Earthquakes and seismic waves

Why the earth shakes. Energy is released along a fault, a sharp break in crustal rocks. Rocks on either side tend to move in opposite directions, but friction locks them. When the push overcomes friction, the blocks slide abruptly, releasing energy as waves in all directions. All natural earthquakes happen in the lithosphere (to about 200 km deep).

• The point where energy is released is the focus or hypocentre.
• The point on the surface directly above it is the epicentre, the first place to feel the waves.
• A seismograph records the waves.

Earthquake waves:

• Body waves start at the focus and travel through the earth's body. P-waves (primary) are fastest, arrive first, are like sound waves, travel through solids, liquids and gases, and vibrate parallel to their direction, squeezing and stretching the rock. S-waves (secondary) arrive later, travel only through solids, and vibrate perpendicular to their direction, making crests and troughs.
• Surface waves form when body waves meet surface rocks; they arrive last and are the most destructive, displacing rocks and collapsing buildings.
• Waves travel faster in denser material and reflect or refract where density changes.

Shadow zones. Seismographs within 105° of the epicentre record both P and S-waves. Beyond 145°, only P-waves arrive. So 105°-145° is a shadow zone for both, and the whole area beyond 105° gets no S-waves: a little over 40 per cent of the earth's surface. Because S-waves cannot pass through liquid, this shows the outer core is liquid.

Types of earthquake: tectonic (sliding along a fault; the most common); volcanic (near active volcanoes); collapse (mine roofs falling in mining areas); explosion (chemical or nuclear blasts); reservoir-induced (near large dams).

Measuring earthquakes: magnitude on the Richter scale (energy released, 0-10); intensity on the Mercalli scale (named after an Italian seismologist; visible damage, 1-12). Quakes of 8+ are rare (once in 1-2 years); tiny ones occur almost every minute.

Effects: ground shaking, differential settlement, land and mud slides, soil liquefaction, ground lurching, avalanches, ground displacement, floods from dam failures, fires, structural collapse, falling objects and tsunamis (only when the epicentre is under the ocean and the magnitude is high). A quake lasts seconds, but one above 5 on the Richter scale can be devastating.

Example (new): a seismograph station 60° from an epicentre records a P-wave and, some minutes later, an S-wave; a station 120° away records nothing clear; a station 160° away records P-waves only. Together they outline the shadow zone and the liquid core.