Four Hours or Twenty Minutes: Write a Tsunami Warning Procedure

Investigation question: Using earthquake data, deep-ocean sensor readings and the rule that tsunami speed depends on water depth, what step-by-step procedure should a coastal warning office follow so that its harbor town gets enough time to evacuate without crying wolf?

Scientific purpose

Groups act as the warning office for Port Alder, an imagined harbor town on a Pacific coast loosely modeled on California's North Coast. They calculate how long tsunamis from three source zones take to reach the town by dividing each stretch of the path by the speed for its depth, and when the waves will pass two DART stations. They analyze a record of twelve past earthquakes to find which data predict a dangerous wave and derive a rule that turns a DART reading into a forecast height. Then they write a procedure of if-then steps, test it event by event against the record, count misses, false alarms and late warnings, and merge the best steps of their procedure with another group's.

Why this helps students with this chapter

Students tend to think a warning system either 'predicts' a tsunami or fails. Working the numbers shows what warning centers really do: forecast an arrival time from distance and depth once a quake has happened, and confirm the size with deep-ocean sensors before deciding how far to go. The record forces trade-offs. A rule based on magnitude alone is simple but misses two waves that needed alerts, one of them dangerous, and cries wolf three times; adding DART readings fixes most of that. The local source shows that some tsunamis outrun any message, so the plan must rely on people recognizing the shaking. Crescent City's history, where earlier small surges and false alarms taught people to go back too soon, makes the cost of false alarms concrete.

Standards connections

MS-ESS3-2 (assesses, analyzing and interpreting tsunami data (source location, magnitude, depth, sensor readings and travel times) to forecast arrival and size, and using the forecast to design a warning procedure): Students analyze magnitudes, depths, fault motion and DART readings from past events, calculate arrival times from depth and distance, and use the results to set thresholds in a warning procedure, a mitigation technology. The local-source calculation shows the clarification statement's point that some hazards cannot be forecast with enough notice. The task produces direct evidence toward the PE.

MS-ETS1-2 (practice, evaluating competing procedures systematically against criteria): Each procedure is scored against the same twelve events for misses, false alarms and late warnings, and compared with another group's procedure and with a magnitude-only rule. The criteria and record are supplied, so this practices the PE.

MS-ETS1-3 (practice, combining the best steps of two tested procedures): Two groups compare their scores event by event, identify which steps of each prevented misses or false alarms, and write a combined procedure, as the DART engineers combined three fixes. The cycle is short, so the role is practice.

Roles (rotate each session)

• Chief forecaster: Leads the decision steps, makes sure each decision uses only data available at that time, and has the final word on the procedure's wording.
• Travel-time analyst: Organizes the arrival-time calculations, splitting the segments among the group, and checks every division with a calculator.
• Records analyst: Leads the analysis of the twelve past events and derives the rule that turns a DART reading into a forecast height.
• Tester and recorder: Applies the written procedure to each past event exactly as written, fills the test table and totals misses, false alarms and late warnings.

Materials

Estimated cost: 1 to 3 per class (estimate) (an estimate; About four printed pages per student at roughly 2 cents a page; calculators and pencils are classroom stock.)

Safety

• No physical hazards beyond ordinary classroom work.
• This lab discusses real tsunamis that took lives. Preview it, do not require personal stories, and allow a student who is upset to work on the calculation section alone or step out.
• Make clear that Port Alder and its record are invented; for real local guidance, students and families should use their county's tsunami information and official alerts.

Teacher preparation and setup

1. Print the packet single-sided so students can lay the record and the test table side by side.
2. Review the alert definitions, which are simplified from the U.S. Tsunami Warning Centers: a warning when waves of 1 m or more are expected (evacuate the harbor district); an advisory for 0.3 m up to 1 m (clear beaches, docks and the harbor); a watch while the threat is being checked; an information statement when no threat is expected.
3. Decide whether to run the lab in one period (analysis questions for homework) or two (the second for testing, combining and q4-q10).
4. Pair groups in advance for the combining step so each pair has procedures likely to differ.
5. Know your school counselor's referral process; some students may have experienced a tsunami alert, an evacuation or a disaster.

