School โบ Earthquakes โบ Ages 15โ16
Seismic waves, magnitude and locating earthquakes
P- and S-waves, seismometers, locating an epicentre by triangulation, the logarithmic magnitude scale, earthquake frequency and the limits of prediction. With a data investigation to find an epicentre.
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Earthquakes happen when two blocks of rock suddenly slip past one another on a fault. Plates keep moving, the rough fault sticks, strain energy builds up and is released as seismic waves. Transform boundaries such as the San Andreas fault, where plates move past each other at about 5 cm per year, generally produce shallow earthquakes.[1][3]
P-waves are longitudinal: rock is compressed and stretched along the direction of travel, like sound. S-waves are transverse and cannot travel through liquids or air. In the crust P-waves travel at around 6โ7 km/s and S-waves at around 3.5โ4 km/s.[5]
A seismometer works because a suspended mass tends to stay still while the ground moves. On a seismogram the P-wave arrives first, then the S-wave, then the surface waves.[6]
The SโP time gives the distance to the earthquake. With speeds vP and vS: ฮt = d/vS โ d/vP, so d = ฮt รท (1/vS โ 1/vP). Circles of radius d around three or more stations meet at the epicentre (triangulation).[1][6]
Magnitude is logarithmic: each whole-number step is a tenfold increase in amplitude on a seismogram and about 32 times more energy. An earthquake has one magnitude, but its shaking intensity varies from place to place, mainly with distance from the rupture.[2]
Small earthquakes are far more common: per year on average about 49,000 of magnitude 3 (estimated), 800 of magnitude 5, 120 of magnitude 6, 18 of magnitude 7 and 1 of magnitude 8 or more. More small earthquakes are recorded today only because monitoring networks have grown.[4]
No scientist has ever predicted a major earthquake, which would need its date and time, location and magnitude. Scientists instead calculate probabilities for an area over a number of years.[7]
Try it: Data investigation: locate the epicentre
You need
- Graph paper (scale 1 cm = 10 km)
- A pair of compasses and a ruler
- Calculator
- Station positions (x, y in km): A (145, 174), B (36, 140), C (196, 39)
- SโP times: A 5.0 s, B 10.0 s, C 15.0 s
Safety
No practical hazards: this is a data investigation. Take care with the compass point.
What to do
- Using vP = 6.0 km/s and vS = 3.5 km/s, show that d โ 8.4 km for every second of SโP time.
- Calculate the distance from each station to the earthquake.
- Plot the three stations on the graph paper and draw a circle of the right radius around each.
- Mark the epicentre where the three circles meet and read off its coordinates.
- Repeat the distance calculation with vP = 7 km/s and vS = 4 km/s. How much do the distances change?
- Write a conclusion with the epicentre and the main sources of uncertainty.
What happens, and why?
S-waves are slower than P-waves, so the gap between their arrivals grows with distance: 1/3.5 โ 1/6.0 โ 0.119 s per km, so each second of SโP time means about 8.4 km. The three circles (42 km, 84 km and 126 km) meet near (120, 140). With 7 and 4 km/s each second means about 9.3 km, so the wave speeds you assume are a major source of uncertainty.[5][1][6]
Play: P-wave, S-wave or both?
Is each statement true for P-waves, S-waves, or both?
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6โ7 ยท Ages 10โ11