How Earthquakes Work
An earthquake is one of the most terrifying phenomena that nature can dish up．We generally think of the ground we stand on as 'rock-solid' and completely stable．But an earthquake can shatter that perception instantly．Up until relatively recently, scientists only had unsubstantiated guesses as to what actually caused earthquakes．Even today there is still a certain amount of mystery surrounding them, but scientists have a much clearer understanding．There has been enormous progress in the past century: Scientists have identified the forces that cause earthquakes, and developed technology that can tell us an earthquake's magnitude and origin．The next hurdle is to find a way of predicting earthquakes.
Shaking Ground
An earthquake is a vibration that travels through the earth's crust．Technically, a large truck that rumbles down the street is causing a mini-earthquake, if you feel your house shaking as it goes by, but we tend to think of earthquakes as s that affect a fairly large area, such as an entire city．All kinds of things can cause earthquakes:
-volcanic eruptions
-meteor(流星) impacts
-underground explosions (an underground nuclear test, for example)
-collapsing structures (such as a collapsing mine)
But the majority of naturally-occurring earthquakes are caused by movements of the earth's plates, as we'll see in the next section.
We only hear about earthquakes in the news every once in a while, but they are actually an everyday occurrence on our planet．According to the United States Geological Survey, more than three million earthquakes occur every year．That's about 8000 a day, or one every 11 seconds! The vast majority of these 3 million quakes are extremely weak.
Sliding Plates
The biggest scientific breakthrough in the history of seismology(地震学)—the study of earthquakes came in the middle of the 20th century, with the development of the theory of plate tectonics(板块构造)．The basic theory is that the suce layer of the earth—the lithosphere—is comprised of many plates that slide over the lubricating mantle(地幔) layer．At the boundaries between these huge plates of soil and rock, three different things can happen:
-Plates can move apart—If two plates are moving apart from each other, hot, molten rock flows up from the layers of mantle below the lithosphere.
-Plates can push together—If the two plates are moving toward each other, one plate typically pushes under the other one．At some boundaries where two plates meet, neither plate is in a position to subduct under the other, so they both push against each other to form．mountains.
-Plates slide against each other—At other boundaries, plates simply slide by each other—they are pushed tightly together．A great deal of tension builds at the boundary.
Where these plates meet, you'll find faults—breaks in the earth's crust where the blocks of rock on each side are moving in different directions．Earthquakes are much more common along fault lines than they are anywhere else on the planet.
Faults
Scientists identify four types of faults, characterized by the position of the fault plane, the break in the rock and the movement of the two rock blocks:
-In a normal fault (see animation below), the fault plane is nearly vertical．These faults occur where the crust is being pulled apart, due to the pull of a divergent plate boundary.
-The fault plane in a reverse fault is also nearly vertical, but the hanging wall pushes up and the footwall pushes down．This sort of fault forms where a plate is being compressed.
-A thrust fault moves the same way as a reverse fault, but the fault line is nearly horizontal．This is the sort of fault that occurs in a converging plate boundary.
-In