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Early Warning Race - P Wave Versus Alert

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About This MicroSim

Earthquake early warning is routinely misheard as prediction. It is not, and racing three things across one map makes that unmistakable. A banner says so at the top of every frame: the earthquake has already happened; the alert is simply faster than the shaking.

Three rings leave the epicentre. The P wave at 6 km/s, harmless and first. The S wave at 3.5 km/s, which is the one that does the damage. And, once the nearest station has felt the P wave and the system has spent its processing time, an alert that travels at the speed of light and therefore fills the map instantly.

Four cities at 20, 50, 100 and 200 km each get a card showing when the alert reaches them, when the shaking reaches them, the difference, and what that many seconds actually buys — three seconds is getting under a table, ten is stopping the trains and closing the gas valves, twenty-five is halting surgery, fifty is shutting down a factory line.

At the default settings the model reproduces the chapter's warning-time table closely, and one result is the whole reason the blind zone matters:

City Warning
Nearport, 20 km none
Midvale, 50 km 7.8 s
Farhaven, 100 km 22.0 s
Distant City, 200 km 50.5 s

Nearport is the city closest to the rupture and it receives nothing at all. The shaded red circle is the blind zone, and it is drawn rather than described because it is the honest limit of the technology. Drag the processing delay from 1 to 10 seconds and watch it grow from 5 km to 47 km, swallowing Midvale on the way.

Two controls push back against it. A denser station network detects the P wave sooner, which is exactly the argument for putting MEMS accelerometers in phones and buildings: at the dense setting with a one-second delay, the blind zone shrinks to 5 km and even Nearport gets 3.3 seconds. And a deeper earthquake gives everyone more warning, because the waves have further to travel to reach anyone at all.

How to Use

  • Press Play and watch the three rings race, or drag the Timeline to freeze any instant.
  • Move the Processing slider and watch the blind zone grow and shrink with it.
  • Set Stations to Dense and Processing to 1 s to see the best case a real system reaches.
  • Push Depth to 120 km and note that everybody, including Nearport, now gets warned.
  • Read each city's action line, not just its number of seconds.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://iowerx.github.io/pi-env-monitor/sims/early-warning-race/main.html"
        height="636px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Grade Level

6-12

Duration

20-25 minutes

Bloom's Taxonomy Level

Evaluate (L5)

Prerequisites

  • Chapter 11 sections on P and S waves and on earthquake early warning
  • Chapter 5 on trilateration, for how stations locate the epicentre

Activities

  1. Why it is not prediction (5 min): Scrub to t = 0, then to the moment of detection. Write two sentences explaining what has already happened by the time any alert exists.
  2. Grow the blind zone (8 min): Record the blind zone radius at processing delays of 1, 3, 5 and 10 seconds. Which cities fall inside at each? What would it take to remove it entirely?
  3. Judge the system (7 min): For each city, decide whether its warning time is useful, and justify the answer using the action line rather than the number. Then argue for or against funding a denser network.

Assessment

  • Explains that early warning detects an earthquake in progress rather than predicting one.
  • Computes warning time as S wave arrival minus alert time.
  • Justifies why a blind zone exists and cannot be eliminated.

References

  1. USGS ShakeAlert - the US West Coast system, live since 2019.
  2. Wikipedia: Earthquake early warning system - Japan's nationwide system, Mexico's SASMEX, and the blind zone.
  3. Wikipedia: Seismic wave - P and S wave speeds in the crust.
  4. Japan Meteorological Agency: Earthquake Early Warning - the system that has been running since 2007.