F09.110.Midterm 1 review

Here are some suggestions for studying for the midterm exam.

The following list should be used as your study guide for the midterm. I will use this list to construct the midterm questions. These materials should have been discussed in my lecture and should also have the key points listed in my lecture notes.

The context of this content probably will make the most sense wheh view with the viewgraphs for each lecture.

Layering of Earth- know that Earth is density-layered.

How does that heat escape? (recall mantle convection, which drives plate tectonics)

Faults- know the following: hanging wall, foot wall, strike, dip.  Know the meaning of dip-slip faults (normal and reverse) and strike-slip faults (note that if you face the fault and the sense of motion on the other side of a strike-slip fault is to your right, then we call that a right-lateral strike-slip fault). 

Consider the relative influence of gravity in each of these three senses of displacement (is it the principal force causing the displacement, an intermediate force, or the weakest force?)

Plate tectonics- familiarize yourself with the concept of plate tectonics (Earth’s lithosphere moves as distinct rigid plates, with earthquakes occurring mostly at the plate boundaries). Recall the movie that shows the movement of tectonic plates through the last several hundred million years. Recall the break up of the super continent Pangaea (all the land). Recall that the Atlantic Ocean formed following the break up of Pangaea and it has grown while the Pacific Ocean lithosphere has been recycled (subducted). Note the difference between the Pacific Ocean spreading ridge (not centered) and that of the Atlantic Ocean (centered).  Why is there a difference?

Know the three types of plate boundaries (convergent, divergent, and transform) and the types of faults associated with each. Consider the consequences of each tectonic plate boundary in terms of other hazards (e.g. convergent boundaries with subduction produce volcanoes; divergent plate boundaries produce less significant earthquakes than do convergent or transform boundaries, because divergence (extension) is accompanied by thinning of the lithosphere through normal faculty (where the greatest stress acting there is gravity.

Know how the San Andreas Fault formed (recall the short cartoon that showed the collision of North America with the Pacific spreading ridge), and what kind of plate boundary it is.

What type of boundary existed along the west coast of California prior to the formation of the San Andreas fault?

Recall that the geology of western California (west of the Central Valley) is primarily the remains of the accretionary wedge from the former subduction zone (i.e. it's scraps of rock scraped off the subducted Farallon Plate). This material is now known as the Franciscan Complex.

Seismic waves- know the focus, epicenter, elastic rebound theory, primary, secondary and surface (Love and Rayleigh) waves (their relative speeds, general deformation of materials as they pass through, their relative amplitudes when measured by seismometers, and the concept of refraction).

Know what is recorded on a seismogram (i.e. that the suspended pendulum with pen tip remains stationary while the rest of the seismograph shakes along with the bedrock around it). Know what the Richter, Moment and Mercalli intensity scales measure, and when each is useful to use. Review how Richter magnitude is determined (remember the figure of the seismogram- the recording on the seismograph?) Richter magnitudee requires knowledge of the largest amplitude wave and the difference in arrival time of the P and S waves. If this is not familiar to you, review the lecture viewgraphs).

Earthquake damage - liquefaction, tsunamis, falling structures. Note that people are killed by falling materials, not from seismic shaking.

Recall where you want to be during an earthquake if you are in a large, concrete and metal buildlng (hint- NOT under a sturdy object).

Where in the world do most large earthquakes occur (Answer: at plate boundaries...and note that divergent boundaries have fewer large earthquakes).

How does the hardness of materials affect the transmission of seismic waves (e.g. dense, rigid granite versus loose, unconsolidated sediment).

Q: In order to increase one magnitude level on Moment or Richter magnitude scales, (say a change from a M3 to a M4) how much increase must there be in the energy released (Answer: 32X) and amplitude of shaking (Answer: 10X) ?

Why do seismic waves refract (bend), and why does this cause seismic energy to concentrate at the surface of the planet (where we live) and in porous material (like unconsolidated alluvium or mud)? 

Recall the water wave analogy for refraction. Refraction occurs in the direction of materials that transport waves more slowly.  

