Chapter 9: Earthquakes 9.1: Earthquakes occur along faults 9.2: Earthquakes release energy 9.3: Earthquake damage can be reduced

9.1: Earthquakes occur along faults Review, true or false? Earth’s lithosphere consists of the uppermost mantle and core? Earth’s lithosphere consists of the crust and uppermost mantle Earth’s lithosphere is hot and liquid? Earth’s lithosphere is cold and rigid Tectonic plates move over the asthenosphere? True

9.1: Earthquakes occur along faults Rocks move along faults Fault: fracture, or break, in Earth’s lithosphere, along which blocks of rock move past each other Rocks may slide smoothly or stick or lock together Rocks may bend as stress is applied Stress: force exerted when an object presses on, pulls on, or pushes against another object Sudden release of stress in the lithosphere causes…and earthquake! Earthquake: shaking of the ground caused by the sudden movement of large blocks of rock along a fault Video-abc video – haiti science Video-haiti science 2

Rocks move along faults Most faults are located along tectonic plate boundaries What does this mean for earthquake locations? The blocks of rock moving during an earthquake are much smaller than the plate Block of rock may move only a meter Earthquake strength depends on: How much stress builds up before the rocks move Distance the rocks move along the fault

Rocks move along faults 80% of all earthquakes occur in a belt around the edges of the Pacific Ocean! US: San Andreas Fault: boundary between North American and Pacific Plates

Rocks move along faults Stress along a plate’s boundary can cause rocks to break and move along weak areas toward the middle of the plate All earthquakes occur…in the lithosphere Why may a few occur below the normal depth of the lithosphere? Subduction: plates are still cool enough to break

Faults are classified by how rocks move Rock movement along faults depend on the kinds of stress they are under Three main faults: Normal Reverse Strikeslip Type of fault depends on whether plates are pulling apart, pushing together, or scraping past one another Movement along normal and reverse faults is mainly vertical – the blocks move up and down Movement along strikeslip faults is horizontal – the blocks move sideways Investigate p47 two triangular blocks of wood, masking tape, marker

Normal Faults Stress that pulls rocks apart cause normal faults Common where tectonic plates are moving apart Ex: Great Rift Valley of Africa

Reverse Fault Stress that presses rocks together causes reverse faults Can occur near collision-zone boundaries between plates Ex: Himalaya Mts, where Indian Plate is pushing into the Eurasian Plate

Strike-Slip Faults Stresses that push blocks of rock horizontally cause earthquakes along strike-slip faults Can occur where plates scrape past each other Ex: San Andreas Fault

faults