Unit 06 · Earthquakes & Earth’s Interior
An earthquake is rock releasing strain it could no longer hold, and its waves are the only direct probe we have of the deep interior. This unit covers elastic-rebound theory — how rock stores strain along a fault and lets go in a sudden rebound — the three wave types (P-waves, S-waves, and surface waves) and how each travels, locating an epicenter by triangulating arrival-time differences from three seismograms, and how the waves reveal the layered interior: crust, mantle, liquid outer core, and solid inner core. Mastery means you can read a seismogram, find the quake, and explain what the waves prove about the planet beneath you.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Elastic-rebound theory | Thinks earthquakes strike with no buildup or cause. | Describes stress on a fault but not the sudden rebound. | Explains how rock stores elastic strain and releases it in a rebound, generating the earthquake and its aftershocks. |
| Seismic wave types | Treats all earthquake waves as the same. | Names P- and S-waves but confuses their motion or speed. | Distinguishes P-, S-, and surface waves by their motion, speed, and the materials each can pass through. |
| Locating an epicenter | Cannot use arrival times to find a quake. | Reads one seismogram but cannot triangulate. | Triangulates an epicenter from the P–S arrival-time gap at three stations and defends the single point where the circles meet. |
| Earth’s interior from seismic evidence | Describes the layers as guesses with no evidence. | Names the layers but not how waves reveal them. | Uses the S-wave shadow zone and wave refraction to argue for a liquid outer core and a solid inner core. |
| Lab technique (earthquake triangulation) | Cannot measure arrival-time differences off a trace. | Measures the P–S gap but plots the distance circle carelessly. | Reads P- and S-wave arrivals precisely, converts the gap to distance, and triangulates a defensible epicenter. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“The S-waves never arrived at the far station — that’s the shadow zone, which means the outer core is liquid and won’t carry a shear wave. I measured the P–S gap at three seismographs, drew a distance circle from each, and the epicenter is where all three cross.”
“The needle wiggled, so there was an earthquake somewhere. P and S are just two kinds of waves. I’d need the map to already have the dot on it.”
You demonstrate this unit by triangulating a real epicenter from three seismograms — measuring the P–S arrival gap, converting it to distance, and drawing the circles aloud, not on a multiple-choice test. A criterion counts as mastered only when your three circles close on one point and you can explain what the shadow zone proves about the core. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.