earthquakes! natural disasters unit five

The movement of earthquake waves through the Earth’s interior has given us a better picture of the Earth’s 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 103º -143 º from the focus

S-waves can’t travel through liquids.

When S-waves hit the outer liquid core, they stop. This creates a S-wave shadow at locations greater than 103º 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 (20 - 90 km) or under ocean floors (5 to 10 km).

*Speed also increases with depth in the mantle except for a low velocity zone at 100-250 km. This corresponds to the asthenosphere.

• Focus: where “slippage” first occurs

• Epicenter: directly above the focus on the Earth’s 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

1970’s• 1811-1812: 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 amountof energy released by an earthquake;Richter Magnitude scale (1 to 10);each step in the Richter Scale is an increaseof 10-fold in movement and a 30-foldincrease 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