Shake, Rattle, and Rupture: Understanding Earthquakes

Imagine you are sitting in a café in Tokyo, Japan. Suddenly, the windows rattle, your coffee ripples, and the building sways gently for thirty seconds. Then everything goes still. No storm, no thunder — just the ground itself moving. How is that possible? In 2011, an earthquake near Japan's coast was so powerful it actually shifted the entire island of Honshu about 2.4 metres to the east. The ground beneath us is not the solid, unchanging surface it appears to be. Let's find out why.

The Earth's outer layer, called the lithosphere, is broken into about fifteen large pieces called tectonic plates. These plates float very slowly on the semi-molten rock below, called the mantle. They move only a few centimetres per year — roughly the rate your fingernails grow — but over millions of years that movement reshapes continents. Plates interact at their edges, called plate boundaries. There are three types of boundary: • Convergent boundary — two plates push towards each other. One may be forced beneath the other in a process called subduction. • Divergent boundary — two plates pull apart, creating a gap. • Transform boundary — two plates slide horizontally past each other, grinding and locking. At transform and convergent boundaries especially, plates do not glide smoothly. Friction locks them together. Stress — that is, squeezing and stretching force — builds up over decades or centuries. When the stress finally overcomes the friction, the plates lurch suddenly. That sudden movement releases enormous amounts of energy in the form of seismic waves, which travel outward through the rock in all directions. We feel those waves as an earthquake. The exact point underground where the rupture begins is called the focus (or hypocentre). The point on the Earth's surface directly above the focus is called the epicentre. The epicentre is usually where shaking is felt most strongly. Seismic waves come in different types. Primary waves (P-waves) are compression waves — they squeeze and stretch rock and can travel through solids and liquids. Secondary waves (S-waves) move rock side to side and are slower; they cannot travel through liquid. Surface waves travel along the ground's surface and cause most of the damage we see in buildings and roads. Scientists called seismologists use an instrument called a seismograph to detect and record seismic waves. Earthquake size — its magnitude — is measured on the Moment Magnitude Scale (Mw), which has largely replaced the older Richter scale. Both scales are logarithmic, meaning each whole number represents about 32 times more energy than the one below it. A magnitude 7.0 earthquake releases roughly 32 times more energy than a magnitude 6.0. Earthquakes cluster along plate boundaries. The Pacific Ring of Fire — a horseshoe-shaped zone that runs around the edge of the Pacific Ocean, through Japan, the Philippines, the west coast of the Americas, and New Zealand — experiences about 90% of the world's earthquakes. Other active zones include the Alpide Belt, which runs from the Mediterranean region through Turkey, Iran, and into South Asia.