Earthquakes by Izzy Essi

Earthquakes are caused by the movement of plate tectonics from convection currents in the mantle. When the plates move they are either converging at a convergent boundary or diverging at a divergent boundary. Converging means that the plates collide and diverging means the plates pull apart. These forces that cause the plates to move are examples of stress. The stress and energy is stored into the rock until the rock eventually breaks. Shearing, tension, and compression are three different types of stress that change the shape of a rock. Shearing is when rock slips past each other in different directions, causing the rock to slip and break apart or change its shape. Tension is when rock is slowly tearing or stretching apart making the middle thin. Compression is when the rock is pushed together or squeezed until the rock folds or breaks.

The plates are also moved by the convection currents in the mantle. Inside the Earth, in the asthenosphere are convection currents heated from the Earth’s core and the mantle below the asthenosphere. When convection currents move towards each other, it causes the plates to move towards each other at a convergent plate boundary. If the convection currents move away from each other, it causes the plates to break apart or move even farther away from each other, at a divergent boundary. Convection currents and stress are what cause plate motion, and plate motion causes earthquakes. Earthquakes are more likely to happen near plate boundaries with tectonic motion because when the stress builds up at a plate boundary it causes a fault which is when the crust breaks, causing an earthquake.

Faults are where earthquakes mostly occur. Faults are a break in the crust which causes the Earth’s crust to move. There are four different kinds of faults; strike slip, normal, reverse, and oblique. A strike slip fault is caused by the stress of shearing. Along a strike slip fault line, the rock slides past each other sideways with little up and down motion. Strike slip faults form along a transform boundary. An example of a strike slip fault is the San Andreas Fault in California.

Another type of fault is a normal fault caused by the stress of tension.  In a normal fault the fault is at an angle so half of the rock is above the fault line and the other half is below the fault line. The half that is above the fault line is the hanging wall and the half that is below the fault line is a footwall. At a normal fault the hanging wall slides downward. Since the stress is tension, the plates diverge and break apart. An example of a normal fault is along the Rio Grande rift valley.

Reverse faults are caused by the stress of compression. The reverse fault also has a hanging wall and a footwall but they go in different directions. The footwall and the hanging wall are then pushed together in the stress of compression, causing the hanging wall to go over the footwall along the fault line. An example of a reverse fault is in the majestic peaks at the Glacier National Park in Montana.

The last type of fault is an oblique fault. An oblique fault has two types of stress of shearing and tension, which occur at the same time. The motion of this type of fault is a normal fault and a strike slip fault combined. So the hanging wall slides down the the fault line while the footwall and hanging wall slide past each other.

Strike Slip Fault

http://michaelgivens84.edublogs.org/ch-11-science/

Normal Fault

http://www.williamsclass.com/SixthScienceWork/FoldFault/FoldFaultGeologyNotes.htm

Reverse Fault

http://nees.cornell.edu/education/level1pages/level2pages/reverse.html

Oblique Fault

http://scec.usc.edu/internships/useit/eqbasicinfo

Seismic waves carry the energy from the focus of an earthquake and go through Earth’s interior and across the surface. The focus of an earthquake is where the rock actually breaks along a fault line. The epicenter of an earthquake is when the seismic waves reach the surface of the Earth. There are four different types of seismic waves, P waves, S waves, Rayleigh waves, and Love waves. Every earthquake has P and S waves. P waves can travel through solids and liquids inside the Earth. P waves travel faster than S waves, so they arrive first compressing and expanding the ground in a vertical motion.

After the P waves come to the epicenter, the S waves arrive, traveling in the same path as the P waves. S waves can only travel through solids and not liquids. When the S waves reach the surface they shake it back and forth or in a side to side motion and up and down at the same time.

Along with P and S waves, there are also Rayleigh and Love waves, known as surface waves. These surface waves travel along the Earth’s surface, rather than through the Earth. Rayleigh waves make the ground shake in an elliptical motion. Rayleigh waves travel slower than Love waves. Love waves make a horizontal motion or side to side motion perpendicular to the direction the wave is traveling.

