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### Phase II Introductory Group Activity: Speed of Tsunami

The main aim of the Phase II Introductory Group Activity is to examine records showing changes in water levels on March 11 2011 at various tidal stations located on Pacific Islands and the U.S. West Coast. The aim is to test an hypothesis posed as a question:

1. How fast do tsunamis travel and is their speed uniform across an ocean?

The Phase II Introductory Group Activity tests the hypothesis using data on water levels obtained for specific locations within two different regions, compiling a report on the data collected and using the collective data to calculate the speed of tsunami, thereby evaluating the question posed in this activity.

Each of these hypotheses is first tested using data obtained for different regions of the Pacific, which involves compiling data collected and evaluating the question-based hypotheses based on these data. Information from these different regions is then combined and compared to assess the general validity of the hypotheses based on different ocean paths for the tsunami. Thus, the assignment is divided into three sections as follows:

Sections I and II: Section I focuses on data compilation and evaluation from Pacific Islands and Section II focuses on data compilation and evaluation from the U.S. West Coast and Alaska.

Section III: Involves answering questions based on overall assessment of the validity of the hypothesis based on the data collected, in this instance determining whether the speed of tsunamis is uniform and oceanic controls on their speed (bonus).

The activities associated with these tasks specified by the questions in the Googledoc therefore involve: (i) data compilation, (ii) data evaluation and discussion, and (iii) review. The Activity Log is intended to record individual student activities in each of these areas, with an expectation that more than one student will be involved in each.

There are three resources provided to accomplish the task.

1. Specific recommendations for tidal station locations to consider, and links to the resources for examining data on water levels (from Module 11) and calculating distances between points on the Earth’s surface.
2. Links to the group Googledocs that each group needs to complete, also provided below.
3. A video guide to the exercise activity.

## Recommendations

How fast do tsunamis travel? Is their speed uniform across an ocean?

The evidence needed to evaluate this questions derives from the information provided with a map that identifies the location (epicenter) and time of the earthquake off Honshu on March 11 2011 coupled with information on the timing of the arrival of the tsunami wave at tidal stations in the Pacific Ocean, such as that for Apra Harbor, Guam, and on the West Coast or Alaska. The distance between the two locations can be determined using an on-line calculator (Great Circle Distance (Links to an external site.)Links to an external site.) and the combination of time and distance enables the speed of the tsunami to be calculated in km/h.

Bonus: What ocean characteristics limit the speed of tsunamis?

The understanding needed to evaluate this question is based on consideration of the fact that tsunami have the characteristics of shallow-water waves.

Resources

## Tsunami-Triggering Earthquake

The earthquake offshore Honshu occurred at 05:46:24 UTC on March 11 2011 at 38.29°N, 142.37°E (see Figure 1 below).

## NOAA Water Levels

The interactive water level for specific tidal stations can be generated at the NOAA Tides website (Water Levels (Links to an external site.)Links to an external site.) using an approach similar to that followed in the Tides assignment exercise, except that the focus is on observed water levels rather than tide predictions. The specific procedure for this question requires the following sequence:

• At the Water Levels website scroll down to select a tidal station listing under the heading Pacific Islands, and subsequently for locations in Hawai’i, on the West Coast (under California, Oregon, or Washington), and for Alaska.
• For example, select Apra Harbor, Guam (1630000) or Monterey, CA (9413450).
• Under Options select the dates for from: as Mar 10 2011 and To: as Mar 12 2011 and select the Plot button to the right to redraw the desired graph.
• In the graph the predicted water level is shown in blue and the verified level in green (see Figure 2).
• Moving your mouse over the line provides a data readout for specific times and enables determination of the time that the first tsunami wave arrived and how long it took to travel across the ocean from Japan. The time is given in UTC, the same as the earthquake so no time conversion is necessary, except to ensure that the time is converted from hours and minutes to hours.
• Above the graph there is a tab entitled Station Info. Selecting this tab enables selection of the Station home page, which when displayed provides the co-ordinates (latitude, longitude) for the station.
• The co-ordinates for the tidal station and the earthquake epicenter can then be entered into the on-line distance calculator (Great Circle Distance (Links to an external site.)Links to an external site.) to determine their distance apart (see Figure 3).
• The distance between the earthquake epicenter and the tidal stations at Apra Harbor, Guam and Monterey, CA about 2773 km and 7994 km, respectively.

The two tables in the template require data for a series of water stations located on Pacific Islands & Hawai’i and the West Coast & Alaska. Each needs to be selected from the listing, with at least five from both series of locations, including stations in Hawai’i and Alaska. (Note: Once an image with the desired time range is displayed it is possible to change the station number in the URL to switch displays to a different station. For example, changing 1630000 to 1820000 in the URL will switch the display from Apra Harbor, Guam, to Kwajalein, whereas entering 9413450 will switch to Monterey, CA.

