Some claim that animals — dogs, cats, horses and snakes — know when an earthquake is coming. If so, we humans haven’t learned to read their predictions. Is there any machine that can tell when earthquakes are going to happen or warn us?
Machines called seismographs can measure the intensity of earthquakes as they’re happening. But no machine can predict in advance when an earthquake will occur.
Earthquakes happen because Earth’s surface is divided into huge “tectonic plates” — continent-sized slabs of rock that move over time. Where the plates push against each other, stresses gradually build. Cracks can appear — called faults. When the stress gets too great in a given area, a piece of rock might slip or break — and that’s when you get an earthquake.
Scientists know where the large faults are — they have a pretty good idea where earthquakes will happen. When is another story. As stress at a fault increases, the likelihood of an earthquake might also increase. But evidence indicates that faults are sensitive to even small shocks from nearby faults — which sometimes build stress and sometimes relieve it. That’s why predicting earthquakes is so difficult.
Even if we had machines to monitor stress along all faults, scientists debate whether they’ll ever learn to predict earthquakes precisely.
- A very helpful reference to understand all this is: Self-Organized Criticality, 1991, P. Bak amd K. Chen, Scientific American, January, pp. 46-53
- Stein, Ross S., “The role of stress transfer in earthquake occurrence”, Nature 402, 605 – 609 (09 Dec 1999)
- Earth Science: Parkfield’s unfulfilled promise Ross S. Stein Nature419, 257 – 258 (19 Sep 2002) News and Views
A large earthquake occurs when a rupture that starts on a small segment loads other segments and causes a continuous cascade of ruptures break through segment boundaries to involve a large fault area. However, even in a high stress state small and medium earthquake can occur which do not cascade into large earthquakes.
A worldwide network of seismographs – machines that measure geologic movement – detects about 1 million small earthquakes per year.
The earth is very uneven; tiny, small, medium, and large fault segments are distributed through the crust. Plate tectonics is continuously stressing the system, and segments break. Exactly which segment will break next is practically impossible to predict. We don’t even know where all these faults are, let alone details about their strength or stress states, and small differences can determine whether or not a segment breaks.
Most large earthquakes occur on long fault zones around the margin of the Pacific Ocean. This is because the Atlantic Ocean is growing a few inches wider each year, and the Pacific is shrinking as ocean floor is pushed beneath Pacific Rim continents.