A fault's width, not its length, caps how big an earthquake can get
Most of an earthquake's energy never reaches us as shaking: perhaps 10 percent or less radiates as seismic waves, the rest cracks rock and heats the fault. And the biggest quakes happen where plates dive at shallow angles, giving a rupture surface up to 100 kilometres wide. Steep sideways faults simply lack the room.
Plates grind past one another, but rough patches on a fault, called asperities, make the two sides stick. Strain builds in the surrounding rock until it overwhelms the snag, and the fault lurches, releasing stored energy in a sudden slip. This cycle of slow loading and abrupt failure is the elastic-rebound theory. Only the cold, brittle upper crust and sinking slabs can store energy this way; rock hotter than about 300 °C flows instead of breaking. The rupture begins at the hypocentre, and the point on the surface above it is the epicentre.
Faults come in three main styles. Normal faults, where crust is stretched, rarely produce quakes above magnitude 7, and in Iceland the brittle layer is only about six kilometres thick. Strike-slip faults, where blocks slide sideways, stand nearly vertical and reach about 10 km deep, which limits them to roughly magnitude 8. Reverse or thrust faults at converging plates host megathrust quakes, nearly all events of magnitude 8 or more and about 90 percent of the seismic moment released worldwide.
Magnitude scales steeply. Each whole step means roughly 30 times more energy, so a magnitude 7 releases about 1,000 times the energy of a 5, and a magnitude 8 matches some 5,600 Second World War atomic bombs. The largest ever measured struck near Cañete, Chile, on 22 May 1960 at magnitude 9.5, releasing about twice the energy of Alaska's 1964 Good Friday quake. Ruptures can run for about 1,000 km, as in Sumatra in 2004.
Deadliness depends less on size than on setting. The 1556 Shaanxi earthquake in China was catastrophic because many people lived in yaodongs, homes dug into soft loess hillsides, and the 1976 Tangshan quake was the deadliest of the 20th century. Rare but powerful quakes, weak building codes and nearby coasts where tsunamis can form raise the toll. Humans also trigger tremors through mining, fracking and nuclear tests, and similar quakes shake Mars and the Moon.
Source: Earthquake
