The movement of earthquake waves through the Earths interior has given us a better picture of the Earths interior. P-waves are faster than S-waves. The density and elasticity of rock affects the speed of the waves. When P-waves hit the core, they are refracted and slow down. This creates a P-wave shadow 103 -143 from the focus S-waves cant travel through liquids. When S-waves hit the outer liquid core, they stop. This creates a S-wave shadow at locations greater than 103 from the focus of the earthquake. Speed of Seismic Waves: *Boundary between crust and upper mantle (Moho) *P-waves travel at 6.75 km/sec in the crust and 8 km/sec below this boundary. *The depth varies depending if it is under continents ( km) or under ocean floors (5 to 10 km). *Speed also increases with depth in the mantle except for a low velocity zone at km. This corresponds to the asthenosphere. Focus: where slippage first occurs Epicenter: directly above the focus on the Earths surface The arrival times of P and S waves determines the distance to an earthquake The difference in arrival times of P and S waves determines the distance of the earthquake to a seismograph station. A circle representing the distance is drawn around the station. This is done with three stations, and where the circles cross is the exact location of the epicenter. But they are clustered along the Pacific Coast. Why? Movement along the San Andreas Fault Tectonic Creep is the slow continuous movement along a fault zone that is not accompanied by felt earthquakes. Locked faults are sections that are not moving (locked due to friction). Pressure builds up in these sections until it overcomes the friction and the energy is released in an earthquake (elastic rebound). Offset fence Point Reyes 150 miles long Illinois, Missouri, Arkansas, Kentucky, Tennessee Faults formed when N.A. began to split (failed) Flooded by ancient sea (covered by sediment) Thousands of E-Q since 1970s : 4 of largest NA major E-Qs felt Two lithospheric plates come together Yellow squares indicate earthquake depth Oceanic plate subducts beneath continental (more dense) Older oceanic plate subducts beneath younger oceanic plate Shallow to deep earthquakes No subduction occurs when a continent collides with another continent (same density); folding and buckling occurs along the continental margins Shallow earthquakes Transform boundaries (plates slide past one another) also create relatively shallow earthquakes Plates separate Mid-Atlantic Ridge (MAR) New crust formed Shallow earthquakes Generally less frequent The strength of an earthquake can be measured in two different ways: Intensity: qualitative assessment of the extent of damage done by an earthquake; subjective; Modified Mercalli Intensity scale ( I to XII) Magnitude:quantitative measurement of the amount of energy released by an earthquake; Richter Magnitude scale (1 to 10); each step in the Richter Scale is an increase of 10-fold in movement and a 30-fold increase in energy Fault Scarp - Central California scarp Linear ridge Offset stream Linear valley Earthquake Damage depends on many factors: The size of the Earthquake (magnitude) The distance from the focus of the earthquake The types and properties of the materials at the site The nature of the building Soil and Earthquake Damage: Soil thickness: shallow soils may not perform as well as deep Soil saturation: saturated soils perform less well than dry Soil grain size and sorting: well-sorted, fine grained sands and silts are the most likely to liquefy Types of bedrock: unweathered igneous rocks are better than weak fractured rock Areas where ground may settle or slide Nature of building and Earthquake Damage: Type of construction (size and use of building) Seismic design considerations Architectural simplicity of building Rock, earth or debris flows on slopes due to gravity Major geologic hazard because widespread $1-2 billions/year in damage Fisheries, tourism, timber harvesting, mining and energy production impacted As development , damage Appliances, furniture and gas, chemical and electrical hazards present Water pools can fire hazard Leaking gas lines, damaged or leaking propane containers, and leaking gas tanks San Francisco, 1906 Earthquake struck at 5:13 AM Liquified ground beneath the city exacerbated potential for fires Fires burned for four days Occur when faulting or landslide occur on the ocean floor Seafloor permanently uplifted or down-dropped, creating sloshing of water Displaced water gains height as approaches shore Floating debris and strong currents create damage Indonesia, 12/26/2004 Picture taken by newlyweds vacationing when tsunami stuck; both were killed when tsunami came inland The destruction of coral reefs along shorelines (for shrimp farms) increased destruction from tsunami as waves took longer to break It is difficult to accurately predict earthquakes. Analysis of past earthquake patterns, measurement of movement, and the distribution of faults have allowed scientists to create seismic risk maps. Along the San Andreas fault, geologists are looking for areas along faults that are currently inactive. This probably means that these regions are locked and that energy is building up. Tilting of rocks on either side of fault-lines is also a sign of pressure build-up. Green: Stable bedrock Orange: Unstable bedrock Yellow: Unconsolidated Soil Red: Mud and Fill Earthquake risk in the Bay Area