Resource: Earthquakes: The Prehistoric Record

Print Background Essay

People have known for centuries that some regions of the world are more prone to earthquakes than others. In some locations, large earthquakes occur frequently enough for residents to experience several in a lifetime, and many are recorded in local histories. Seismic events also create their own historical record through the fractures, or faults, that they generate along Earth's surface. These faults provide physical evidence of where earthquakes occurred in the past and indicate where they are likely to occur in the future. Until recently, scientists thought this evidence said little about when a future earthquake might occur. However, that changed in the late 1970s when a geologist began exploring a dry creek bed whose path had been diverted by successive earthquakes along the San Andreas Fault.

Earthquakes result from the movement of tectonic plates. At one type of plate boundary, called a transform boundary, plates slide roughly past each other in opposite directions. When the plates become temporarily locked along these boundaries, forces that drive plate motion cause stress and strain to build up in rocks that make up the plates. When this strain exceeds the strength of the rocks involved, the rocks fracture along faults, temporarily relieving the strain and causing the vibrations we feel as earthquakes.

One of the most active faults in the world is California's San Andreas Fault. More than 1,000 earthquakes occur along this 1,300-kilometer-long (800-mile-long) fault system that forms part of the transform boundary between the Pacific and North American plates. Most of these quakes are too small to be felt. Others, such as those that occurred along the San Andreas in 1857 and 1906, shake the earth violently. The 1906 earthquake and resulting fires killed hundreds and all but destroyed San Francisco.

Large earthquakes such as these cause significant shifts in the position of plates along a fault. During the 1857 and 1906 earthquakes, the Pacific and North American plates shifted by as much as six meters (20 feet) in opposite directions. These shifts accumulate over time and are recorded as separate breaks in layers of sediment. By dating organic material laid down in the sediment, geologists can determine when earthquakes have occurred, how frequently, and the amount of plate movement that resulted from each event. From such evidence, geologists have determined that large earthquakes have occurred on the San Andreas Fault approximately every 132 years for the past 2,000 years or so. These findings suggest that the probability of a large earthquake striking this area in the next 50 years is high.

To learn more about the effects of earthquakes, check out Earthquake Prediction, Earthquakes: San Francisco, and Earthquakes: Los Angeles.

To learn more about interactions between tectonic plates, check out Mountain Maker, Earth Shaker and Tectonic Plates, Earthquakes, and Volcanoes.

Print Questions for Discussion

• Why has the geologist searched for evidence of past earthquakes along this section of the San Andreas Fault?
• Think of a way (or a model) to demonstrate the build-up of strain on a fault, and how an earthquake will suddenly release that strain.
• Discuss the types of prehistoric evidence that geologists have used to understand earthquakes.
• Name and discuss all the different methods you saw that the geologists used in this video.
• Do you think geologists will ever find a way to accurately predict when a big earthquake will occur?