Types of Faults and seismic waves

What is a fault? A fault is a break in the rocks that make up the Earth’s crust, along which rocks on either side have moved past each other. The direction of movement along the fault plane determines the fault type.

GEOLOGICAL CONCEPTS TYPES OF FAULT Footwall/Hanging wall from Mining geology Miners put their lamps on the hanging wall and walk on the foot wall.

Three major Faults Normal Reverse Strike-slip

Normal Fault The hanging wall moves down relative to the foot wall. This fault type is caused by tensional stress.

Normal Fault

Normal Fault

Normal Fault

Reverse Fault The hanging wall moves up relative to the foot wall. This fault is caused by Compressional stress.

Reverse Fault

Reverse Fault

Reverse Fault

Strike-Slip Fault The fault exists between two pieces of crust and the Movement occurs horizontally where the sides slide past each other. This fault type is caused by shear stress.

Strike-Slip Fault

Strike Slip Fault

Strike-Slip Fault

Fault Movement = Earthquakes An earthquake is caused by a sudden slip on a fault. The tectonic plates are always slowly moving, but they get stuck at their edges due to friction. When the stress on the edge overcomes the friction, there is an earthquake that releases energy in waves that travel through the earth's crust and cause the shaking that we feel.

Focus- where the slip happens below ground Epicenter-where the shaking is first felt above ground directly above the focus.

Slippage along fault initiates seismic waves in all directions radiating from the focus. Seismic waves are vibrations that travel through the Earth carrying the energy released during earthquakes

There are two categories of seismic waves: Body Waves: Can travel through Earth’s inner layers (core, mantle, crust)- Primary and Secondary waves Surface Waves: Can only travel through the surface layers (crust) Love and Rayleigh

Primary Waves P waves- are the first waves to arrive at a seismograph station. -fastest form of wave, sometimes called compression waves. Can move through both liquids and solids. These waves cause rock particles to move back and forth in the same direction as the wave is traveling (push-pull).

Secondary Waves S-Waves- arrive after the primary waves at the seismograph station. -Can travel through solids only. -Cause particles to move back and forth at right angles to the line of wave movement

SEISMOLGY EARTHQUAKE LOCATION Travel time versus distance plots Can construct distance versus travel time for earthquakes knowing properties of earth’s interior Note that with increasing distance the arrival times for the various waves separates. The 3 min 45 sec difference between the arrival time of the P and S waves corresponds to a distance of 2000 km.

SEISMOLOGY LOCATING EARTHQUAKES Triangulation -Need three different seismic stations to locate an earthquake. -Measure the P-S arrival times and convert these to distance. -Triangulate using radius given by P-S travel times. -Triangulation for position of an earthquake near New Madrid using stations in Columbus, St Louis and Memphis

Surface Waves Rayleigh wave-surface wave causing the ground to shake in an elliptical motion Because of its motion, it moves the ground up and down, and side-to-side in the same direction that the wave is moving Most of EQ shaking is due to this type of wave

Love wave- produce entirely horizontal motion Moves the ground side-to-side Named after A.E.H. Love a British mathematician

http://web. ics. purdue. edu/~braile/edumod/waves/WaveDemo http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm#Particle_Motion

Measuring Earthquakes Magnitude- the amount of energy released during an EQ Richter Scale- based on the largest seismic waves generated by the EQ on a factor of 10. Moment Magnitude Scale- considers the size of the fault rupture, the amount of movement, and rock’s stiffness

Modified Mercalli Scale Measures the intensity of the EQ using a rating system of the damage Scientists compile information from various observers within different zip codes to get a better understanding of the devastation caused by an EQ to determine its intensity.