Figure 1: Map for Japanese earthquake on March 11 2011 (note the data are given as year month day hh:mm:ss) specifying its location co-ordinates and time (UTC)

Figure 2: Graph of verified (green) versus predicted (blue) water levels for March 11-12 2011 at Apra Harbor, Guam.

Figure 3: Great circle website with data for Apra Harbor, revealing that its distance from the earthquake epicenter is 2773 m.

Section I: Pacific Ocean & Hawai’i

Section I Task 1 – Hypothesis: How fast do tsunami travel?

Background information: The activity focuses on tsunami generated by the Japanese earthquake that occurred on March 11 2011 at 05:46:24 UTC at 38.29°N, 142.37°E

a. Pacific and Hawai’i Locations (4 points)

Complete the table with names and coordinates (i.e. latitude, longitude) for appropriate tidal stations, recording the time of arrival (UTC) of the tsunami on March 11 2011 based on the disturbance of water levels. Use this information to determine both the travel time of the tsunami, the distance from epicenter and hence the calculated speed (distance/time), as shown by the example for Apra Harbor, Guam.

(A strong answer will provide information for at least four additional station locations on islands in the Western Pacific and including at least two from Hawai’i, making the necessary calculations, while ensuring that travel time is converted into hours from hours and minutes.)

 Name of Tidal Station Latitude Longitude Arrival Time (UTC) Travel time (hr) Distance (km) Speed (km/hr) 1 Apra Harbor, Guam 13° 27’N 144° 39’E 09:30 3.73 2773 743 2 3 4 5 6 7

b. Evaluation & Discussion (3 points)

Explain the range of your data and estimate the average speed for the waves from this tsunami.

(A strong answer will evaluate the data for tsunami speeds, commenting on any significant differences between the various locations examined – for example is the tsunami speed uniform for all stations throughout the West Pacific?)

Section II: US West Coast & Alaska

Section II Task 2 – Hypothesis: How fast do tsunami travel?

Background information: The activity focuses on tsunami generated by the Japanese earthquake that occurred on March 11 2011 at 05:46:24 UTC at 38.29°N, 142.37°E

a. West Coast and Alaska Locations (4 points)

Complete the table with names and coordinates (i.e. latitude, longitude) for appropriate tidal stations, recording the time of arrival (UTC) of the tsunami on March 11 2011 based on the disturbance of water levels. Use this information to determine both the travel time of the tsunami, the distance from epicenter and hence the calculated speed (distance/time), as shown by the example for Monterey, CA.

(A strong answer will provide information for at least four additional station locations – excluding Monterey – on the West Coast (California, Oregon & Washington) and including at least two from Alaska, making the necessary calculations, while ensuring that travel time is converted into hours from hours and minutes.)

 Name of Tidal Station Latitude Longitude Arrival Time (UTC) Travel time (hr) Distance (km) Speed (km/hr) 1 Monterey, CA 36° 36’N 121° 53’W 16:00 10.23 7994 781 2 3 4 5 6 7 8

b. Evaluation & Discussion (3 points)

Explain the range of your data and estimate the average speed for the waves from this tsunami.

(A strong answer will evaluate the data for tsunami speeds, commenting on any significant differences between the various locations examined – for example is the tsunami speed uniform for all stations throughout the U.S. West Coast and Alaska?)

Section III: Synthesis

Section III Task 3 – Hypothesis: How fast do tsunamis travel and is their speed uniform across an ocean?

a. Evaluation and Discussion (6 points)

Consider the estimates for the speeds of the waves from the 2011 Japanese tsunami across the Pacific and compare the values calculated for the tsunami waves within the West Pacific and Hawai’i with those for waves that traveled across the Pacific to the U.S. West Coast or in a northeastern direction to reach Alaska. Are there any specific trends in these data that suggest specific controls on the speed of tsunami waves – for example is there evidence that they slow down with distance traveled?

(A strong answer will evaluate the data for tsunami speeds, commenting on any trends – for example whether the speed is higher for stations in the West Pacific closer to the earthquake epicenter, or uniform for all locations irrespective of their distance, and how other factors perhaps influence the speed of tsunami waves?)

Section III – Bonus Hypothesis: What ocean characteristics limit the speed of tsunamis?

a. Reflective Question (3 bonus points)

How does comparison of the data for the speed of tsunamis across the Pacific Ocean help evaluate and recognize possible controls on their speed?

(A strong answer will discuss the data for the speed of tsunamis based on their arrival times at different locations as examples of the behavior, drawing parallels with shallow water waves to explain controls on their speed.)