Unit 2 EARTHQUAKES AND EARTH’S INTERIOR. BIG IDEA: A model of the structure and dynamics of Earth’s interior, where the transfer of heat from the interior.

Unit 2

EARTHQUAKES AND EARTH’S INTERIOR

BIG IDEA:• A model of the structure and dynamics

of Earth’s interior, where the transfer of heat from the interior towards the surface causes slow motion of Earth’s tectonic plates, is based largely on recordings of seismic waves.

I CAN….

• List and describe the layers of the Earth.

• Draw and label each layer.

• Name, compare, and contrast the two types of crust.

• Compare and contrast the lithosphere and asthenosphere.

• Explain convection and its relationship to Earth’s layers.

• Explain how scientists think the magnetic field is generated, and where.

EARTH’S LAYERS AND MAGNETIC FIELD

• The Earth is composed of concentric layers. Working from the outside to the inside, there is the crust, mantle (lithosphere and asthenosphere), outer core and inner core.

• The crust is composed of continental crust, oceanic crust, or both. • It is solid.

• Continental crust made of lighter, granitic rock, is thicker and older than oceanic crust, which is younger, thinner, denser (meaning it sinks more easily), and more uniform and consistent in composition.

• Oceanic crust underlies, or encircles the earth, while continental crust rides on top of oceanic crust where the continents are located

• Oceanic crust is created where we have spreading boundaries such as the mid-Atlantic ridge, and destroyed at subduction boundaries.

• Continental crust is created through volcanic eruptions and lava flows and destroyed at subduction boundaries and through erosion. It can be deformed in areas where two plates with continental crust at the edges meet and pile up to create mountains.

• The boundary between the crust and mantle is called the Mohovoricic discontinuity, or Moho.

• The mantle is composed of both hard, cool rock and softer, weaker, rock.

• The lithosphere includes the crust and the top, cool, hard rock of the mantle. The asthenosphere is composed of the softer, pliable, weaker rock of the mantle.

• Convection is occurring in the asthenosphere, which is believed to be the cause of the moving plates on Earth’s surface. The plates are moving in a conveyor belt fashion riding on the convection currents of the mantle as thermal energy escapes from the core.

• The outer core is thought to be more liquid-like and the inner core solid rock.

• Earth’s magnetic field is thought to be generated by the outer core; in fact we know it is generated from the interior of the Earth. Although it is not well understood, the field is believed to be generated by the spinning of liquid iron and nickel creating electrical currents, and creating the magnetic field. This is called the dynamo effect.

I CAN…

• Explain how earthquake waves are evidence for Earth’s layers, including arrival times and shadow zones.

• Describe how earthquake waves are different, including type of wave, speed, and affect on the surface.

• Explain other evidence for Earth’s layers.

EARTHQUAKES WAVES AND EVIDENCE FOR LAYERS

• Scientists have used the information gleaned from seismic waves to determine that the above layers exist.

• Seismology, or the study of earthquakes waves is 2000+ years old. Currently we use seismographs to record earthquake waves, which when an earthquake event occurs produces a seismogram, or the actual paper recording of the waves .

• There are three types of earthquakes waves: primary waves (p-waves), secondary waves (s-waves) and surface waves (l-waves).

• Primary waves are body waves or waves that travel through the Earth. P-waves are longitudinal waves the travel like a compressed slinky. These waves are able to travel through solids, liquids and gasses because these substances all spring back once a force is removed.

• P-waves are the first to be felt and recorded at a seismic station, so they travel the fastest (6-11 km/s and 1.7x faster than s-waves).

• Secondary waves are also body waves. S-waves are transverse waves that travel like a rope shaken up and down. Solids are the only materials that will transmit s-waves, so they will not travel all the way through the Earth.

• Secondary waves travel at about 3.5 km/s and arrive second at a recording station. We can use the time difference in the arrival times of P and S waves to determine the distance a recording station is from an epicenter.

• Surface waves are more complex than P or S waves. They move similar to water waves at the surface of the ground. Surface waves are extremely damaging to structures that are man made.

• These waves give us clues to the interior of the Earth.

• Because p-waves can travel through all states of matter, they can be recorded on any seismograph assuming that there is enough energy released for the wave to reach it. These waves will not be stopped by the changes from solid to more liquid substances in Earth’s composition .

• S-waves remember cannot travel through liquids, so some seismographs will not be able to record s-waves because of their position on the Earth relative to where the earthquake occurred.

• This is some evidence of the Earth having layers. Other clues come from seismic wave arrival times.

