Earthquake 1

Earthquake • Vibration of Earth produced by the rapid release of energy • Earthquake triggers– slippage along fault (primary cause)– volcanic eruption– nuclear blast • focus- source at depth from which energy radiates as seismic waves in all directions– site of initial rupture • epicenter- surface location above source (focus)

Earthquake waves are like waves radiating in water from the focus of the earthquake

Seismic waves are the elastic energy that radiates in all directions from the focus

Elastic Rebound Causes Earthquakes • 1. Rock initially bent (rather than fractured) by tectonic forces.– Bending stores elastic energy in the rock • 2. Frictional “lock” (resistance) holding the rock together is overcome.– slippage occurs at the weakest point (the focus) • 3. Elastic rebound - strain is released and rock snaps back to flat shape – causes earthquake • 4. Slippage occurs along the fault. – Cycle begins again until next earthquake

Fault creep versus stick-slip faults • fault creep – gradual fault movement releases elastic energy so little is stored– regular slippage = small earthquakes– Faults with this situation are not a danger. • Stick-slip fault– fault locks storing elastic energy for hundreds of years and its release produces large earthquake– Faults with this situation are a great danger.

Surface Expression of Faults • vertical offset: produces a fault scarp – cliff • horizontal offset – with no vertical offset displaces structures along the fault. • no offset : very deep faults will have no surface expression

Earthquake Duration • Earthquake - last seconds to minutes • Foreshocks- days to years before a major earthquake small earthquakes occur– monitor to predict earthquakes - poor results • Aftershocks- for days after main earthquake smaller earthquakes occur

Rate of Movement • Fault movement is listed as movement pre year, even if the fault only moves once every 100 years. • For example, the San Andreas fault of San Francisco moves about 3 cm per year, but that is calculated from actual offset that occurs of 300cm every 100 years.