Earthquakes

Defining Earthquakes

• Originate with sudden shifting of rock
  • When rocks are under significant pressure, and this pressure continues to build until it exceeds the strength of the rocks, they break and shift, releasing the built-up pressure
  • This released energy travels outward from the point of breakage and travels in a wave-like motion
  • On the ground surface, the wave of motion is felt as the ground shaking, or an earthquake

This process is also known as elastic rebound

• Elastic rebound: when the rocks release pressure, they rebound to their undistorted (though now broken) condition
• The breaking and shifting of rocks results in the creation of faults
  • Fault: a crack in the rocks along which there has been some kind of movement of the rocks
  • A crack is only a fault of the rocks have shifted along the fault
• Faults may shift vertically or horizontally
  • The continuous vertical shifting of rocks along faults can produce significant mountain ranges
  • Eg. the Teton Mountains of Wyoming
• Every movement of rocks along a fault produces an earthquake; bigger shifts tend to produce bigger earthquakes
  • Most earthquakes originate from fault shifts; most fault shifts occur underground
  • The point where the fault moves (the point from which the earthquake originates) is called the focus
  • The point on the ground surface, directly above the focus, is called the epicenter
  • This is marked on most maps as the geographic location of earthquakes
  • The energy originates from the focus and travels outward in all directions with wavelike motions
  • Seismic waves: the waves of energy traveling through the surrounding rocks
• There are multiple kinds of seismic waves
  • Surface wave: a seismic wave that travels on the ground surface
  • Sometimes have visible vertical movements in intense earthquakes
  • Body wave: a seismic wave that travels through the earth
  • Two main types of body waves are p- and s-waves (primary and secondary)
  • P-wave: a body wave that travels by compressing and stretching the rocks in a push/pull motion
  • S-wave: a body wave that travels through a shaking motion (vertical or horizontal)
  • P-waves travel faster than s-waves
• Earthquakes may also result from the movement of magma rising under a volcano
  • Rocks shift, having been deformed by the rising magma and release built-up energy

Measuring Earthquakes

• Seismology: the study of earthquakes
• Seismologist: a geologist who studies earthquakes
• Seismograph: a device used to measure the intensity of earthquakes
  • A simple scientific device based on physics (specifically the principle of inertia)
  • When an earthquake happens, the ground shakes, shaking the support arm and the wire; the suspended weight remains motionless (inertia)
  • If a marking device (eg. a pen) is attached to the weight, the pen also remains motionless
  • If paper is added to move steadily below the pen, the paper records the movement of the ground
  • Seismogram: the paper record of ground motion recorded by a seismograph
• Seismographs produce direct records of the amount of ground motion, recording amounts of such motion and timing of seismic events
• Seismometer: a modern version of the seismograph replacing a pen and paper with an electronic transducer
  • Ground motion is recorded as a digital signal which can be transmitted by radio to a central office
  • Can be set up anywhere in the world

Locating Earthquakes

• Measurement devices record the arrival of surface, p-, and s-waves
  • Speed of waves (fastest to slowest):
  • P-wave
  • S-wave
  • Surface wave
  • 
• Based on known travel times, the time difference between the p- and s-waves can be used to estimate the distance from the seismometer to the earthquake’s epicenter
  • This just measures distance, and does not indicate which direction the quake came from
• To determine epicenter location, distance measurements are needed from at least three seismic stations to triangulate the center
  • Because the current seismic network is so large, it’s possible to develop three-dimensional information about earthquakes’ locations

The Size and Strength of Earthquakes

• Seismic data also provides methods of estimation for earthquake size
  • The two traditional measurements are intensity and magnitude
• Earthquake intensity: the amount of ground motion and force of shaking
  • Intensity measurements can be estimated from seismic measurements, but are more commonly estimated from damage done by earthquakes
  • Traditional scale — Mercalli Scale (1-12)
  • Because of how seismic waves spread from the epicenter, the intensity dissipates farther from the epicenter (modified somewhat by geological characteristics)
  • Intensity measurements provide useful information about the immediate effects of an earthquake but are not helpful in comparing earthquakes from place to place or time to time
• Earthquake magnitude: the amount of energy released when the fault shift releases the built-up pressure that had been stored in the rocks
  • Not possible to directly measure this energy release (fault movements happen far underground)
  • Estimates the energy release by the indirect evidence of the seismic waves
  • Richter Scale: the traditional method of measuring magnitude (1-10)
  • Richter used seismograph records and measured the size of the tracing of ground motion on the seismogram
  • Connecting the two measurements on the graph yields the magnitude estimate on the middle line
  • The Richter scale is logarithmic; each step of 1 is a multiple of 10 on the log scale
    • Eg. an earthquake with a magnitude of 4 is…
    • 10x smaller than a 5
    • 100x smaller than a 6
    • 1,000x smaller than a 7
    • And so on
  • Recently, there’s been some dissatisfaction with the Richter scale → large earthquakes tend to cluster close together and are difficult to differentiate
  • Moment Magnitude Scale: a new magnitude scale just like the Richter scale, but larger earthquakes are more spread out
  • Measurements are based on:
    • The amount of displacement along the fault (how much the fault moves)
    • The size of the fault line
    • The density of the rocks around the fault line (harder rocks emit energy more efficiently than softer rocks)
  • The necessary measurements are more complicated but can be estimated from seismic data with modern technology