warning
What it shows
P waves travel through the crust at about 6 km/s and S waves at about 3.5 km/s, so the farther a station is from an earthquake, the longer the gap between their arrivals: distance ≈ 8.4 km/s × (S – P lag). One station gives only a circle of possible epicentres; three circles meet at the epicentre, the point on the surface above the focus. Richter's local magnitude comes from the largest swing on a standard seismograph and the distance; the moment magnitude Mw comes from the fault area and slip. A tsunami crosses the ocean at √(g·d). Speeds, attenuation and fault sizes are simplified textbook models.
How to use
In Locate the epicentre, drag the blue P and red S markers onto the start of each wave, drag the round handle of each station to draw its circle until the graph matches its S – P lag, then press Record reading. Tap where the circles meet and press Check epicentre. In Magnitude, pick a Station and tap the distance and amplitude scales of the nomogram. In Intensity, tap a town. In Tsunami, set Moment magnitude Mw, Ocean depth and Warning issued after, then press Start the earthquake.
Parameters you can change
- Screen Locate the epicentre, Magnitude, Intensity, Tsunami
- Earthquake Earthquake 1, Earthquake 2, Earthquake 3 (offshore)
- Focus depth 5–70 km
- Moment magnitude Mw 7.5–9.3
- Ocean depth 1000–7000 m
- Trench to coast 100–1000 km
- Warning issued after 2–30 min
Questions to explore
- Why does one seismograph station tell you how far away an earthquake was, but not in which direction?
- How much more energy does a magnitude 7 earthquake release than a magnitude 5 earthquake?
- Why does a tsunami slow down and grow taller as it reaches shallow water near the coast?
