ESS 202: Earthquakes

Lecture and Discussion Questions Weeks 5 and 6,  2003

The third quiz will be on Friday, May 9.  It will cover material discussed in class as well as from the reading assigned up to May 7.

Here are some questions to help you review:

What three things are required for an earthquake prediction?

Some earthquakes, like Nisqually, are entirely underground.  Some reach the surface, but only the top few meters (the fault scarp) are visable, while the fault may continue 10,000 meters underground.  Geophysical methods such as gravity, magnetics, reflecting seismic waves and seismic tomography allow scientists to learn more about deep earthquake faults.  Explain how these three methods help us understand the Seattle Fault and/or other faults in the western Washington.

How does GPS work?  Describe the silent (or slow) earthquakes that are observed in western Washington.
What do we mean by saying that the megathrust zone is 'locked'?  What motions of the earth's surface does locking produce in GPS observations.  What happens when it unlocks?  What direction is western Oregon moving relative to Canada.  What happens to western Washington.  

How much has sea level risen since the last ice age (during the last 18,000 years)?  Why is this important to understanding the Seattle fault?  
Why are faults in western Washington so hard to find?  How has Lidar (which produces high-resolution images of surface topography) helped us find faults?  Name one fault that it has helped discover.

Where earthquakes occur and why

What is the epicenter of an earthquake? What is the hypocenter of an earthquake? What 4 parameters define the "location" of an earthquake? Where do most of the earthquakes occur?


Aftershocks and Foreshocks
What is an aftershock? What is a foreshock? Did the Nisqually have more or fewer aftershocks compared with the Northridge earthquake?

Seismic waves

What is a compressional (P) wave? What is a shear (S) wave? What is a ray? What is a wavefront? What is the difference? What is the direction of particle motion for compressional (P) wave relative to the direction of the ray? What is the direction of particle motion for shear (S) wave relative to the direction of the ray? Is a sound wave a compressional wave or a shear wave?

Seattle Fault

Discuss evidence for the Seattle Fault and the earthquake that apparently occurred on it about 1000 years ago. Draw a cross section showing the direction of motion on this fault. Give 2 examples of observations used to determine when the Seattle fault occurred. Give 2 examples of observations used to argue that the uplift at Restoration Point occurred during a short time.

Lecture and Discussion Questions Weeks 3 and 4,  2003

The second quiz will be on Monday, April 28.  It will cover material discussed in class as well as from the reading assigned up to April 25.

Here are some questions to help you review:



Volcanos
What is a 'hot spot' volcano? Name two of them?  How do they differ from "island arc" volcanos?

Subduction Zones
Draw a cross section of a subduction zone. Label the scale of the drawing. Where do the 'megathrust' or 'subduction zone' earthquakes occur, and at what depths? What is the magnitude of the largest earthquake, where and when did it occur? What is the size of its fault surface? How much displacement occurred during this fault? Where are the 'Wadati-Benioff' or 'deep' earthquakes. How deep are the deepest earthquakes? Why do these deep earthquakes occur in subduction zones, but not other places around the world? Where are the 'island arc volcanos'? Why do we get magma coming out behind a subduction zone if the subducting slab is cold?
In general, where are the very deep oceanic trenches around the world? Why are they where they are? How deep is the deepest trench? Where is it?

Spreading centers
Draw a picture (map view) of an oceanic spreading center including transform faults. Indicate the relative motions of the rocks at all the boundaries. Where do the largest earthquakes in these systems occur? Why?
Draw a cross section of a spreading center indicating the bathymetry of the seafloor, oceanic crust, and oceanic mantle.  Draw a picture showing the direction the two plates are moving, and the direction material directly under the ridge is moving.

Plate Boundaries and Earthquake Focal Mechanisms
What are the three types of plate boundary?
Give the names of specific convergent, divergent and transform plate boundarys.
Draw a cross section of a normal (thrust) fault showing direction of motion across the fault and the direction of the forces that cause the earthquake.
Draw a mapview of a strike slip fault.  
Which kind of earthquake faulting do you get at each of the three kinds of plate boundaries.  


Tectonics of the Pacific Northwest

Draw a map and a cross section of the Pacific Northwest showing the location of the megathrust earthquake, crustal earthquakes (including the Seattle Fault), Wadatti-Benioff zone earthquakes, trench, and Cascade volcanoes. How deep are the deepest earthquakes in this subduction zone? Why are they not as deep as in other subuduction zones such as Japan or Sount America? Explain why the Cascade volcanoes are where they are, for example, what is the process that produces volcanoes that are associated with subduction zones.

Evidence for large earthquakes in the Pacific Northwest

Discuss the evidence for a large subduction zone earthquake along the Cascadia subduction zone. Explain why the coastal areas were down dropped. What happened to the trees? The last big earthquake occured on January 26, 1700. How do we know the exact date? What corraborating evidence do we have locally that can date this event to within a year?

