Every fact in the three earthquake lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the lessons simplify: seismic wave speeds vary with the rock: the British Geological Survey gives about 6–7 km/s for P-waves and 3.5–4 km/s for S-waves in the crust, so the 15–16 lesson uses 6 and 3.5 km/s and shows how the choice changes the calculated distance. The 'about 32 times more energy' per magnitude step is the USGS rounded value. The yearly earthquake counts are long-term averages since 1900 and vary from year to year. The 1992 Roermond earthquake has several magnitudes on the Royal Observatory of Belgium page (ML 5.8, MS 5.4, MW 5.3); the lesson uses the local magnitude ML 5.8. The block-tower and elastic-band experiments are models: they show shaking and the build-up and sudden release of stored energy, not real rock. Safety advice follows the CDC; local rules in your school come first.
All sources
[1] The Science of Earthquakes
USGS Earthquake Hazards Program · Reliability: High
Type: Government science agency education page
An earthquake is what happens when two blocks of the earth suddenly slip past one another; the surface where they slip is the fault; the energy released spreads as seismic waves that shake the ground; the hypocenter is where the rupture starts underground and the epicenter is the point on the surface directly above it; mainshocks always have aftershocks that can continue for weeks to years; the Earth's outer shell is made of plates that keep slowly moving, sliding past and bumping into each other, and most earthquakes happen at plate boundaries; the edges of plates are rough, so they get stuck while the rest of the plate keeps moving, and the stored energy is released when the fault slips; a seismograph records ground motion on a seismogram; P waves are faster than S waves, so the time between their arrivals gives the distance, and circles around three or more stations meet at the epicenter (triangulation); the size of an earthquake is its magnitude.
[2] Earthquake Magnitude, Energy Release, and Shaking Intensity
USGS Earthquake Hazards Program · Reliability: High
Type: Government science agency education page
Magnitude is the size of the earthquake and each earthquake has one magnitude, while shaking intensity varies from place to place, mostly with distance from the fault rupture; each whole-number increase in magnitude is a tenfold increase in measured amplitude on a seismogram and about 32 times more energy release; the United States uses the Modified Mercalli Intensity Scale.
[3] This Dynamic Earth: Understanding plate motions
US Geological Survey (USGS) · Reliability: High
Type: Government science agency booklet
Divergent boundaries: plates move apart and new crust is created by magma; convergent boundaries: plates move toward each other and one may be subducted; transform boundaries: plates slide horizontally past one another, e.g. the San Andreas fault, where the Pacific Plate has moved past the North American Plate at about 5 cm per year; transform faults are generally defined by shallow earthquakes; the Mid-Atlantic Ridge spreads about 2.5 cm per year; the collision of India with Asia pushed up the Himalayas; strong earthquakes are common along the Peru-Chile trench.
[4] Are earthquakes becoming more frequent? (Earthquake FAQ)
British Geological Survey (BGS) · Reliability: High
Type: National geological survey FAQ
There is no evidence that earthquakes are becoming more frequent; more small earthquakes are recorded because monitoring networks are expanding; USGS yearly averages since 1900: magnitude 8 and higher about 1, 7–7.9 about 18, 6–6.9 about 120, 5–5.9 about 800, 4–4.9 about 6,200 (estimated), 3–3.9 about 49,000 (estimated), magnitude 1–2 about 8,000 per day.
British Geological Survey (BGS) · Reliability: High
Type: National geological survey education page
P-waves travel fastest and consist of contractions and expansions like sound waves, with particle motion parallel to the direction of travel; S-waves are slower, transverse waves with particle motion at right angles to the direction of travel; S-waves cannot travel through air or liquids; in the Earth's crust P-waves travel at around 6–7 km/s and S-waves at around 3.5–4 km/s.
British Geological Survey (BGS) · Reliability: High
Type: National geological survey education page
A seismograph or seismometer is the instrument that makes a seismogram; a suspended mass tends to stay still when the ground moves, so the relative motion measures the ground motion; the P-wave arrives first, followed by the slower S-wave and then the surface waves; the time between the P- and S-wave arrivals gives the distance of the earthquake from the seismograph.
No: neither the USGS nor any other scientists have ever predicted a major earthquake, and they do not expect to know how in the foreseeable future; scientists can only calculate the probability of a significant earthquake in a specific area within a certain number of years; a prediction must give the date and time, the location and the magnitude.
Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public health agency safety page
During an earthquake indoors: DROP down onto your hands and knees, COVER your head and neck (and your body if possible) under a sturdy table or desk, HOLD ON until the shaking stops; outside: stay outside, move away from buildings, utility wires and fuel and gas lines, get down low and stay there until the shaking stops.
[9] Roermond (NL), 13 April 1992 (Earthquakes in Belgium catalogue)
Royal Observatory of Belgium, Seismology · Reliability: High
Type: National observatory earthquake catalogue
Earthquake near Roermond (Netherlands) on 13 April 1992 at 03:20 Belgian time; magnitudes ML 5.8, MS 5.4, MW 5.3; depth 19.0 ± 1.4 km; maximum intensity VII.