EQ Mechanisms Bay Area Faults EQ Magnitude

Earthquake Waves Frequency 0.1 Hz to 10 Hz (outside human sensory range) Types of Motion P waves S waves surface waves velocityamplitude

What is an Earthquake? A release of energy stored on a fault What is a fault? A roughly planar surface where rock has broken and separated Why does an earthquake happen? Built-up energy exceeds frictional resistance on the fault

How does “slip” on the fault happen? Elastic rebound Rocks accumulate stress as two sides of fault move past each other Elastic strain is built up in rocks as they deform Stress = force per unit area Strain = change in shape of rocks due to stresses Elastic = returns to original shape when released Remember the fence

from: and Fault Geometry (focus)

Normal Fault Reverse Fault Strike-slip FaultThrust Fault Faults Types from:

How do geologists.... Find faults?

How do geologists find faults?

How do geologists.... Find faults? The Hayward Fault is open for you to walk down into until October 31, 2006 See:

How do geologists....determine whether a fault is active? State of California (A-P act): An active fault is one that has slipped once in the last 11,000 years (or 2 or more times in the last 700,000 years) Consider this schematic roadcut/seacliff: fault #1fault #2fault #3

Spaced-based measurements (VLBI* and GPS) show that PAC-NA motion in CA is ~50 mm/yr. *Very Long Baseline Interferometry

Earthquakes in California & Nevada,

Some of the Bay Area’s active faults

The San Andreas is NOT “the PAC-NA plate boundary.” This diagram applies at the latitude of Bakersfield or San Luis Obispo. Red arrow: predicted motion: 50 mm/yr Blue arrows: subsets of the motion that “add up” to the predicted motion.

? > ? 17? 23? ? 1-3? Farallon Islands ~Stockton About 36 mm/yr happens on the San Andreas in central CA, but northwest of Hollister, things are a LOT messier. The ~36 mm/yr must be divided up on many faults. Geologists study each to determine individual rates. Let’s add up the slip on faults along four paths to see whether we’ve found the ~36 mm/yr.

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