PLease please answer all the questions , thank you . Earthquakes in the Pacific

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PLease please answer all the questions , thank you .

Earthquakes in the Pacific Northwest           

Geologic Setting and Earthquakes. Read the Geologic Setting of the Pacific Northwest –at the site below. There is a link to this online document in the weekly folder of the course website.

http://www.iris.edu/hq/files/programs/education_and_outreach/aotm/22/1b.EarthquakesVolcanoesInThePacificNW.pdf

Use the information in this document to answer the following questions.

What is the name of the subduction zone under Seattle and the name of the 2 plates involved? Indicate which plate is being subducted (sinking below the other). (2)

Explain the 2 locations where magma is rising in association with the Juan de Fuca plate. (2)

Describe the 3 general parts of the Cascadia Subduction zone where earthquakes occur.

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How big (magnitude) can “subduction-zone” earthquakes be, and how often do they occur along the Cascadia subduction zone? (2)

What is the Date and Time of the last big Cascadia Subduction zone earthquake?

Draw a cross-sectional (vertical slice into Earth’s interior, or side view) diagram showing the subduction zone and the 3 general locations of earthquakes. Label each of the 3 earthquake locations with A, B or C to correspond to the 3 locations that you described in question #3. In addition to the earthquake locations, label the overriding plate and the subducting plate and the trench. Indicate the relative motion between the 2 plates and indicate whether each plate is oceanic or continental along the boundary. You can use the diagram(s) from the article for reference, but do not copy them. Create your own cross-section; you do not need to include a map-view – just the profile/side-view. (3)

If you hear that there was a Magnitude 6 earthquake with a focus depth of about 10 km (6 mile), at which of the 3 general locations do you think it was generated? Explain your reasoning.

How is it possible to know the timing of the quake if it occurred long before the introduction of seismographs or any written record? Summarize 2 lines of evidence given in this report. Be sure to describe/explain how this evidence records the earthquake timing and magnitude. (2)

Explanation / Answer

1. Name of the subduction zone under Seattle is 'Cascadia Subduction Zone'. The names of the 2 plates involved are Juan de Fuca and North American plate.  Juan de Fuca oceanic plate is being subducted.

2.  The 2 locations where magma is rising in association with the Juan de Fuca plate are Juan de Fuca Ridge and the Cascade Range.

3. Earthquakes in the Pacific Northwest are generally thought to occur in three different parts of the Cascadia subduction zone 1. Within the deforming part of the North American plate; 2. Along the subduction zone fault between the two plates, sometimes called the interplate thrust or megathrust fault; and 3. Within the down-going Juan de Fuca Plate beneath the North American Plate.

Subduction-zone earthquakes, which can be as large as magnitude 9.0 (M9.0), recur every few hundred years on the shallow part of the Cascadia Subduction Zone fault that lies off the coast of Washington and Oregon. Deep earthquakes of M6 to M7 recur every 30 years or so within the subducting Juan de Fuca Plate beneath western Washington.

4. The last great Cascadia earthquake occurred just over 300 years ago during the evening of January 26, 1700 Between 9:00 PM and 10:00 PM.

6. If a magnitude 6 earthquake with a focus depth of about 10 km (6 mile) was generated then it was within the deforming part of the North American plate because its depth is less than 15 miles and low intensity earthquakes occur here.

7. Ancient trees, once submerged along PNW coastal beaches, put an accurate date on a likely seismic event occurring in the Pacific Northwest.

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