Notes
Faults and earthquakes
- Normal faults come from tension, reverse from compression and strike-slip from shear.
- Earthquakes release stored energy by elastic rebound. The focus is underground; the epicenter is above it on the surface.
Seismic waves and location
- P waves are fastest and move through solids and liquids. S waves are slower and move only through solids. Surface waves are slowest and most damaging.
- A larger S-P gap means a greater distance. Three stations locate the epicenter by triangulation.
- Time $\displaystyle = \frac{\text{distance}}{\text{speed}}$.
Magnitude and volcanoes
- Each magnitude step is about 32 times more energy and 10 times more amplitude. Mercalli intensity measures damage.
- Volcano types: shield (gentle), composite (explosive), cinder cone (small). Hazards include lava, ash, pyroclastic flows, lahars and tsunamis.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Compression pushes the hanging wall up.
Shear waves need a solid.
Two steps of a factor of 10 each give 100.
$6 \times 40 = 240$ km.
Sticky, gas-rich magma makes explosive eruptions.
Lahars are volcanic mudflows.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Three seismograph stations record an earthquake. At Station A, the P wave arrives 50 seconds after the earthquake begins. Assume P waves travel at 8 km/s and S waves at 4 km/s in this simplified problem. The S wave reaches Station A at 100 seconds after the earthquake.
(a) Calculate the distance from Station A to the epicenter.
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$d = 8 \text{ km/s} \times 50 \text{ s} = 400$ km. (Check: $4 \times 100 = 400$ km.)
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(b) What is the S-P time gap at Station A, and what happens to this gap at a station twice as far away?
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$100 - 50 = 50$ s. At 800 km, P arrives at 100 s and S at 200 s, so the gap doubles to 100 s.
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(c) Explain why data from three stations are needed and how scientists use them.
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Each station gives only a distance, so a circle of possible locations. The circles from three stations intersect at one point, the epicenter.
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