Earthquakes DYNAMIC EARTH

What is an earthquake? Used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip Caused by volcanic or magmatic activity, Caused by other sudden stress changes in the earth.

What causes earthquakes? Tectonic plates move past each other causing stress. Stress causes the rock to deform Plastic deformation – does not cause earthquakes Elastic deformation – rock stretches then reaches a breaking point, releasing energy.

Elastic Rebound – deformed rock goes back to its original shape http://www.uwgb.edu/dutchs/EarthSC-102VisualsIndex.HTM

Focus – point inside the Earth where an earthquake begins Focus – point inside the Earth where an earthquake begins. Earthquakes that cause the most damage are those with shallow foci. This is because most of the energy has not dissipated by the time the vibrations reach the surface. Epicenter – point on Earth’s surface above focus; is located by subtracting P-waves from S-waves.

How seismographs work There are three sensing devices on a seismograph One device records the vertical motion of the ground. The other two devices record horizontal motion-one for east-west, another for north-south.

How seismographs work Seismographs record motion by tracing wave-shaped lines on paper or by translating the motion into electronic signals. A tracing of earthquake motion that is recorded by a seismograph is called a seismogram.

Typical Seismogram http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Seismic Waves The rocks release energy in the form of vibrations. These waves travel outward in all directions from the focus through the surrounding rock. Two Types: Body: these are waves that travel through the body of a medium. Surface: these waves travel along the surface of a body rather than through the middle.

Primary Waves (P Waves or Compression waves) A type of seismic wave that compresses and expands the ground; the more rigid the material, the faster the wave moves The first wave to arrive at an earthquake http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Secondary Waves (S Waves) A type of seismic wave that moves the ground up and down or side to side Second Wave to arrive http://daphne.meccahosting.com/~a0000e89/insideearth2.htm

Comparing Seismic Waves

Surface Waves Move along the Earth’s surface Produces motion in the upper crust Motion can be up and down Motion can be around Motion can be back and forth Travel more slowly than S and P waves More destructive

How do scientists calculate how far a location is from the epicenter of an earthquake? Scientists calculate the difference between arrival times of the P waves and S waves The further away an earthquake is, the greater the time between the arrival of the P waves and the S waves

Magnitude: is determined by measuring the amount of ground motion caused by an earthquake. Intensity: depends on the earthquake’s magnitude, the distance between the epicenter and the affected area, the local geology, the earthquake’s duration, and human infrastructure.

How are Earthquakes Measured How are Earthquakes Measured? Richter Scale: measures the ground motion from an earthquake to find the earthquake’s strength.

How are Earthquakes Measured? Mercalli Intensity Scale Click Link for Interactive Demo http://elearning.niu.edu/simulations/images/S_portfolio/Mercalli/Mercalli_Scale.swf

Mercalli Intensity Scale This scale expresses intensity in Roman numerals from I to XII and provides a description of the effects of each earthquake’s intensity.

Formation of a tsunami http://isu.indstate.edu/jspeer/Earth&Sky/EarthCh11.ppt

Stress The amount of force that is exerted on each unit of area. Types: Compression: Squeezes and shortens a body. It pushes rocks higher up or deeper down into the crust. Mostly occurs at or near convergent boundaries. Tension: stretches and pulls a body apart. Mostly occurs at or near divergent boundaries. Shear: distorts a body by pushing parts of the body in opposite directions. Mostly occurs at transform boundaries.

Three Types of Faults Normal: A fault in which the hanging wall moves downward relative to the footwall. Generally seen in divergent boundaries. Three types of faults Form depending on type of plate motion and complex reaction of earth’s lithospheric blocks Strike-slip Normal Thrust

Thrust (Reverse): The hanging wall moves upward relative to the footwall. This occurs at convergent boundaries. Three types of faults Form depending on type of plate motion and complex reaction of earth’s lithospheric blocks Strike-slip Normal Thrust

Strike-Slip: The rock slides (slips) parallel to the direction of the length (strike) of the fault. These faults occur at transform boundaries. Three types of faults Form depending on type of plate motion and complex reaction of earth’s lithospheric blocks Strike-slip Normal Thrust