Fault

A fault is a geologic term describing a fracture at which two bodies of rock have been displaced relative to each other. Bedrock faults are those in which bodies of rock meet; small, local movements may occur on bedrock faults. Much larger movements or displacements occur along Faults where plates of Earth's crust abut each other. Faults may be inches (centimeters) to hundreds of miles (kilometers) in length, and movements or displacements have the same range in length. Major fault systems are typically found where plates meet; for example, the San Andreas Fault in California, is really a fault system including many smaller faults that branch off of the main trace of the San Andreas as well as faults that parallel the main fault. It may be more accurate to call these systems "fault zones" or "fault belts" that contain known and unknown faults. The Northridge earthquake in the Los Angeles, California, area in January 1994, occurred along a thrust fault that had not previously been known but is within the San Andreas zone. A fault zone may be hundreds of feet (meters) wide and each has a unique character; some include countless faults and others have very few.

The San Andreas Fault

The San Andreas Fault may well be the best known fault in the world. It marks a major fracture in the Earth's crust, passing from Southern through Northern California for a length of about 650 mi (1,050 km) and then traversing under part of the northern Pacific Ocean. The San Andreas does mark a plate boundary between the Northern Pacific and North American plates, and, because this transform fault extends to the surface in a heavily populated area, movement along the fault causes major earthquakes. The forces that cause these movements are the same ones responsible for continental drift. The Great San Francisco Earthquake of 1906 occurred along the main San Andreas, and the Loma Prieta earthquake of 1989 was caused by movement on a branch of the San Andreas. The motion of the Northern Pacific plate as it grinds past the North American plate causes strike-slip fault movements. The plate is moving at an average of about 0.4-in (1 cm) per year, but its speed accelerated during the 1900s to between 1.6–2.4 in (4–6 cm) per year as it pushes Los Angeles northward toward San Francisco. Much more rapid jumps occur during earthquakes; in 1906, movements as great as 21 ft (6.4 m) were measured in some locations along the San Andreas Fault.

The San Andreas Fault is infamous for another reason. The major cities of California including Los Angeles, Oakland, San Jose, and San Francisco, home to millions of people, straddle this fault zone. Such development in this and other parts of the world puts many at risk of the devastation of major fault movements. Sudden fault movements fill the headlines for weeks, but, over the course of geologic time, they are relatively rare so the chances to study them and their effects are limited. Similarly, our knowledge and ability to predict fault motions and to evacuate citizens suffers. An estimated 100 million Americans live on or near an active earthquake fault.

The New Madrid Fault is more properly called a seismic zone because it is a large fracture zone within a tectonic plate. It is a failed rift zone; had it developed like the East African Rift Valley, it would have eventually split the North American continent into two parts. The zone crosses the mid-section of the United States, passing through Missouri, Arkansas, Tennessee, and Kentucky in the center of the North American Plate. The zone is about 190 mi (300 km) long and 45 mi (70 km) wide, and it lies very deep below the surface. The zone is covered by alluvial material (soil and rock carried and deposited by water) from the Mississippi, Ohio, and Missouri rivers; because this alluvial material is soft and unstable, movement within the fracture zone transmits easily to the surface and is felt over a broad area.

On December 16, 1811, and January 23 and February 7, 1812, three earthquakes estimated to have measured greater than magnitude 8.0 on the Richter scale had their epicenters near the town of New Madrid, Missouri, then part of the American Frontier. An area of 3,000–5,000 sq mi (7,800–13,000 sq km) was scarred by landslides, fissures, heaved-up land, leveled forests, and lakes, swamps, and rivers that were destroyed, rerouted, or created. These earthquakes were felt as far away as the East Coast, north into Canada, and south to New Orleans.

On January 16, 1995, the city of Kobe, Japan was struck by a magnitude 7.2 earthquake that killed more than 4,000 people and left almost 275,000 homeless. Like the California cities along the San Andreas, Kobe is a port city, so the earthquake also caused tremendous losses to the economy of the region. Also like Oakland and San Francisco, California, Kobe is located next to a deep bay. Osaka Bay is encircled by a host of faults and fault zones with complicated relationships. The Nojima Fault on Awaji Island appears to have been the fault that hosted the Hyogogen-Nambu Earthquake of 1995. The North American Plate, Pacific Plate, Eurasian Plate, and Philippine Sea Plate all impact each other near the islands united as Japan. Thick, relatively young deposits of alluvial soil overly the faults that pass under Osaka Bay; these amplified the earth's movements along the fault in this highly populated area.