As an engineered structure, the Earth’s crust is not up to code. From time to time, its design problems cause it to fail, and the result is an earthquake.
The principal cause of crustal weakness is geothermal heat. Isotopes of radioactive elements within the Earth decay to other isotopes, producing heat that is trapped beneath the surface. Because of this trapped heat, the crust is warmer with increasing depth, as anyone knows who has ever descended into a deep mine. Geothermal heat warms the City of Klamath Falls, Oregon, heats the hot springs of the Pacific Northwest, and, on rare occasion, causes the eruption of great volcanoes like Mt. St. Helens.
Just as iron becomes malleable in a blast furnace or hot silica glass becomes soft enough for a glassblower to produce beautiful bowls, the rock becomes weak, like saltwater taffy, when the temperature gets high enough (Figure 2-1). Rock that is soft and weak under these conditions is said to be ductile. At lower temperatures, rock is brittle, meaning it deforms by shattering, accompanied by earthquakes.
Increased temperature tends to weaken rock, but, on the other hand, increased pressure tends to strengthen it. With increasing depth, rock is subjected to conditions that work in opposite directions. The strengthening effect of increased pressure dominates at low temperatures within ten to twenty miles of the Earth’s surface, whereas the weakening effect of higher temperature kicks in rather abruptly at greater depth, depending on the type of rock. The strength of rock, then, increases gradually with increasing depth, and the strongest rock is found just above the depth where temperature weakening takes over (Figure 2-1), a depth called the brittle-ductile transition.
Think about a bridge with a layer of asphalt and concrete overlying a framework of strong steel. If the bridge collapses, it will be because the steel frame fails, not the weaker layers of concrete or asphalt on top. So it is with the Earth’s crust. The crust fails when its strongest layer breaks, just above the brittle-ductile transition where the temperature begins to weaken its minerals. Earthquakes tend to originate in this strongest layer. When this layer fails, shallower and deeper rock fails, too.