Earthquakes

Earthquake The shaking that results from the movement of rock beneath Earth’s surface There are about _____ earthquakes per day, worldwide. 8,000

Where do Earthquakes Occur? Earthquakes occur at tectonic plate boundaries, and along fracture zones (fault lines). Red lines = plate boundaries Orange lines = fracture zones

The movement between plates and along faults is not smooth The movement between plates and along faults is not smooth. The plates move in jerks, giving rise to earthquakes. The locations of earthquakes throughout the world mark the major tectonic boundaries. The red dots indicate areas where earthquakes have occurred.

The movement of Earth’s plates creates powerful forces that ___ or ___ the rock in the crust. squeeze pull Stress Stress: A force (push or pull) that acts on rock to change its shape or volume

Compression Stress that squeezes rock until it folds or breaks

Tension Tension: Stress that stretches rock so that it becomes thinner in the middle

Shearing Shearing: Stress that pushes a mass of a rock in opposite, horizontal directions

Deformation Deformation: A change in the volume or shape of Earth’s crust (which causes it to bend, stretch, break, tilt, fold or slide) Most changes in the crust occur so slowly that they cannot be observed directly

Review: How do stress forces affect rock? The three kinds of forces that affect rock are: Shearing The rocks break and slip apart Tension The rock stretches and becomes thin in the middle Compression The rock squeezes until it folds or breaks These stresses work over millions of years to change the shape and volume of rock

Faults A break in the Earth’s crust, no more than 10 miles deep, where slabs of rock slip past each other Faults occur when enough stress builds up in rock to make it move.

Why do faults form and where do they occur? Faults usually occur along plate boundaries or at fracture zones, where the forces of plate motion compress, pull, or shear the crust so much that the crust breaks (deforms) Rocks on both sides of the fault can move up or down, or sideways

Strike-Slip Faults A type of fault where rocks on either side move past each other sideways with little up or down motion. Shearing causes these types of faults

Normal Faults (sometimes called Thrust Faults) A type of fault where the hanging wall slides downward Tension forces cause normal faults

Hanging Wall & Footwall Hanging wall: The block of rock that forms the upper half of a fault Footwall: The block of rock that forms the lower half of a fault

Reverse Faults A type of fault where the hanging wall slides up Compression forces cause reverse faults

Review: What are the three types of fault Review: What are the three types of fault? What force of deformation produces each type? Strike-slip faults Produced by shearing Normal faults Produced by tension Reverse faults Produced by compression

Friction exists because… What is friction? A force that opposes the motion of one surface as it moves across another surface Friction exists because… surfaces are not perfectly smooth.

Describe what occurs when the friction along a fault line is low. The rocks on both sides of the fault slide by each other without much sticking

Describe what occurs when the friction along a fault line is moderate. The sides of the fault jam together From time to time they jerk free Small earthquakes occur

Describe what occurs when the friction along a fault line is high. Both sides of the fault lock together and do not move The stress increases until it is strong enough to overcome the force of friction Larger and/or more frequent earthquakes will occur

The San Andreas fault in California is a transform boundary that contains ___ stress. high

Fault-Block Mountain A mountain that forms where a normal (thrust) fault uplifts a block of rock

How does the process of a fault-block mountain begin? Where two plates move away from each other, tension forces create many normal faults When two of these normal faults form parallel to each other, a block of rock is left lying between them As the hanging wall of each normal fault slips downward, the block in between moves upward When a block of rock lying between two normal faults slides downward, a valley forms

Focus The point beneath Earth’s surface where rock breaks under stress and causes an earthquake

Epicenter The point on Earth’s surface directly above an earthquake’s focus

Seismic Waves

Seismic Wave A vibration that travels through Earth, carrying the energy released during an earthquake

At what point do seismic waves first reach the surface? The epicenter

What determines how much the ground shakes during an earthquake? How close a location is to the epicenter The types of rock and soil surrounding the epicenter determines how much the ground shakes

There are three types of seismic waves: Primary Waves (P waves) Secondary Waves (S waves) Surface Waves An earthquake releases two types of waves: P waves and S waves When the waves reach Earth’s surface at the epicenter, surface waves develop.

