Learn about the geophysics behind earthquakes, how they are measured, and where the most powerful earthquake ever witnessed occurred. According to the USGS, some earthquakes also have foreshocks, or smaller quakes that precede a larger earthquake.Įarthquakes can leave behind incredible devastation, while also creating some of the planet's most magnificent formations. A quake measuring 3 to 4.9 is considered minor or light 5 to 6.9 is moderate to strong 7 to 7.9 is major and 8 or more is great.Įarthquakes are always followed by aftershocks, which are smaller quakes that strike after the main quake and can continue for weeks-or even up to years in some cases. Scientists assign a magnitude rating to earthquakes based on the strength and duration of their seismic waves. The Santa Clara Valley south of San Francisco holds a fault prone to oblique motions, for example, seen in a 1999 quake. Geological Survey sees the fault as posing a risk of more magnitude 7.0 earthquakes.įaults that combine sideways with up-and-down motions are called oblique by seismologists. One magnitude 7.0 quake along the fault perhaps 550 years ago dropped the ground on one side of the fault by three feet (a meter). A reverse is, well, just the reverse.Īn example of a normal fault is the 240-mile (150-kilometer) long Wasatch Fault underlying parts of Utah and Idaho, again caused by the Pacific plate driving under western North America. A normal fault occurs where the deeper part of the crust is pulling away from an overlying part. Up-and-down motions in earthquakes occur over so-called "dip-slip" faults, where the ground above the fault zone either drops (a normal fault) or is pushed up (a reverse fault). The sideways motion of the fault's branches is caused by the Pacific Ocean's crustal plate moving to the northwest under North America's continental crust. The most famous example is California's San Andreas Fault, which stretches some 600 miles (1,000 kilometers) from southern California to north of San Francisco. When portions of the Earth's crust moves sideways, the result is a horizontal motion along a "strike-slip" fault. There are several different types of faults, including a normal dip slip fault, reverse fault, and strike-slip fault.
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