Unit 4 Earth Science Section 4 3 Seismology

Unit 4: Earth Science Section 4. 3 Seismology

4. 3 Seismology – The Study of Earthquakes • An earthquake is a shaking of the ground as the result of a sudden release of energy in Earth’s crust Focus – the location where there is the energy is released, below the surface of the earth. Epicentre – point on the surface that is directly above the focus.

• Earthquakes occur at various depths, depending on the plates involved. • Earthquakes near the surface tend to cause more damage.

I. Causes of Earthquakes • Rock masses on either side of a fault are pushing past one another. • Rock masses bend and compress until there is enough stored energy to overcome friction between the rock masses; • this energy is then quickly released as kinetic energy during an earthquake – Elastic Rebound Theory.

II. Earthquake Waves • P Waves – primary wave • S Waves – secondary wave • Love Waves – surface wave

Primary waves (P waves) • pressure wave; • particles move in the same direction as the wave. • Almost twice as fast as S waves. • Travel through solids and liquids (crust, mantle and core)

Secondary waves ( S wave) • transverse wave; • particles move perpendicular to the direction of the wave. • Travel slower than P waves. • Travel only through solids (lithosphere)

Surface waves (Love waves): • ground motion is a rolling action, like ripples on a pond. • Travels along surface of the Earth. • Slowest and last waves to arrive after an earthquake • Usually cause the most structural damage

III. Locating an Earthquake The seismometer • Instrument used to measure the ground motion resulting from earthquakes • Produces a zigzag tracing on a paper by a pen • Tracing is called a seismogram

Determining the Distance to the Epicentre • P waves are faster than S waves; therefore, P waves arrive at a seismograph station before the S waves. • The time interval between the arrival of P and S waves at a seismograph is used to determine the distance to epicenter. • Time-distance graph is used as a tool to calculate distance to epicenter

Locating the Epicentre • Need THREE different locations using the time -distance graph to calculate the epicenter. • Draw a circle around each seismograph with a radius equal to the distance to the epicenter. • The location where three circles meet is the epicenter.

• Depth to the focus is found by the lag time of the L wave – the longer the lag time, the deeper the focus.

IV. Measuring an Earthquake – Earthquake Magnitude • The Richter Scale is a standardized scale of earthquake magnitude (amount of shaking) • Each point represents vibrations that are 10 times greater than the point below it. • Each tenfold increase in vibrations means 30 times more energy released.

V. Earthquake Hazards 1. Ground shaking • The result of the waves set in motion by the earthquake • Some vibrations move up and down while others move side to side • Most buildings can withstand large up-anddown vibrations, but not side-to-side (cause collapse)

2. Liquifaction • Vibrations cause groundwater to rise, turning solid ground into a liquid-like material. • Buildings built on solid rock experience little damage; buildings located on bog muds or soft fill suffer severe damage

3. Tsunamis • Large ocean waves produced when earthquakes occur under water. • Caused by a large displacement in the water column

VI. Vancouver’s Next “Big Earthquake” • The Juan de Fuca Plate, Explorer Plate, and Gorda Plate are all subducting under the North American Plate • Each of the three plates have transform fault lines • Will most likely be produced by stress and strain generated at a transform fault or along the subduction zone under the North American Plate.