Science Focus Topic 5 Notes: Earthquakes | Print |
Earthquakes occur when tectonic plates move suddenly

Measuring Earthquakes

- scientists called seismologists use a seismograph to record the intensity of an earthquake
- the seismograph must be attached to
bedrock (the solid rock that lies beneath the soil and looser rocks) to feel the vibrations on the plate
- a marking pen, inside the seismograph, records the vibrations on a rotating drum (modern seismographs are electronic)
- the measurement scale used is called the
Richter scale (table 5.3, p. 396)

Earthquake Waves

- seismic waves are the energy waves that travel outward from the source of the earthquake.
aftershocks are actually smaller earthquakes

Types of Earthquake Waves

- Primary or p waves are the fastest and can push through solids, liquids and gases
Secondary or s waves travel more slowly and can only pass through solids
Surface waves are the slowest of all, but their rolling motion can be very destructive (like a ripple effect on water)
- primary waves are bent or refracted as they travel (the area where they do not come through the other side of the earth is called a
shadow zone - Figure 5.52, p. 398)

Locating an Earthquake

- it is possible to determine the location of an earthquake by the interval between the p waves and the s waves (the farther apart they are, the further away the earthquake is
- the source of an earthquake deep in the crust is called the
focus, where the p waves and s waves originate
- the surface waves come from the
epicentre (the location on the surface directly above the focus)

Earthquake Zones

- the zones of greatest intensity (8 or more on the Richter scale) 1 off Canada's west coast, 8 in Mexico, and 8 in Alaska

Types of Rock Movement in Earthquakes

- where the plates meet, the rock is under great pressure, which can make it bend and stretch - when the pressure is too great, the rock breaks suddenly creating a fault
- there are three types of movement, of the tectonic plates, along a fault (see Figure 5.54, p. 403):
Normal Faults, (pulling action, which breaks rocks apart) - North Atlantic
Reverse Faults (compression, where rocks are squeezed, causing them to bend and break) - Marianas Trench, near Japan
Strike-Slip or Transform Faults (shear causes slipping, which makes the jagged edges break off) - Pacific Plate

Preparing for Earthquakes

- stabilize furniture, storage of heavier items close to the floor, earthquake-resistant designs (allow building to bend a little)

Other Effects of Earthquakes

- tsunamis (Japanese word meaning 'harbour wave') are huge waves that happen when an earthquake occurs under the ocean
avalanches or rock slides occur in mountains as a result of an earthquake
- type of foundation upon which building are constructed can have an effect on the severity of the earthquake (


- Ancient predicting device used a pot with eight dragon heads on it, with little balls inside the heads and frogs to catch the balls (the frog that caught the ball indicated the direction of the earthquake) p. 395
- Animals: rabbits hop wildly for several minutes, deep-sea fish swim close to the surface, catfish jump out of the water, bees evacuate their hives, and mice are dazed before an earthquake