Lesson Plan: The Deep
Subject areas: Earth science, physics, history of science Recommended level: Ages 13 to 18 (lower and upper secondary), adaptable for introductory undergraduate Duration: One to two 50 minute lessons
Learning objectives
By the end of this lesson, students will be able to:
- Describe P waves and S waves and explain how the time gap between them is used to locate an earthquake's distance and depth.
- Explain why earthquakes were long thought to be confined to the shallow brittle crust, and what changes with depth (heat, pressure, ductile flow).
- Recount how Kiyoo Wadati discovered deep-focus earthquakes in the 1920s and how his inclined zone of seismicity later supported plate tectonics.
- Summarise the 2026 confirmation of continental mantle earthquakes beneath Utah and explain why they are a deeper puzzle than subduction-zone deep earthquakes.
- Discuss how scientific credit is assigned and why important results can be overlooked for decades.
Background for the teacher
Earthquakes radiate two main body waves: the fast P wave (a push-pull compression) and the slower S wave (a shear). Because the two travel at different speeds, the delay between them grows with distance, and with careful geometry that delay also reveals depth. In the late 1920s the Japanese meteorologist-seismologist Kiyoo Wadati used this S minus P method on hundreds of records to prove that some earthquakes begin hundreds of kilometres down, far below the cold brittle crust where rock was thought too hot to fracture. He also found the deep quakes line up on a plane that dips beneath Japan. Overlooked in the West for twenty years, this inclined zone (now the Wadati-Benioff zone, sharing credit with Hugo Benioff, 1949) became key evidence in the 1960s for plate tectonics: it traces a cold slab of ocean floor sinking into the mantle.
How rock breaks at such depth, where pressure should prevent ordinary cracking, is still debated; the leading ideas (mineral phase changes, dehydration embrittlement) all rely on having a cold subducting slab. In 2026 a University of Utah team led by Keith Koper confirmed nine continental mantle earthquakes at 68 to 90 km depth beneath northern Utah and Wyoming, where temperatures top 700 degrees Celsius and there is no subducting slab at all. Their proposed trigger is mantle flowing around the cold deep root of the Wyoming Craton. The events revive, in pure form, the century-old question Wadati first raised.
Warm up (10 minutes)
Drop a book flat on a table and have students notice they hear the bang (fast) before they feel the table shake (slower). Use this to introduce two kinds of waves travelling at different speeds, then connect to lightning and thunder: the longer the gap, the farther away the source.
Core activities
Activity 1: Timing the quake (15 minutes). Give students a simple seismogram (or simulated arrival times) and have them measure the S minus P gap to estimate distance, then discuss how combining several stations also fixes depth. Emphasise that this is arithmetic on careful measurements, the same tools Wadati had.
Activity 2: Brittle or ductile (15 minutes). Compare a snapped stick of chalk (brittle, fast) with slowly stretched putty or warm toffee (ductile, flows). Discuss how rock becomes more ductile with heat and depth, and why an earthquake deep in hot rock seemed impossible.
Activity 3: Two kinds of deep quake (15 minutes). Using a cross-section sketch, students mark a subduction-zone deep earthquake (inside a cold sinking slab) and a continental mantle earthquake (hot mantle, no slab) and list what is different about the Utah case. Why does removing the cold slab make the puzzle harder?
Discussion questions
- Why did scientists assume earthquakes could not happen deep in the mantle? What evidence forced them to change their minds?
- Wadati published important work that was ignored for twenty years. Why do good ideas sometimes get overlooked, and how should credit be shared?
- The Utah earthquakes were recorded in 1979 but doubted until 2026. When should scientists trust a strange measurement, and when should they distrust it?
- If earthquakes can begin deeper than our hazard maps assume, how should that change the way we prepare?
Extension
- Research the Wadati-Benioff zone and how it helped confirm plate tectonics in the 1960s.
- Investigate one proposed mechanism for deep earthquakes (dehydration embrittlement or phase-transformation faulting) and explain why it needs a cold slab.
Glossary
- P wave: the fast compressional seismic wave, first to arrive.
- S wave: the slower shear seismic wave, arriving after the P wave.
- Focal depth (hypocentre): how deep below the surface an earthquake begins.
- Crust / mantle: the thin outer rock layer, and the hotter, thicker layer beneath it.
- Brittle vs ductile: breaking suddenly (brittle) versus flowing slowly (ductile).
- Subduction zone: where one tectonic plate sinks beneath another into the mantle.
- Craton: an ancient, stable, cold core of a continent, with a deep rock root or keel.
- Continental mantle earthquake: a rare earthquake that ruptures in the upper mantle beneath a continent, away from any subduction zone.