While we in earthquake-prone places like California tend to get blase about the ground shaking ( aw hell, my coffee spilled! ), even the most unaffected of us feel that little niggle of fear whenever the walls start to shake... will this be a big one? These are some of the most basic things about earthquakes: what we know about them, why they happen and what they can do. This is not a safety list, but more of a list of facts about the most basic elements of an earthquake... what kinds there are, what causes them, the types of faults, etc.
Something to note is that it really is only recently (within the last 30-40 years) that we have really even begun to understand these dangerous natural events. Imagine what it must have been like 3,000 years ago when you had to not only deal with your hut collapsing on top of you, but having to wonder what you did that made your gods get so pissed off. Peruse this fascinating stuff about earthquakes, many of which are great earthquake facts for kids, and learn up! That Cascadia fault isn't going to stay put for long!
Our Lithosphere is comprised of many plates that slide over the lubricating athenosphere layer. At the boundaries between these huge plates of soil and rock, three different things can happen:
1. 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. This magma comes out on the surface (mostly at the bottom of the ocean), where it is called lava. As the lava cools, it hardens to form new lithosphere material, filling in the gap. This is called a divergent plate boundary.
2. Plates can push together If the two plates are moving toward each other, one plate typically pushes under the other one. This subducting plate sinks into the lower mantle layers, where it melts. 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. The lines where plates push toward each other are called convergent plate boundaries.
3. Plates slide against each other At other boundaries, plates simply slide by each other -- one moves north and one moves south, for example. While the plates don't drift directly into each other at these transform boundaries, they are pushed tightly together. A great deal of tension builds at the boundary.
Normal fault. These are described as being nearly vertical and occur in areas where earth's plates are pulled apart because of a divergent plate boundary nearby. On this fault, the hanging wall pushes down on the footwall. For reference, the hanging wall is the rock pushed above the fault plane and the footwall is the rock below the plane. The fault plane is the flat surface representing the fracture line of the fault.
Reverse fault. These are created when the earth's crust is compressed when two plates collide. Here the hanging wall pushes up and the footwall pushes down.
Strike-slip fault is a horizontal fault where the areas of rock slide past one another. These occur in areas where there is a transform plate boundary. The San Andreas fault in California is an example of a strike-slip fault
Thrust Fault When thrust faults are exposed on the surface overburdened material lies over the main block. They are normally associated with areas of folded surfaces and or mountainous regions. The dip angles of thrust faults are normally not as steep as a normal fault.
When you boil it down, despite the simplicity of classification, earthquakes are all about three basic types of elastic waves.
Two of the three propagate within a body of rock. The faster of these body waves is called the primary or P wave. As it spreads out, it alternately pushes (compresses) and pulls (dilates) the rock. These P waves are able to travel through both solid rock, such as granite mountains, and liquid material, such as volcanic magma or the water of the oceans.
The slower wave through the body of rock is called the secondary or S wave. As an S wave moves, it shears the rock sideways at right angles to the direction of travel. If a liquid is sheared sideways or twisted, it will not spring back, hence S waves can only move through solids like rock.
(In most earthquakes, the P waves are felt first. The effect is similar to a sonic boom that bumps and rattles windows. Some seconds later, the S waves arrive with their up-and-down and side-to-side motion, shaking the ground surface vertically and horizontally. This is the wave motion that is so damaging to structures.)
The third type of wave is called a surface wave, because its motion is restricted to near the ground surface. Just like the ripples of water that travel across a lake.bThese kinds of waves can be divided into two types. The first is called a Love wave. It moves the ground from side to side in a horizontal plane but at right angles to the direction of propagation. The horizontal shaking of Love waves is particuly damaging to the foundations of structures. The second type of surface wave is known as a Rayleigh wave. Like rolling ocean waves, Rayleigh waves wave move both vertically and horizontally in a vertical plane pointed in the direction in which the waves are travelling.
The epicenter is directly above the point where the fault begins to rupture, and in most cases, it is the area of greatest damage. However, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone.
The point where the energy is released is called the focus and the focal depth is the depth beneath the earth's surface where the energy release originates. The epicenter is the point on the earth's surface directly above the focus. From here, the energy released spreads out in rings moving across the surface - not unlike those caused when a rock hits still water.
From what we know from recorded history, earthquakes occur in the same general patterns year after year, principally in three large zones of the earth.
The world's greatest earthquake belt, the circum-Pacific seismic belt, is found along the rim of the Pacific Ocean, where about 81 percent of the world's largest earthquakes occur. It has earned the nickname "Ring of Fire". The belt extends from Chile, northward along the South American coast through Central America, Mexico, the West Coast of the United States, and the southern part of Alaska, through the Aleutian Islands to Japan, the Philippine Islands, New Guinea, the island groups of the Southwest Pacific, and to New Zealand. This is a region of young, growing mountains and deep ocean trenches which invariably parallel mountain chains. Earthquakes necessarily accompany elevation changes in mountains, the higher part of the earth's crust, and changes in the ocean trenches, the lower part.
The second important belt, the Alpide, extends from Java to Sumatra through the Himalayas, the Mediterranean, and out into the Atlantic. This belt accounts for about 17 percent of the world's largest earthquakes, including some of the most destructive.
The third prominent belt follows the submerged mid-Atlantic Ridge, which, you know... middle of the ocean.
