On May 22, 1960, a massive earthquake hit southern Chile. For 10 minutes, the ground shook so violently that people could not stay on their feet. Cracks opened in roads and buildings toppled.
The Valdivia earthquake was roughly a magnitude 9.5, the largest ever recorded even to this day. Although it is possible, the chances of a much larger quake are low. It would require an enormous chunk of crust to break all at once-the movement of a fault both extremely deep and extraordinarily long. There are few places on Earth where that could happen, said Wendy Bohon, an earthquake geologist. A 9.5 magnitude earthquake is probably the upper limit for what the planet can generate, she explained, and a magnitude 10 is extremely unlikely.
The magnitude of a quake is dependent on the total area of a fault that breaks. This also depends on how deep the fault goes down into the crust and how long, horizontally, the segment is that breaks. There exists physical limits to how big an area can break. The deepest faults occur at subduction zones, where one tectonic plate pushes under another.
There are limits to the length of a fault segment that can break. Even subduction zone faults don’t break all at once, said Bohon. Usually, something gets in the way-a seamount (an undersea mountain), perhaps, or a change in the type of rock or the geometry of rock that makes one segment of a fault more resistant to stress than its adjacent.
“So many potential faults could have damaging earthquakes,” said Bohon. “But people only think about the big one.”