Geology Cafe

Faults, Earthquakes, and Landscapes

What is a fault? A fault is a fracture in the bedrock in which observable movement has taken place, displacing one side again the other. Faults are observable on many scales. They can small features observable on an outcrop along the road, to large-scale features that extend for hundreds of miles across the landscape. The can be a single fracture, or a complex series of fractures, a "fault zone"that can be many meters to miles wide, depending on observable features.
Types of Faults
What is an earthquake? An earthquake is a shock or shaking created when rocks break and/or when a fault moves. Earthquakes range from small (barely noticable) to great, having the capability of causing tremendous destruction, even changing parts of the landscape. Not all faults are capable of producing earthquakes because they may have formed long ago and no longer active or capable of producing earthquakes. Faults that have a history of earthquakes (observable in the range of modern times to about a million years) are considered "earthquake faults."The San Andreas Fault and Hayward Fault are perhaps two of California's most famous earthquake faults.

San Andreas Fault scarp next to Mission San Juan Bautista
Mission San Juan Bautista was built next to the fault scarp of the San Andreas Fault. The dirt path is part of California's historic road, the El Camino Real. It is preserved in its original appearance within San Juan Bautista State Historical Park. In this view, the road basically follows the trace of the San Andreas faultline.
Selected Resources

Earthquake and Disaster Preparedness
How do faults affect the landscape? Large faults leave apparent traces on the landscape. Great faults, like the San Andreas Fault, have formed and evolved over millions of years, sometimes changing its active path over time as new faults traces form and evolve. The San Andreas Fault is part of a greater fault system that includes many geologically recent earthquake faults. The great faults extend deep into the earths crust, downward to the "brittle-ductile transition zone"- the strongest part of the Earth's crust. In general, the deepest earthquakes are the strongest (highest magnitude) because great amounts of energy are released with rocks break at depth. However, shallow earthquakes can be strong, and even more destructive because of their close proximity to surface features. Earthquakes may or may not create surface ruptures... it depends on the character of the bedrock and the depth and intensity of earthquakes. Brittle-Ductile Zone illustration
Lastest earthquakes in California

Earthquake Magnitude-Intensity Comparison (U.S. Geological Survey)

Shake Maps (from recent earthquakes; USGS)

U.S.Geological Survey, Earthquake Hazards Program

The San Andreas Fault (USGS guide)

Historic United State Earthquakes (USGS)

Quaternary Fault and Fold Database of the United States (USGS)

Quaternary Fault Map of California (USGS and California Geological Survey)

Bay Area Earthquake Probabilities (USGS)

Southern California Earthquake Center (SCEC)

Earthquake Fault Field Guides