A deep earthquake at 500 km depth in a subduction zone registers Mw 7.5. A shallow earthquake at 15 km depth registers Mw 6.5. Assuming both occur the same distance from a city, which is likely more destructive?
AThe deep Mw 7.5 earthquake, because it releases roughly 32 times more energy
BThe shallow Mw 6.5 earthquake, because seismic energy is concentrated near the surface and attenuates less before reaching the city
CThey cause equal damage because the magnitude difference exactly compensates for the depth difference
DDeep earthquakes are always more destructive because they rupture larger fault areas
Depth matters enormously for destructiveness. A shallow earthquake concentrates its energy near the surface, producing intense ground shaking in a smaller area. A deep earthquake at 500 km disperses its energy through a much larger rock volume before reaching the surface, resulting in weaker shaking at any given point. The Mw 6.5 shallow earthquake can easily cause more local destruction than a Mw 7.5 deep earthquake, despite releasing ~32 times less energy total. This is the common misconception: bigger magnitude does not automatically mean more destruction.
Question 2 Multiple Choice
Why do earthquakes with depths greater than 300 km occur only in subduction zones and nowhere else?
ASubduction zones are the only locations with sufficient tectonic stress to generate large earthquakes
BOnly the geometry of subduction zones creates faults that extend to such depths
CThe subducting slab remains cold and brittle enough to fracture at those depths, while surrounding mantle rock deforms plastically
DDeep earthquakes occur globally but seismographs can only detect them at subduction zones
Rock fractures (producing earthquakes) only when it is cold and brittle. At great depths, the surrounding mantle is hot enough to deform plastically rather than fracture. The subducting oceanic slab, however, descends faster than it can equilibrate to ambient mantle temperatures — it remains anomalously cold and therefore brittle down to about 700 km depth. Below that, even the slab has heated enough to deform plastically, and earthquakes cease. This inclined zone of seismicity — the Wadati-Benioff zone — directly images the geometry of the subducting plate.
Question 3 True / False
A magnitude 8 earthquake releases approximately twice the energy of a magnitude 7 earthquake.
TTrue
FFalse
Answer: False
The moment magnitude scale is logarithmic in a specific way: each whole-number increase corresponds to roughly 32 times more energy released (and 10 times the ground motion amplitude). So a magnitude 8 earthquake releases about 32 times more energy than a magnitude 7, and about 1,000 times more than a magnitude 6. This nonlinearity is why great earthquakes (Mw 8–9) release a vastly disproportionate share of total seismic energy compared to the thousands of smaller earthquakes that occur daily.
Question 4 True / False
Shallow earthquakes near populated areas are typically more destructive than deep earthquakes of equal magnitude.
TTrue
FFalse
Answer: True
Seismic energy attenuates (weakens) as it travels through rock. A shallow earthquake at 10 km depth delivers its energy to the surface over a short path, concentrating shaking in a small area at high intensity. A deep earthquake at 400 km spreads the same energy through a vastly larger volume of rock before reaching the surface, resulting in weaker but more widespread shaking. For a given magnitude, proximity of the hypocenter to the surface is one of the strongest predictors of localized destruction.
Question 5 Short Answer
Explain the elastic rebound theory of earthquake generation, and why the hypocenter and epicenter are located at different points.
Think about your answer, then reveal below.
Model answer: The elastic rebound theory holds that tectonic plates move continuously, but friction locks fault surfaces together, causing rocks on either side to slowly deform elastically as strain energy accumulates — like bending a ruler. When accumulated stress exceeds the fault's frictional strength, the rocks snap back to their unstrained configuration, releasing the stored energy as seismic waves. The hypocenter (focus) is the underground point where this rupture initiates; the epicenter is the point on Earth's surface directly above it. They differ because faults are inclined planes at depth, not vertical walls reaching the surface.
This distinction matters for hazard assessment: the epicenter is easy to report on a map, but the hypocenter depth determines how energy is distributed. A magnitude 7 hypocenter at 10 km is far more dangerous to surface structures than one at 200 km. The elastic rebound model also explains aftershocks: after the main rupture, the fault has adjusted its geometry and the surrounding rock continues to settle into new stress configurations, producing smaller subsequent fractures.