Earthquakes

From GeoClasses

Contents 1 Earthquakes 1.1 What is an earthquake? 1.2 How do we compare earthquakes? 1.3 Where do earthquakes occur? 1.4 You need to have faults: 1.5 What are seismic waves? 1.6 Wave terminology 1.6.1 velocity 1.6.2 amplitude 1.6.3 period 1.6.4 frequency 1.7 Why the frequency matters: 1.8 Shaking can be amplified by the ground material: 1.9 Stress and Strain: 1.10 Man-made causes of earthquakes: 1.11 Effects of earthquakes: 1.12 Predicting earthquakes:

Earthquakes

What is an earthquake?

• Overview of earthquakes
• epicenter (on the surface above the focus or rupture)
• shallow focus earthquakes do more damage than deeper ones

How do we compare earthquakes?

• Richter scale/magnitude
  • energy released based on the amplitude of the seismic waves
  • table 5.2 - magnitude and frequency

Where do earthquakes occur?

• Ring of Fire

• At plate boundaries (interplate) - strike slip boundary
  • ex: San Andreas fault, California
  • NASA's Explanation:

    The Earth's surface is broken. Cracks in the Earth's crust known as faults can run for hundreds of kilometers. These faults are frequently the sites of major earthquakes as the tectonic plates that cover the surface of the Earth shift. Pictured above is San Andreas Fault in California, one of the longest and most active faults. Visible as the linear feature to the right of the mountains, San Andreas Fault reaches 15 kilometers deep and is about 20 million years old. The above exaggerated-height image was created by combining radar deployed by the Space Shuttle Endeavour in February with a true-color Landsat picture. Along San Andreas Fault, the titanic Pacific Plate is shifting relative to the huge North American Plate by an average of a few centimeters per year. At that rate, in a few million years, the Earth's surface will look quite different than it does today.

San Andreas marked on an enhanced satellite photo of W North America

enhanced image of San Andreas (linear feature to right of mountain range): see text above

• Within a plate (intraplate)
  • ex: New Madrid

map of earthquakes in the New Madrid fault zone

evidence of New Madrid earthquakes of 1811-1812, small fissure filled with sand

You need to have faults:

• 

  

  major fault types
  • 1. strike-slip
  • 2. compressional = reverse fault (top layer moves up)

a real live thrust fault in China!

cartoon of idealized normal fault and reverse (thrust) fault

• • 3. extensional = normal fault (top layer moves down)

extensional formation of horst and graben features (normal faults)

satellite photo of graben in Afar Depression

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

Why the frequency matters:

• high frequency waves will vibrate and damage low buildings
• low frequency waves damage tall buildings

damage to San Francisco buildings after Loma Prieta earthquake in 1989

demo showing how tall and short buildings respond to earthquake shaking

• the further from the epicenter, the more the higher frequency waves are attenuated (weakened)
• BUT low frequency waves DO NOT attenuate, so tall buildings can be damaged even if far from the epicenter

Shaking can be amplified by the ground material:

• amplification is very low in hard rocks (igneous) but is gradually greater in sedimentary rock - alluvium - silt - mud (fig. 5.16)
• faults can also amplify shaking (once a rock has moved along a fault, it is easier to move there a second time

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

Man-made causes of earthquakes:

• loading the crust (dam or reservoir - water is heavy!)
• pumping water or waste into the ground (through wells)
• underground explosions

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

Predicting earthquakes:

An Overview

• 1. foreshocks - up to several days before the major earthquake
• 2. deformation of ground surface - many years prior, but only few cm in height
• 3. radon gas emissions - months prior, Ra is natural gas emitted from igneous rocks, dissolves in water and travels with the water
• 4. seismic gaps - places where strain is building but hasn't been an earthquake lately (risky location)