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