Investigation design

Student procedure

1. Read the scenario and the timing rules aloud as a group. Underline the three numbers that limit every decision: quake data arrive 5 minutes after the quake, a message takes 5 minutes to reach the public, and the harbor district needs 30 minutes to evacuate.
2. Arrival times: split the eleven path segments among the group. For each segment, look up the speed for its depth in the speed table and calculate time = distance / speed. Add the segments for each source and convert the totals to hours and minutes.
3. DART times: calculate when a tsunami from the North zone passes station D-North and when one from the West zone passes D-West (q1). No station lies between the Local zone and the coast.
4. Analyze the record of twelve past events (q2): look for how wave height at Port Alder relates to magnitude, find the events that break the pattern and what they share, and divide each event's wave height by its DART reading for the North and West zones to find a rule for forecasting height.
5. Write your procedure in the procedure table: no more than six steps, each with when it happens, which data it uses and an if-then rule that ends in an alert level. Every decision may use only data available at that time.
6. Test the procedure: for each of the twelve events, apply your steps exactly as written, without looking at the actual wave until your alert is recorded. Then compare with the alert the actual wave needed, and mark the result correct, miss (alert lower than needed) or false alarm (alert higher than needed). Mark whether each needed advisory or warning reached the public at least 35 minutes before the first surge.
7. Total your misses, false alarms and late warnings, post them on the class chart, and answer q3.
8. Meet your partner group. Compare your procedures event by event, find the step in each that prevented a miss or a false alarm, and write a combined procedure. Test the combined procedure on the events where the two disagreed.
9. Answer q4 through q10 individually, in class or for homework.

Expected observations

• Arrival times at Port Alder: North about 4 h 25 min, West about 9 h 55 min, Local about 20 minutes. D-North is passed about 1 h 24 min after a North-zone quake and D-West about 3 h 46 min after a West-zone quake.
• Wave height rises with magnitude, except E9 (sideways fault) and E12 (300 km deep), which made small waves despite magnitudes of 8.4 and 8.0.
• Wave height divided by DART reading is 0.1 m per cm for every North event and 0.06 m per cm for every West event.
• Procedures that use DART readings to set the alert for distant sources usually reach zero misses and one false alarm (E10, a Local event); magnitude-only procedures score far worse. No procedure gets a Local-zone warning out in time.

Likely difficulties

• Converting decimal hours to minutes (0.419 h is 25 minutes, not 41). Model one conversion: multiply the decimal part by 60.
• Using the actual wave height while 'testing' the procedure, which makes every procedure look perfect. Have the tester cover the last column of the record with a strip of paper until the alert is written.
• Treating a watch as a false alarm. Remind groups that only advisories and warnings move people; a watch that is cancelled is the system working.
• Writing steps that are too vague to apply ('warn if it looks big'). The chief forecaster should insist on numbers in every rule.
• Running out of time. If the period is short, assign half the events to each pair within a group, or move the combining step and q4-q10 to a second period.

