earthquake

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.

Seismology • The study of earthquake and seismic waves

Seismograph (or seismometer) • Seismograph measures seismic waves, by recording the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape.– Seismographs work on the principal of inertia- objects at rest tend to stay at rest unless acted on by an outside force • Seismogram is the record provided by a seismograph

Siesmograph Seismographs

Seismograph types • More than one type of seismograph is needed, because movements are both horizontal and vertical. • Horizontal-motion seismograph– mass suspended from support attached to ground. The support vibrates the hanging mass remains stationary • Vertical-motion seismograph– mass suspended on spring

Types of Seismic Waves • surface waves (L) waves long - travel in the rock just below the ground surface– Two types: and up-and-down side-to-side motions (*these are the most destructive to buildings) • body waves – travel through Earth’s interior– Primary (P) waves– Secondary (S) waves

How do body waves (P and S) give us information on the interior of the earth? • P wave- push-pull waves compress and expand the rock- like toy slinky– Can travel through solid and liquid • S wave- shake the material – like shaking a rope– It changes the shape of the material – shear s it so it cannot travel through liquid in the interior of the earth • Used to identify the liquid outer core.

Earthquake Waves

How does the relative velocity of P and S waves give us information on the location of earthquakes? • The different types of seismic waves travel at different speeds, that results in a lag time that increases with distance from the focus. • Relative speeds: P waves > S waves > surface (L) waves

Typical seismogram, note 5-minute lag between first P wave and first S wave.

Locating the epicenter from the lag time. • Seismic-wave travel-time graph relates lag time to distance from the epicenter– Obtain the distance of the seismograph from the epicenter by matching the lag time between the P and S waves on the seismic-wave travel-time graph • Then use triangulation-– use distance data from 3 different seismograph locations– draw circle with radius of epicenter distance– epicenter where the 3 circles intersect

Use the lag time from three different seismograph location anywhere in the world to obtain their distance from the earthquake source.

Triangulation to find earthquake source

Where Do Most Earthquakes Occur? • Convergent plate boundaries: About 95% of energy released from earthquakes originates in two narrow zones at convergent plate boundaries. Location of the largest most destructive earthquakes.– Circum-Pacific belt and the Alpine-Himalayan belt • Divergent plate boundaries have frequent, shallow weak earthquakes • Transform faults tend to generate large earthquakes

Distribution of 15,000 earthquakes with magnitude greater than 5 between1980 - 1990

Earthquake activity within a continental plate: New Madrid fault zone, 7.7 earthquake of 1811.