A surface rupture: the fault reached the ground. This is the classroom sense of earthquakes: a named landform, process, or object. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCWhy this matters
San Andreas and Cascadia teach the same waves on different machines. The San Andreas is a transform: Pacific plate northwest, North American southeast, strike-slip you can walk on the Carrizo Plain. 1906 San Francisco and 1989 Loma Prieta are this boundary in living memory. Cascadia is a megathrust: Juan de Fuca diving under North America, a locked subduction zone with a 1700 earthquake dated from Japanese tsunami records and drowned forests. Same P, S and surface waves. Different fault, different depth story, different tsunami story. GCSE AQA 3.1.1.4 and NGSS MS-ESS3-2 both need focus versus epicentre before anyone colours a shake map. UK students will not feel these plates under the desk; they will meet them on the paper. Hawaii and Iceland are volcano labs. Katrina and Sandy are water and wind. This lab is slip in the rock and the three-wave sequence that follows. Map dot is not the underground spark.
A second photograph for the projector script — not the lab cover.Wikimedia Commons / NASA / USGSThe mechanism
Slip starts at the focus, also called the hypocentre: a point (really a patch) in the rock. The epicentre is the surface point vertically above it, the map dot, not a cave. Elastic strain stored on a locked fault becomes kinetic energy and heat when the fault slips. Body waves leave first. P-waves are compressional, fastest, and travel through solid and liquid — primary, the first arrival. S-waves are shear, slower, and die in liquid; that fact is the outer-core argument in the interior lab, reused here as a reminder. Surface waves (Love and Rayleigh) travel along the free surface, arrive later, and often do the damage in town because they shake the ground where buildings sit. Play in this lab: P shell grows, then S, then a surface ring and a ripple on the top face. Magnitude is a source-size scale. Intensity is what a place feels — distance, geology, building. Keep that pair qualitative. San Andreas creep versus locked segments, Cascadia’s long recurrence, shallow versus deep: all of that sits on top of focus, epicentre, P, S, surface. Get those five words right and the case pages can work.
How to drive the model
Play from t = 0. The hanging wall and footwall sit either side of a visible fault plane. A spark in the rock is the focus; a cool marker on the top face is the epicentre, joined by a vertical line. P expands as a fast shell. S follows, slower, shearing. Then a surface ring and a ripple on the ground. Orbit so you can see that the epicentre is on the map, not underground. Pause after P and ask who arrived first. Step the camera down to the focus, then up to the epicentre. Click the part: Focus, Epicentre, Fault, P, S, Surface. San Andreas and Cascadia are the locators after the sequence, not before it.
If the model will not spin
Without the canvas the sequence is still the lesson. Focus (hypocentre) is the point of slip in the rock. Epicentre is the surface point above it, the map dot, not underground. P-waves are fastest and travel through solid and liquid. S-waves shear and die in liquid. Surface waves arrive later and often do the damage in town. Magnitude is the source; intensity is what a place feels. San Andreas is transform; Cascadia is subduction. Same waves, different machine. Do not put the epicentre in a cave.
Film
How we know about the Earth's core. Khan Academy. About 6 min. Khan Academy on YouTube. CC BY-NC-SA. Educational embed, autoplay off. Autoplay off.
Misconceptions
The epicentre is underground.
Epicentre is on the map. Focus is in the rock.
The biggest wave arrives first.
P arrives first. Surface waves come later and often do the damage.
Magnitude and intensity are the same number.
One is the source. One is what a town feels.
Focus
Where the slip starts in the rock.
Focus in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. Where the slip starts in the rock. Source: Wikimedia Commons / NASA / USGS / public domain.Wikimedia Commons / NASA / USGS / public domainEpicentre
Surface point vertically above the focus. The map dot.
epicentre in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCP-wave
Fastest body wave. Compressional. Travels through solid and liquid.
p wave in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCS-wave
Shear body wave. Slower than P. Dies in liquid.
s wave in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCSurface wave
Love and Rayleigh waves along the free surface. Often the damage in town.
surface wave in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCMagnitude
A scale for source size. Not the same as intensity.
magnitude in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CCIntensity
What a place feels, given distance, ground and buildings.
intensity in the earthquakes lab — a TERMS crop, not the glossary plate and not the lab cover. A TERMS crop, not the glossary plate and not the lab cover. Source: Wikimedia Commons / NASA / USGS / NOAA, public domain or CC.Wikimedia Commons / NASA / USGS / NOAA, public domain or CC