Earthquakes


Earthquakes:


  1. Earthquakes occur with the passage of large seismic waves.

  2. Earthquakes are the effects of a large seismic wave.

  3. Seismic waves result from slip and rupture (breakage) of rocks along faults.


Faults: Three types


  1. Normal fault: Hanging wall moves down relative to the foot wall. Normal faults develop where the crust is under tension (is being pulled apart).

  2. Reverse fault: Hanging wall moves up relative to the foot wall. A thrust fault is a reverse fault where movement occurs at a low angle. Reverse and thrust faults develop where the crust is undergoing compression.

  3. Strike-slip (Transform faults): Block of crust on either side of the fault slip horizontally past one another. Right lateral: Right side of fault move towards you. Left lateral: Left side of fault moves towards you.





Fault Animation



Earth quakes occur when a fault ruptures and stored energy is released as seismic waves. The point on the fault that first ruptures is the quakes focus. The point on the Earth's surface directly above the focus is the quakes epicenter.



Most earthquakes occur in the upper 10 Km of the crust. It is here that rocks are cold enough to break in a brittle fashion and behave elasticly.


Elastic rebound theory:


  1. Rocks on either side of a fault bend as stress builds up. This stores energy in the rock.

  2. At some point, the rock ruptures (breaks) and the rocks on the two sides of the fault snap back to their original shale (elastic behavior).

  3. Energy is released as seismic waves.





Types of seismic waves:


Body waves; Seismic waves that pass through the Earth.


  1. Primary waves: Compressional waves. Velocity5-8 km/sec

  2. Secondary waves: Shear waves, Velocity 3.5-4.5 km/sec.



Surface waves: Seismic waves traveling parallel to the Earth's surface.


Love waves: Shear waves parallel to Earth's surface.

Rayleigh waves: Rolling waves perpendicular to the Earth's surface.


Body (P and S) waves

Body (P and S) wave speeds

Surface waves

 

Seismic waves are detected and recorded with seismographs. The location of any earthquake cane be determined if three or more seismic stations record the quake.


Seismograph animation



Locating Earthquakes


Primary (P) waves travel through the Earth faster than secondary (S) waves. Because of this, S waves lag behind P waves, and fall further and further behind as the two waves pass through the Earth. Thus the further the waves travel, the greater the time difference between the arrival times of P and S waves at a certain point. Thus the distance to an earthquake can be measured from any seismograph on earth by measuring the time difference between when P and S waves arrive. If the seismic signature of an earthquake is recorded on any three seismic station on Earth, the epicenter of the earthquake can be located.






Locating Earthquakes Animation


Measuring the size of an earthquakes:


Modified Mercalli scale: A scale ranging from I to XII by which people judge the size of an earthquake based on the observed damage, and effects felt or seen during the quake. This is a subjective scale based on human judgment.




Richter scale: A magnitude scale based on measurable characteristics on a seismogram. The Richter scale is based on the amplitude of surface waves, as corrected for distance from the quakes focus or epicenter. The Richter scale is a magnitude scale ranging from 0 to 10. Every increment represents a 10X increase in the amplitude measured on a seismogram, and a corresponding 10X increase in the amount of ground shaking. This is not a direct 1 to 1 relationship with energy. Exact incremental increase in the Richter magnitude corresponds to an approximately 30X increase in energy (see sidebar 3-1 on page 48 of your text).





Moment Magnitude Scale (Mw): A calculated measure used for very large earthquakes; Based on the slip distance (displacement), rupture area, and strength of the rock.


The maximum possible size of an earthquake appears to be Mw 10 based on a fault with a slip distance of 40,000 km, the circumference of the Earth. Since a fault of this size is very unlikely, maximum magnitudes of 9.5 or 9.5 appear to be a practical limit. Earthquakes of this size have occurred (the latest triggered the tsunami in Indonesia last year) but are very rare.


Earthquake risk:


Estimated by recurrence interval (probability of a quake of a certain size occurring in a given year) and ground acceleration (a measure of how quickly the ground moves during a quake). These two considerations are used to ID areas at risk for earthquakes of different sizes (see figure 4-13).




Earthquakes primarily cause damage through:


  1. Shaking

  2. Liquefaction


Shaking: Natural and man made factors can affect the amount of damage caused by ground shaking;


Natural Factors: The effects of an earthquake can be amplified depending on the material upon which structures are built. This is known as ground amplification. Different materials will also transmit and enhance seismic waves of different frequencies.


Material

Degree of Amplification

Seismic wave frequency

Rock

Little amplification

Higher frequencies

Sand and Gravel

Moderate amplification

Intermediate frequencies

Mud

Strong amplification

Low frequencies



Low frequency seismic waves will cause most damage to large, tall structures

High frequency seismic waves will cause most damage to smaller structures



Liquefaction: A process where water saturated sand or silt loose their strength and ability to support structures when affected by shaking. A normally stable sediment or soil will behave like a liquid and structures will sink.



Earthquake predictions and forecasting:


Prediction: a statement of when and where an earthquake will occur.


Forecast: statement of the probability that a quake will occur in a given period of time.


Short term prediction is not possible yet, but is a very active area of research. IT CAN SAVE LIVES.


Earthquakes are often accompanied by precursor events that may be able to provide short term warnings.


  1. Fore shocks

  2. Release of radon gas.

  3. Ground deformation

  4. Drop in water table.


Longer term forecasts may be accomplished through:


  1. Identification of seismic gaps: Sections of a fault that have not experienced an earthquake in recent times where stress may be building up. However, some sections of faults continuously experience small movements (fault creep) and stress does not build up.

  2. Earthquake migration: Rupture does not occur on the entire length of a fault. When a section of a fault rupture and slips, the zone of stressed rock moves to the end of the rupture. This is often the location of the next earthquake. This process is most important on transform faults.

  3. Paleoseismology: The study of earthquake effects preserved in the geologic record can give us an idea of the recurrence interval of large quakes.