Where are restraining bends along the San Andreas Fault? Where are releasing bends? What does each do to the geology of that area (i.e. form mountains or valleys, changes local faulting from strike-slip to reverse or normal faults)? What type of fault will restraining bends form to accommodate this localized change in tectonic stresses? Is the San Andreas best described as a fault or a fault system (the latter meaning many faults accommodating the tectonic stresses between the Pacific and North American plates)?  What does this say about the strength of the rocks in western California (uniform in strength or not?)

The 1906 earthquake. Did the intensity of ground shaking decrease uniformly with distance from the epicenter? What caused the most damage in San Francisco? What sense of fault displacement occurred along the San Andreas Fault?

Was the amount of fault displacement uniform or variable along the 430 km of fault rupture? Depending on your answer, what does this indicate about the western California's rocks' abilities to store elastic stresses? Are the materials uniform or non-uniform?

Why were there so many fires in San Francisco following the earthquake (recall insurance, broken water mains)?

What was seismicity throughout the SF Bay Area like in the 75 years before versus after the 1906 earthquake?

What does this suggest about the Bay Area's future for earthquakes (namely that the last 100 years was much quieter than the 100 years before that, and so future activity may become more intense than we have experienced so far in our lives).

How does earthquake activity on one fault affect activity on other faults (note that thre are several answers- you can have increases, decreases or no change in seismic activity on other faults).

What does this imply about the likelihood of earthquakes clustering in time?

What should you do before, during and after an earthquake?

If you were feeling a major quake, could you tell whether it was a smaller earthquake that had its focus near you versus a larger one with its focus far away?  If so, how?

Which fault will be the most likely to be the location of the next "big" earthquake in Northern California?

Volcanoes- know the relationship of most volcanoes relative to plate tectonic boundaries (i.e. basalt lava volcanism at divergent plate boundaries and rhyolite pyroclastic (ash-producing) volcanism at convergent plate boundaries).

Note that intraplate (within plate) volcanism, such as Hawaii or Yellowstone, is likely to have a mantle plume (hotspot) source of magma.

Know the causes of magma formation (pressure decrease, temperature increase, addition of volatiles) and the tectonic setting where each of these is likely to occur. What controls whether a volcano will produce lava or pyroclastic eruptions? Recall that the density of crust above the magma chamber dictates how much crystallization and crystal settling of heavy minerals must occur before magma can rise to the surface as an eruption (and since the density of continental crust is lower than ocean crust, this means that magma rising to the surface under continents is generally less dense than magma rising under ocean crust.

Know the difference between a shield and a composite volcano. What's a caldera? What's a pyroclastic flow? What’s a lahar (volcanic mudflow- mixture of water and pyroclastic (fragmented ash)- these can travel 10s of km away from a volcano.

What makes lahars less predictable than pyroclastic flows?

Review video : Volcano's Deadly Warning. Recall that a lahar from Nevado del Ruiz killed 25,000 people in Armero, Colombia

Fumaroles- how can they be used to assess a volcano's likelihood to erupt in the near future? (Recall what happened to Stanley Williams and colleagues on Galeras).

Rock fracture seismicity versus volcanic seismicity (A-type and B-type seismicity).

Recall Bernard Chouet's discovery of B-type long-period events, and what they meant was happening in the volcano.

California volcanoes. Know about the potentially active ones- Shasta, Lassen, Long Valley Caldera.

Lahar dangers for Mt. Rainier in Washington State.

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Example Midterm question and essay for full credit

Question: Your house is built on granite, and your neighbor’s house is built on loose sediment. Other than the geology beneath them, the two houses are identical.  An earthquake occurs, and destroys your neighbor’s house, but your house is relatively intact.  Why?

The strength (rigidity) of geologic materials determines how quickly seismic waves will pass through them.  Rigid granite permits seismic waves to pass through quickly, and hence the wave amplitudes will stay small. By comparison, loose sediment causes the waves to slow down, causing the wave amplitudes to increase. Increased wave amplitude means that greater shaking will occur. Also, shaking loose sediment can lead to liquefaction. These physical responses to seismic waves will lead to destruction of buildings.