Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.

Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!

Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always haveaftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

crust

Earth's outermost layer, which varies in thickness from about 5km to 60km and is separated from the mantle by the Moho Discontinuity

epicenter

point of Earth's surface directely above an earthquake's focus

focus

in an earthquake, the point beneath Earth's surface where energy release occurs

inner core

very dense, solid center od the Earth that is made of mostly iron with smaller amounts of oxygen, silicon, sulfur, or nickel

magnitude

measure of the energy released by an earthquake

mantle

largest layer inside Earth, lying directly above the outer core and that is made mostly of silicon, oxygen, magnesium, and iron

normal fault

break in rock due to tension forces, where rock above the fault surface moves downward in relation to rock below the fault surface

outer core

liquid core that surrounds Earth's solid inner core, and that is made mostly of iron

primary waves

waves that travel outward from an Earthquake's focus and cause particles in rocks to move back and forth in the same direction the wave is moving

reverse fault

break in rock due to compression forces, where rocks above the fault surface move upward and over the rocks below the fault surface