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Hot Spots and Earthquakes

Name _______________________

Part 1: The Hawaiian Hot Spot A dramatic aspect of plate tectonics is the estimated 50 to

100 hot spots across the Earth’s surface. These are individual sites of plumes of upwelling

material from the mantle. Hot spots occur beneath both oceanic and continental crust and

appear to be deeply anchored in the mantle, tending to remain fixed relative to migrating plates.

An example of an isolated hot spot is the one that has formed the Hawaiian-Emperor Islands

chain. The Pacific plate has moved across this hot plume for at least 82 million years, building a

string of volcanoes, islands, and seamounts stretching west and north from the hot spot location

beneath the island of Hawai’i (the Big Island). Thus, the age of each island or seamount

increases with distance from the hot spot location.

Hawai’i and the Emperor Seamounts. (Ages are shown in millions of years.)

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The Big Island of Hawai’i took less than 1 million years to build to its present size and is

still growing. Kilauea volcano, for example, has erupted continuously since 1983.

1. What is the approximate distance from the present location of the hot spot to the

Meiji Seamount in kilometers? Use a ruler and the scale at the lower left to

estimate the distance. Measure along the seamount chain, not in a straight line.

2. List the main inhabited Hawaiian Islands in order from youngest to oldest.

3. Where is the newest Hawaiian Island forming (it is still below sea level) and

what is its name?

4. What is the direction of motion of the Pacific Plate in the area of the Hawaiian

Islands today?

5. Approximately how many years ago did the direction of Pacific Plate motion

change (note the “bend” in the Emperor Seamount chain)?

6. What was the direction of motion of the Pacific Plate before the bend occurred?

7. What do you think could cause the Pacific Plate to change, or appear to change,

its direction of motion so abruptly? Speculate wildly!!!

8. The age of Meiji Seamount is about __82 million years__. The distance from

Meiji Seamount to the hot spot is ______________kilometers (from 1 above).

Given these values, what has been the average speed of the Pacific Plate

movement per year (in cm/year, one kilometer = 100,000 centimeters)?

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The following exercises are adapted from a worksheet developed by Floyd McCoy and Toshi Ikagawa at Windward Community College. Part 2: How fast does the Pacific plate move? One way to calculate plate movement is by looking at island chains that form from hot spots. Dating of rocks from the Hawaiian Islands shows that the ages of the volcanoes are progressively older to the northwest.

Table 1. Ages of the volcanoes of the Hawaiian Islands

Island

Age of island (million years)

Distance from Hawai'i (km)

Movement (cm/year)

Hawai'i 0.0

Maui 1.1 140 km

Oahu 2.5 300 km

Kauai 4.0 460 km

Laysan 20.0 1,880 km

Kure 30.0 2,660 km

AVERAGE

9. Calculate the rate of movement of the Pacific plate in Table 1 above. First determine the rate for each island, and then take the average of these values.

10. On the next page, plot two graphs. (1) plot age (horizontal axis) and distance

(vertical axis), and (2) plot age (horizontal axis) and movement per year (vertical axis). Be sure to connect the dots with lines.

11. Are your plot results linear (straight line) or non-linear (curved line) and do they suggest negative or positive relationships?

Plot line Linear or non-linear Positive or negative

Age vs. distance

Age vs. speed of motion

12. Do your movement calculations suggest that the Pacific Plate has been slowing

or increasing in speed in recent geologic time? ___________ Suggest a reason for this change in speed. Speculate wildly!!!

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Part 3: Where was the epicenter of this earthquake? Earthquake waves originate within, or below, the Earth’s crust at a location called the focus. The point on the Earth’s surface that is directly above the focus is called the epicenter. The waves radiating through the Earth away from the focus can be recorded by an instrument known as a seismograph, which produces a record known as a seismogram. Two of these waves, P-waves (longitudinal waves) and S-waves (transverse waves), are used to locate earthquake epicenters. L-waves (surface waves) are not used in this task. Travel-time curves are graphs that show how long it takes for a particular type of seismic wave to travel a certain distance (see next page). The difference between the S-wave arrival time and the P-wave arrival time is a function of the distance of the seismograph from the focus of the earthquake. This time difference can be converted readily into distance, using the travel-time curves.

For example, in the figure above (a sample seismogram recorded at Dallas), zigzags indicate the arrival of P-waves and S-waves, as labeled. The P-waves began to arrive at 08.2, and the S-waves began to arrive at 11.3 (3.1 minutes later than the P-waves). The difference in arrival times between the P-waves and S-waves is 3.1 minutes. To determine the distance, use the big graph of time gap (y) vs. distance from epicenter (x). First find the length of 3.1 minutes on the vertical axis (dotted arrow next to the vertical “Time” axis). Then find where this vertical length occurs between the S and P curves. For this example, this occurs at 1,000 miles (dotted arrow between the S and P curves). This (1,000 miles) is the distance between Dallas and the focus for the earthquake. This distance approximates the distance to the earthquake epicenter. In other words, if you are 1,000 miles away from the focus, S-waves arrive 3.1 minutes later than P-waves.

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To answer the following questions, refer to seismograms of an earthquake recorded at three stations (above).

13. Estimate, to the nearest tenth of a minute, the time of first arrival of the P-waves and S- waves at each station. Then, using the “S minus P” times, estimate the corresponding distance from the epicenter.

Table 2: Distance from earthquake epicenter to recording station

P arrival (min)

S arrival (min)

S minus P (min)

Travel Distance (x-axis, miles)

Sitka, AK

Charlotte, NC

Honolulu, HI

Travel-time curves for P-waves, S-waves, and L-waves

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Length of 3.1 minutes

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14. Find the location of the earthquake epicenter, using the distances obtained. Using a drafting compass (be careful not to poke holes in the table, please), draw a semi-circle about each seismic station with a radius representing the travel distance determined above in Table 2. For each arc, open up your drafting compass to the proper travel distance in Table 2 using the map scale below. 15. The arcs you draw should intersect near a single point - the epicenter. The location of the epicenter is: Latitude: ____________________ Longitude: _____________________

Sitka, AK

Charlotte, NC

Honolulu, HI