The ground upon which people stand is neither as stable nor as still as it seems. The earth’s surface, or crust, is in constant motion though at a rate many hundreds of times slower than even a snail’s pace. The crust is the outermost of the three layers comprising the planet’s structure and covers the mantle which, in turn, surrounds the core. The crust is not a solid casing enclosing the mantle but is broken into a number of plates, known as tectonic plates, which “float” on the mantle similar to ice floes on the ocean surface. The movement of these plates causes many earthquakes.
The areas between the plates create fault zones and most earthquakes occur along these faults. Some of the plates are moving in opposite directions away from each other which can cause large cracks or fissures called rift valleys to form. Others are moving toward each other and one plate may slide over top of the other causing an uplifting of the plate or they may simply force the rock at the point of abutment to buckle upwards. Another type of fault occurs when two plates are sliding along each other in opposing directions. The San Andreas Fault along the California coast is the most notable of this type. A highway constructed across this type of fault could become misaligned by a few inches or several feet during an earthquake.
The friction of these plates grinding and pushing against each other builds up enormous levels of stress and, once the force of the movement is sufficient to overcome the friction, a sudden and violent shifting of the plate occurs. It is much like opening a window that is stuck; once enough pressure is applied the window will suddenly move. These movements, deep within the earth, cause vibrations called seismic waves.
Seismic waves radiate from the point of movement, called the focus or hypocenter, in a spherical pattern like an expanding balloon. The point at which the first wave reaches the surface is called the epicenter and is typically directly above the focus. The strength or magnitude of the energy released in these waves, measured on the Richter scale, is determined by the amount of potential energy stored due to the friction, the distance the rock in the plate moved and the ability of the surrounding rock to transmit the waves.
When these waves reach the surface the ground vibrates, often causing man-made structures such as buildings and bridges to shake or even collapse. Damage can also occur to other important infrastructure such as power lines, water or gas mains and roads. This type of damage can severely impede the efficacy of emergency services and rescue efforts. When an earthquake occurs near or beneath the ocean it can cause a tsunami which will often result in greater devastation than the quake itself.
Earthquakes can also cause a phenomenon known as liquefaction. The violent shaking can transform water bearing ground to a liquid-like slurry incapable of supporting a building causing the foundation to tilt or sink. Some earthquakes can be linked to volcanic activity as large masses of molten rock called magma move beneath the earth’s surface.
The USGS estimates that there are nearly 1.5 million detectable earthquakes worldwide each year but fewer than 20 of these are magnitude 7 or higher on the Richter scale. This suggests that our planet is in constant motion and that the earth is not just a big rock floating in space but an ever changing and evolving entity.