Geologic evidence, which in retrospect is obvious, has been sitting there for decades. Why haven't we seen it before?

What are the two properties of the Cascadia subduction zone that lead scientists (in the early 1980's) to think that, even though we clearly have subduction, we may not ever get big subduction zone earthquakes?

Lecture and Discussion Questions Weeks 1 and 2, 2003

The first quiz will be on Monday, April 14.  It will cover material discussed in class as well as from the reading assigned up through April 11.

Here are some questions to help you review.

What is the distance from the equator to the North pole?

Science
What is science? What is the scientific method? How is a hypothesis different from a theory or different from an explanation?  What distinguishes the Elastic Rebound Theory of earthquakes from former myths and legends?  or the theory of Plate Tectonics from Continental Drift.  Might these theories be incorrect?

Earthquakes
What is an earthquake? What is a fault?  If you are not actually standing on a fault how does the energy get from the fault to you? Describe the "Elastic Rebound Theory" that was proposed by Reid to explain the 1906 San Francisco earthquake.  Draw a picture to clarify the main concepts.  

Continental Drift and Plate Tectonics
Discuss geologic evidence from early 1900's that continents drift.
Why was the Continental Drift hypothesis not widely accepted?
How is the theory of Plate Tectonics (developed in the 1960's) different from Continental Drift (developed in the 1920's)?
What is the energy source that is thought to drive Plate tectonics?

Lithosphere and Tectonic Plates:
What are the key defining properties of a lithospheric plate? How is the asthenosphere different from the lithosphere? Why is the lithosphere important to plate tectonics? What is the relation among lithosphere, asthenosphere, oceanic crust and oceanic mantle? Draw a cross section of the earth to illustrate the relative positions (including the depths) of these layers.
What is the thermal structure of the mantle and how does this relate to the lithosphere and asthenosphere? How much does temperature increase across the 100 km thick lithosphere, how much across a 100 km thick boundary layer at the base of the mantle, how much across the rest of the mantle?
What are the average thicknesses of the oceanic crust, continental crust, mantle, and lithosphere?   What is the elevation of much of the continents in regions that are far removed from plate boundaries?  What is the average water depth above old oceanic crust.  Why is the elevation of the continents so different than the elevation (bathymetry) of the ocean floor?
What are the three types of plate boundary?

Rheology
We have discussed 3 types of rheologies:  1) Elastic, 2) Brittle, 3) Ductile (or Viscous).  Describe what is meant by these three rheologies.  Give an example in which each is important to understanding plate tectonics or earthquakes.

GEOLOGIC TIME
What was the date of the last Cascadia subduction zone earthquake?
When, approximately, was the last earthquake on the Seattle fault?
Place these two events in the framework of human history elsewhere in the world.

How fast is strain accumulating on the Cascadia subduction zone (CSZ) as the oceanic plate dives under North America?

Learn that part of the geologic time scale that includes the Tertiary and Quaternary periods and the Pleistocene and Holocene epochs.  How long ago did the Pleistocene start, and how long ago did the Holocene start?

What is the age of the following geologic events in the Northwest?
    a.  Eruption of Mt. Mazama to form Crater Lake.
    b.  Retreat of the glacial ice sheet from Puget Sound region (range of ages)
    c.  Missoula floods across eastern Washington and down the Columbia River.
    d.  Eruption of basalts of Columbia Plateau.
    e.  Extinction of dinosaurs and great asteroid impact.  (beginning of Tertiary Period).
    f.  Age of the Earth
Rates of plate motion about the same as the rate your fingernails grow.

Understand the difference between the time of strong shaking in an earthquake, say 30 seconds for the Seattle fault, or 4 minutes for the CSZ, and the length of time it takes to build up strain between earthquakes, say 600 years for the CSZ, unsure for Seattle fault but clearly thousands of years.

How is carbon-14 formed?  What is its half life as it decays to carbon-12?  What does half life mean?

What is the difference between radiocarbon years and calendar years?  Why do we have to make this conversion?  Note that we commonly give ages as BP, meaning Before Present, but "Present" actually means AD 1950, when nuclear-explosion carbon messed up our dating scales.

FAULTS

What is a normal fault, reverse fault, thrust fault, strike slip fault; difference between left-lateral and right-lateral strike slip faults.  What are hanging wall and footwall of a fault?  What is a blind thrust?  What is an anticline?

On the San Andreas fault, how do we go about determining the slip rate based on offset streams?

What is a sag pond, shutter ridge, fault scarp, colluvial wedge?

If the long-term slip rate on the San Andreas fault where we studied it is 35 mm/yr, and the 1857 earthquake produced strike slip of 10 meters, how long would it take to build up enough strain for the next 1857-type earthquake?

Why do strike-alip faults appear on the map as straight lines whereas normal and reverse faults may have a curving map trace?

In the Crane Lake trench on the Toe Jam Hill fault, how does a geologist go about limiting the age of the most recent surface rupture? 

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