Earthquakes II

From GeoClasses

Contents 1 What are seismic waves? 2 Wave terminology 2.1 velocity 2.2 amplitude 2.3 period 2.4 frequency 3 Stress and Strain: 4 Effects of earthquakes:

What are seismic waves?

• P wave
  • slinky = compression
  • moves through solid, liquid, or gas

• S wave
  • rope waves = shear waves
  • moves only through solids

cartoon of pathways of P and S waves through interior of Earth

• R wave
  • rolling surface wave
  • causes the most damage

• P waves are the fastest (but are slower through liquid than solid)
• S waves are 1/2 the speed of P waves
• R waves are slowest but have the largest amplitude

• you can calculate the location of the epicenter based on the time between the P and S waves IF you have 3 seismograms from 3 different locations: see how

Wave terminology

velocity

• speed that wave moves through material
  • USGS explanation: On this example it is obvious that seismic waves take more time to arrive at stations that are farther away. The average velocity of the wave is just the slope of the line connecting arrivals, or the change in distance divided by the change in time. Variations in such slopes reveal variations in the seismic velocities of rocks. Note the secondary S-wave arrivals that have larger amplitudes than the first P waves, and connect at a smaller slope.

amplitude

• height of the wave

period

• the time it takes for a single wave (from crest to crest) to pass a fixed point

frequency

• the number of periods completed per unit of time, or cycles/second, measured in Hertz

S wave moving through a grid

Stress and Strain:

• stress (pressure, such as compression) placed on a rock causes elastic strain = bending.
• if the stress is released, rock will return to its original shape
• if stress continues, rock will break (fault) = earthquake
• takes time to build up enough strain again

Effects of earthquakes:

• shaking and faulting
  • watch an earthquake simulation along Hayward Fault near San Francisco
  • photo gallery of faults from 1999 earthquake in Izmit,Turkey, with nice examples of earthquake ground features

buckling of ground along fault within arid landscape (photo by Chris Walls for USGS)

• liquefaction - if dry ground, causes compaction; if wet ground, causes quicksand

damage cause by soil liquefaction after earthquake in Niigata, Japan

Sand boil or sand volcano measuring 2 m (6.6 ft) in length erupted in median of Interstate Highway 80 west of the Bay Bridge toll plaza when ground shaking transformed loose water-saturated deposit of subsurface sand into a sand-water slurry (liquefaction). Vented sand contains marine-shell fragments. (J.C. Tinsley, U.S. Geological Survey; Loma Prieta earthquake)

• landslides

Landslide debris blocks both eastbound lanes of Highway 17 near Summit Road. Foreground material is damaged lane separators. (photo by C.E. Meyer, U.S. Geological Survey)

debris flow (landslide) caused by 2001 El Slavador earthquake

• fires, disease

aftermath of 1906 San Francisco earthquake and fires

• tsunamis - called seismic sea waves if the epicenter is under the ocean

how an underwater earthquake causes tsunamis

people running from tsunami in Hawaii (breaking waves in the background)

2004 Indonesian earthquake and resulting tsunami