Earthquakes

Types of Stress• Stress: a force that acts on rock to change its shape or volume.

– Because stress is a force, it adds energy to the rock. The energy is stored in the rock until the rock changes shape or breaks.

– Most changes in the crust occur so slowly that they cannot be observed directly. But if you could speed up time so a billion years passed by in minutes, you could see the crust bend, stretch, break, tilt, fold, and slide.

• Tension: pulls on the crust, stretching rock so that it becomes thinner in the middle.– Occurs when two plates are moving apart (divergent boundary), but can also

happen inside a plate, such as the Great Basin (Basin and Range region) in Nevada and Utah’s western boarder.

• Compression: squeezes rock until it folds or breaks. – Occurs when two plates are moving together (example Himalayas, convergent

boundary) and during folding and faulting inside the plates.• Shearing: stress that pushes a moss of rock in two opposite directions.

– Occurs when two plates are moving pass each other in opposite directions (transform boundary). Also can happen during faulting and folding in a smaller size scale.

Types of Stress

Earthquake features

• Hang and foot wall– The blocks of rock involved in faults– Hanging wall: touch you head on– Foot Wall: walk on

Earthquake features Continues

• Fault Scarp– When the energy build up enough the fault will

cause a break in the Earth’s crust, and a fault scarp can occur.

Type of Earthquakes• Most faults occur along plate boundaries, where the forces of plate

motion push or pull the crust so much that the crust breaks.• Normal fault:

– tension in Earth’s crust pulls rock apart– Happens at an angle– Footwall moves up while the hanging wall moves down– Example: Wasatch Front, Yes where you live!

• Reverse fault– Compression pushes rock together– Footwall moves down while the hanging wall moves up– Example: northern Rocky mountains in the western United states and Canada.

• Strike-Slip fault– Rocks on either side of the fault slip past each other sideways, with little up or

down motion.– Shear stress– Right lateral and left lateral – Example: San Andreas fault in California

Types of Faults

Changing Earth’s Surface• Folding of Earth’s Crust

– Folds are caused by compression, which cause rocks to bend without breaking– Anticline: when the rock folds upward into an arch– Syncline: when the rock folds downward to form a valley– Examples of folds:

• Appalachian Mountains in Pennsylvania are folded mountains made up of parallel ridges(anticlines) and valleys (synclines)

• Himalayas in Asia and Alps in Europe: formed when pieces of the crust folded during the collision of two plates.

• Stretching Earth’s Crust– Fault-block mountain forms when two normal faults cut through a block of

rock.– Happens when tension forces pull two plates apart creating many normal

faults. When two of these normal faults parallel each other, a block of rock is left lying between them.

– Example: Great Basin• Uplifting Earth’s Crust

– Force the raise mountains can also uplift plateau• Plateau: is a large are of flat land elevated high above sea level. Has many different flat

layers and is wider than it is tall.• Example: Colorado Plateau

Types of folds

Question

• What conditions would allow older sedimentary rock to lie in layers above younger sedimentary rock?– A. rocks that formed in desert environments– B. places where fossils were located– C. areas that were underwater for long periods of

time– D. areas of faulting and folding

• Which location shows older rocks above younger rocks?– A. A– B. B– C. C– D. D

A

B

D

C

• Based on the diagram, what force most likely created the changes to the bottom three layers?– A. compression causing folding– B. compression causing stretching– C. tension causing faulting– D. tension causing waving

A

B

D

C

General information about Earthquakes

• Earthquakes: shaking and trembling that results from the movement of rock beneath Earth’s surface.

• Being in the lithosphere within about 100 kilometers of the Earth’s surface

• Focus: is the area beneath Earth’s surface where rock that is under stress breaks, triggering an earthquake. This is the point in which the stored energy is released in the form of waves.

• Epicenter: the point on the surface directly above the focus

Focus and Epicenter

Types of Seismic waves• Types:

– Primary: P waves• First waves to be felt• Faster than the S waves• Compress and expand the ground• Move through solids and liquids

– Secondary: S waves• Second waves to be felt• Slower than the P waves• Vibrate from side to side as well as up and down• Can only move through solids

– Surface wave• A surface wave is a seismic wave that is trapped near the surface of the earth.• Two types

– Rayleigh waves: is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.

– Love wave: is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Type of Seismic Waves

Recording Earthquakes-Energy• Geologist use a seismograph to record energy released

by an earthquake onto a seismogram.• From this data scientist can see when the P and S

waves arrived, which they then use to pin point where the earthquake started in a process know as triangulation.– Triangulation requires three different location in which the

earthquake was felt. – Also requires the difference in time between the P and S

wave arrive. Remember the P wave is the fastest of the two. • Greater the difference in time the further the epicenter is and the

shorter the difference of time the closer the epicenter is.

Seismogram

Triangulation

Measuring Earthquakes• Mercalli Earthquake Intensity Scale

– Developed to measure the level of damage at a give place– 12 steps

• Richter Magnitude Scale– Is a rating of an earthquake’s magnitude based on the size of the

earthquake’s seismic waves.– Waves are measured by a seismograph– Works well for nearby earthquakes, but not as well for large or distant

earthquakes• The Moment Magnitude Scale

– A rating system that estimates the total energy released by an earthquake– Often used today because it can measure far and near earthquakes– Most the time the new will quote the Richter scale, but most the time it is

actually the Monument Magnitude Scale they are reading.– Helps to determine how much movement was along the fault and the

strength of the rocks that broke when the fault slipped.

Earthquake maphttp://earthquake.usgs.gov/earthquakes/recenteqsus/Maps/US2/40.42.-113.-111.php