- What is the difference between the focus and the epicentre?
- The focus (hypocentre) is the point underground where the rock breaks and the waves start. The epicentre is the point on the ground directly above it. Shaking is usually strongest near the epicentre, but damage also depends on the depth, the magnitude and the ground under the buildings.
- What are P and S waves?
- P (primary) waves are compression waves: they push and pull the rock like sound, travel about 6 km/s in the crust and can pass through liquids. S (secondary) waves shake the rock from side to side, travel about 3.5 km/s and cannot pass through liquids. Surface waves travel along the ground more slowly and cause most of the damage.
- How do seismologists find the epicentre?
- They measure the S–P gap, the time between the P and S arrivals, at three or more stations. A bigger gap means a station further away; about 8 km for every second of gap is a good estimate. Drawing a circle of that distance around each station, the circles cross at the epicentre.
- What does magnitude measure?
- Magnitude measures the energy released at the focus on a logarithmic scale. Each whole step is about 32 times more energy (10 to the power 1.5), so a magnitude 7 quake releases about 32 times the energy of a magnitude 6. Intensity is different: it describes how strongly the ground shakes in one place.
- Why are shallow earthquakes more damaging?
- A shallow focus is close to the surface, so its waves reach the ground over a short path and less of their energy is spread out before they shake buildings. Deep quakes of the same magnitude are usually felt over a wider area, but more weakly.
- What do the fault types mean?
- Strike-slip faults slide sideways, as on the San Andreas Fault. Thrust faults push one block up over another, as in the Himalayas. Normal faults pull the crust apart so that one block drops, as in the Basin and Range of the western USA.