Primary Waves (P Waves) A type of seismic wave that compresses and expands the ground The first wave to arrive at epicenter

Secondary Waves (S Waves) A type of seismic wave that moves the ground up and down or side to side Second wave to arrive at epicenter

How are P waves different from S Waves? P waves compress and expand the ground P waves can travel through solids, liquids and gases S waves move the ground up and down or side to side S waves can only travel through solids

Surface Waves A type of seismic wave that forms when P waves and S waves reach Earth’s surface Surface waves are the most destructive of the 3 types of waves, because they move both sideways and up-and-down.

Why do you think surface waves produce more severe ground movements than P waves and S waves? Surface waves travel through loose soil, sand, gravel, mud, and small rocks (not solid rock) These looser substances are more likely to shift and slide

How does the energy of an earthquake travel through Earth? Seismic waves carry the energy of an earthquake from the focus, through Earth’s interior, to the epicenter, and across the surface

Detecting Seismic Waves

Seismograph or Seismometer A device that records ground movements caused by seismic waves as they move through Earth iPhone seismometer-- yes, there’s an app for that! The old-fashioned kind

Describe how a mechanical seismograph records ground movement. A heavy weight attaches to a frame by spring or wire A pen connected to the weight rests its point on a rotating drum During an earthquake the seismic waves cause the drum to shake while the pen stays in place The pen records lines on the paper around the drum

Measuring Earthquakes

Magnitude The measurement of an earthquake’s strength based on seismic waves and movement along faults Intensity the strength of ground movement in a given place.

Mercalli Scale A scale that rates earthquakes according to their intensity and how much damage they cause Developed in the early twentieth century

The Mercalli scale has ______ steps and describes how an earthquake affects ________, ________, and the ________. 12 People Buildings Land surface

How would you rate the damage to the Foligno city hall on the Mercalli scale? The damage would probably rate VII - VIII

Richter Scale A scale that rates the amplitude (height) of seismic waves measured by a seismograph It is a logarithmic scale, which means each level has 10 times the magnitude of the level below it.

What is a strength of the Richter scale when measuring earthquakes What is a strength of the Richter scale when measuring earthquakes? What is a weakness? Strength: Provides accurate measurements for small, nearby earthquakes Weakness: Does not measure larger and more distant earthquakes well

How are the Mercalli scale and the Richter scale similar How are the Mercalli scale and the Richter scale similar? How are they different? Both measure the strength of an earthquake The Mercalli scale measures the strength in terms of the amount of damage caused and the amount of shaking that people experienced The Richter scale measures the size of an earthquake’s seismic waves

Moment Magnitude Scale A scale that rates earthquakes by estimating the total energy released by an earthquake

Why is the moment magnitude scale used today by geologists to measure earthquakes? The moment magnitude scale determines the total energy released by an earthquake This scale uses a electronic seismograph that can measure earthquakes that are big or small, and near or far Geologist examine movement along the fault and the strength of broken rock These two measurements give a more accurate measurement of an earthquake

On which scale would an earthquake’s strength vary from one place to another? Explain. The Mercalli scale, because the amount of shaking that people would feel and the damage to objects would be greater in a place closer to the earthquake’s epicenter

Review: What are three scales for measuring earthquakes Review: What are three scales for measuring earthquakes? Explain what each one measures. Mercalli Scale Measures earthquakes on how much damage they cause Richter Scale Measures seismic waves using a seismograph Measures small and nearby earthquakes Moment Magnitude Scale Looks at the total energy released Measures large and distant earthquakes Helps scientists predict how much fault movement there was

Locating the Epicenter

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

Figure 17: Use the map scale to determine the distances from Savannah and Houston to the epicenter. Which one is closer? Houston 800 Km Savannah 900 km

Seismicity of the Caribbean plate

Haiti earthquake 1-12-2010 7.0