Structured debrief

1. Put every group's score on the board. Ask the groups with zero misses and the fewest false alarms to read their DART step aloud. In most classes the best procedures use quake data only to start a watch for distant sources, then let the DART reading set the alert level, which is close to how real warning centers work.
2. Run the magnitude-only rule together (q6): two misses (E2 and E11) and three false alarms (E7, E9, E12). Ask why E9 and E12 were harmless despite their size: a sideways fault barely lifts the seafloor, and a quake 300 km down does not move the seafloor much at all. Tell students real warning centers face exactly this problem in the first minutes, which is why their first message is cautious and later messages are refined with sea-level data.
3. Spend time on the Local zone. The arithmetic shows the wave arrives about 20 minutes after the quake, but data plus a message take 10 minutes and evacuation takes 30. No procedure can close that gap, so the mitigation must happen before the quake: hazard maps, marked evacuation routes, drills, possibly tall evacuation structures, and teaching that strong or long shaking at the coast is the warning. This is the clarification statement of the standard in action: some hazards allow reliable forecasts and some do not.
4. Close with real numbers. In 1964 the first surge reached Crescent City about four hours after the Alaska quake, and the fourth surge did the most damage after people had gone back. In 2011 the forecast arrival at Crescent City was 7:23 a.m. and the observed arrival 7:34 a.m., about ten hours after the Japan quake. Ask what their Port Alder procedure would have done in each case.

Recording sheet: what students fill in

Lab 3 recording sheet: the Port Alder warning office

Port Alder is an imagined harbor town on a Pacific coast. Its harbor district is low and flat, and its seafloor and harbor shape focus tsunami energy, much like Crescent City's. Three zones can send it tsunamis: the North subduction zone (far), the West trench (very far) and the Local fault zone just offshore. Timing rules: earthquake location, depth and magnitude reach the office 5 minutes after a quake; any message takes 5 minutes to reach the public; the harbor district needs 30 minutes to evacuate. Alert needed: warning if the wave is 1 m or more; advisory if 0.3 m up to 1 m; none if under 0.3 m. A watch means 'prepare while we check' and does not evacuate anyone.

Analysis questions: answer key

Individual evidence of learning

Each student submits the recording sheet (arrival-time table, q1, q2, the procedure, the test table, the scores and q3) and answers to q4 through q10. The arrival-time table with q1, q4 and q5 is the evidence for forecasting arrival from depth and distance; q2, q6 and the procedure test are the evidence for analyzing hazard data to inform a mitigation; q7 and q10 show judgment about the limits of forecasting.

Assessment criteria

Accessibility adaptations

• Provide a speed-lookup strip with only the depths used in the paths, and a worked first row of the arrival-time table.
• Offer a procedure template with blanks ('At ___ minutes, if ___ is at least ___, then issue a ___') for students who need structure.
• Students who find long tables hard to track can use a ruler or card to follow one row at a time; enlarge the record to 14-point type for low vision.
• Allow the procedure to be presented as a flowchart drawing instead of a table, and accept spoken answers to q7 and q10.

Low-cost alternative

The lab is already paper only. Without a printer, project the data tables and have groups copy only the rows they need; the arrival-time table can be split so each student computes one source and the group shares results.

Model limitations

This activity has not been tested with students by the authors. Port Alder, its paths and its record are invented, though the speeds follow the real long-wave rule and the alert levels are simplified from the U.S. Tsunami Warning Centers. Real tsunamis bend around seafloor features and do not follow straight segments of one depth; real arrival-time forecasts use detailed seafloor maps and computer models. Real coastal heights depend on the shape of each beach and harbor and are not a fixed multiple of a DART reading. Real warning decisions also use many DART stations, coastal tide gauges and updated earthquake information over time.

Teacher notes

Check the arrival-time table first; most later errors come from decimal-hour conversions. In the test table, insist that the alert is written before the actual wave is uncovered. The record is built so that the DART ratios are exact (0.1 and 0.06); real coastal amplification varies by place and event. A model procedure that scores 0 misses and 1 false alarm: (1) at 5 minutes, for a shallow Local quake of M7.0 or more, issue a warning at once; (2) for a shallow North or West quake of M7.5 or more, issue a watch, otherwise an information statement; (3) when the DART reading arrives, forecast the wave (x 0.1 North, x 0.06 West) and issue a warning for 1 m or more, an advisory for 0.3 m to under 1 m, or cancel; (4) send every message within 5 minutes; (5) end alerts only after 3 hours and calm gauge readings. Applied to the record it gets E1-E9 and E12 right, false-alarms on E10 and is late on E11.