P Waves

http://www.universetoday.com/85000/p-waves/

S Waves

https://www.esgsolutions.com/english/view.asp?x=857

Rayleigh Waves

http://earthquake.usgs.gov/learn/glossary/?term=Rayleigh%20wave

Love Waves

http://allshookup.org/quakes/wavetype.htm

When you track these seismic waves you use a seismogram. A seismogram records and tracks the movements of the waves as they travel through the Earth. It mainly records the arrival times, the strength of the seismic waves, and their motion. The P wave is the primary wave, or, as recorded the first wave that arrives going faster than the S wave. The P wave has more energy recorded vertically than horizontally. S waves are secondary waves, which arrive after P waves as they travel slower. S waves have more energy recorded horizontally than vertically. The surface waves, Rayleigh and Love waves, are the strongest waves out of all of them even though they move slower than P and S waves.

Example of a Seismogram

http://www.bgs.ac.uk/discoveringGeology/hazards/earthquakes/howWeMeasureThem.html

Geologists use a moment magnitude scale to estimate how much energy was released by an earthquake. The moment magnitude scale is used to rate earthquakes everywhere no matter how big and far away they are. Before rating an earthquake on the moment magnitude scale, geologists must study the data from the seismogram. When they study this data they can figure out what seismic waves were produced and how strong they are. They also try to figure out what motion occurred along the fault line and the strength of the rocks when they broke along the fault line. When geologists gather all of this information they can rate the earthquake on the moment magnitude scale. They rate the earthquake on the moment magnitude scale using numbers. If they rate the earthquake a 5.0 or below on the moment magnitude scale then the earthquake was small and had little damage. If the earthquake was above a 5.0 then it caused a lot of damage. A magnitude of 6.0 produces 32 times as much energy than a 5.0 magnitude, and almost 1,000 times as much energy as a 4.0 magnitude earthquake.

Subduction zone earthquakes are normally the most destructive and dangerous earthquakes. They occur at a converging plate boundary, causing a lot of the stress to build up before the fault breaks from the pressure of two plates colliding, which is why it’s so damaging. A subduction zone is when an oceanic plate and a continental plate converge and the denser oceanic plate subducts into the mantle. Subduction zone earthquakes can often create tsunamis if they’re located on a coastline.  The continental plate overlies the oceanic plate and they get stuck together because of high friction and the continental plate drags backwards. The thin layer above the oceanic plate gets scraped off of the plate and onto the ledge of the continental plate creating a wedge of ocean sediments and seamount. The continental plate responds to the stress building upon it with the coast range rising and the inland subsiding, while the oceanic plate is still subducting. Over a long period of time the overriding continental plate will go through elastic rebound pushing the plate outward into the ocean causing a large earthquake and tsunami. The tsunami occurs because the ground beneath the ocean is displaced from the elastic rebound. As the depth of the ocean increases the wave speed and wavelength increase as well. When the wavelength increases the wave amplitude decreases. When the tsunami travels towards the shore, its wavelength shortens causing the amplitude to grow.

Subduction Zone

http://www.webanswers.com/science/earth-sciences-geology/what-type-of-tectonic-plate-boundary-sometimes-has-a-subduction-zone-395c8e

Tsunami Wavelength and Amplitude from Ocean to Shore

http://www.sms-tsunami-warning.com/pages/wave-shoaling-process#.UUfGs6V8vdk

Along subduction zones the earthquake depth can vary. The farther the earthquake is from the subduction zone, the deeper the earthquake. Meaning the closer the earthquake is to the subduction zone, the shallower the earthquake. Earthquakes only occur on the plate that is not subducting into the mantle. If you wanted to know the slope of the subducting plate you would take the depth of the center of an earthquake and measure the distance from the focus of that earthquake to the ocean trench.

Diagram of Earthquake Depth Along Subduction Zones

http://blogs.agu.org/mountainbeltway/2011/03/11/japan-m8-9-quake-tsunami/

Here is a short animation to give you a better understanding of how earthquakes cause tsunamis.

http://www.youtube.com/watch?v=qQ9Mw_rtDng