• When earthquake waves arrive at a seismograph, there is a time difference. P-waves travel faster than s-waves, and arrive first.

• The density and depth of the rocks that the waves travel through does affect travel times of all the types of seismic waves. Changing the density (liquid to solid, solid to liquid) has a great effect on the energy traveling by seismic wave, slowing it down, speeding it up, or reflecting the energy altogether.

• Scientists can look at this information and make inferences as to what the internal structure of the earth looks like. Questions about Earth’s interior still arise though, as we cannot drill down deep enough to look at the separate layers. We cannot directly look at the interior, so educated guesses must be made.

• Scientists use the data gathered from earthquakes, meteorites, and exposed mantle rocks to make intelligent decisions about Earth’s structure.

• However, it only takes one valid investigation to prove information wrong. We have not had any yet on the topic of Earth’s layers, so it is believed that we have the concentric layers that we learn about.

I CAN…

• Compare and contrast the Mercalli and Richter scales of earthquake measurement.

• Give 4 factors that determine earthquake damage and describe why they are important.

• Explain how tsunamis occur during only some earthquakes.

• Explain the danger of an earthquake to humans.• Explain the elastic rebound theory and give

evidence from the real world showing how it works.

MEASURING EARTHQUAKES

• When an earthquake occurs, there are a few different ways it can be categorized and measured.

• Mercalli developed the first scale based on the damage that occurred at a location. The Mercalli scale measures the intensity of an earthquake. The level of intensity depends on the strength of the earthquake, the distance from the epicenter, type of surface materials, and design of the structures on the surface.

• The Mercalli scale does not give an accurate measure of an earthquakes actual strength for the reasons above, and because if they are under the ocean or at great depth, there is usually no damage at all to assess.

• Mercalli scales use Roman numerals I-XII (1-12).

• Another scale used to measure earthquakes is the Richter scale. Most people are more familiar with this scale.

• The Richter scale measures magnitude instead of intensity. This scale is used worldwide today.

• Richter magnitude is determined by measuring the amplitude of the largest seismic wave recorded. Adjustments are made for the distance away from an epicenter a recording station is.

• Richter scale uses numerals with decimals to categorize earthquakes.

• The Richter scale uses a logarithmic scale to show magnitude.

• So, an increase of one (4.3 to 5.3) is an increase 10x in wave amplitude. And a 31.7x increase in the amount of energy released.

• So the difference between a 4.3 and a 5.3 on the Richter scale energy-wise could be shown like this:

• 4.3 = 31.74 5.3 = 31.75

EARTHQUAKE DESTRUCTION• The destruction from earthquakes depends

on several factors:• Intensity – how strong is the earthquake

• Duration of vibration – how long is the shaking occurring

• Nature of the ground (sand, rock, etc.) liquification can be a problem

• Design of structure on earth – earthquake code construction or not

• Unreinforced masonry buildings are the most vulnerable to destruction. Why?

• Most people are injured in an earthquake by falling debris. The Earth does not open up and swallow people. However, earthquakes can hurt humans in other ways too. Tsunamis, fires, landslides and ground subsistence can also cause problems.

• Tsunamis, (NOT tidal waves) or seismic sea waves, are caused by vertical motions of the ocean floor or by landslides displacing a large amount of water.

• Out at sea tsunamis are not dangerous as there is a large amount of water to absorb the energy. However at shorelines there is not a lot of water to spread the energy out, and the water piles up onto the shore causing destruction.

• One sign a tsunami is approaching is the withdrawal of water from the shoreline.

• Fires are started during earthquakes when gas and electrical lines are severed. If the water lines are broken this can compound the problem.

• Landslides and ground subsistence occur when the ground is shaken and cannot tolerate the vibration. It can slide or settle into new locations causing damage to what was sitting on top if it prior to the earthquake.

THE ELASTIC REBOUND THEORY

• Elastic rebound theory explains how pressure builds up in a fault until the pressure must be released, and then returns close to original position without the strain.

• This is similar to how a rubber band behaves when it is stretched out and then released. It does not stay stretched out; however it may not be as tightly bound as it once was.

• This theory seeks to explain how earthquake pressure builds up, and then gives under the pressure, and returns the ground somewhat back to normal, passing the released energy into the surrounding rock.

• Fault creep is slow, gradual displacement and can occur rather smoothly either creating small earthquakes or unnoticeable movements.

• Fault scarp is vertical movement along a fault.

• Elastic rebound can be shown with fences that are offset after earthquake activity, or orchards or cropland that is no longer in neat rows.

• We can demonstrate elastic rebound with a wooden pencil.