6.3 Seismology & earthquake principles

Elastic Rebound and Earthquake Anatomy

An earthquake is the sudden release of energy stored in rock that has been strained beyond its elastic limit. Elastic rebound theory, formulated by H. F. Reid after the 1906 San Francisco earthquake, explains the process: stress slowly deforms the rock on either side of a locked fault; when frictional strength is finally exceeded, the rock ruptures and snaps back toward its unstrained shape, radiating seismic waves in every direction.

The point at depth where rupture begins is the focus, or hypocenter; the point on the surface directly above it is the epicenter. Small foreshocks may precede the main shock, and aftershocks follow it, sometimes for months, as the crust settles into its new configuration.

Seismic Waves

Seismic energy travels as body waves through Earth's interior and as surface waves along the surface.

• P-waves (primary) are compressional; particles vibrate parallel to the direction of travel in a push-pull motion. They are the fastest waves, arrive first, and pass through solids, liquids, and gases alike.
• S-waves (secondary) are shear waves; particles vibrate perpendicular to the direction of travel. They are slower than P-waves and, crucially, cannot travel through liquids.
• Surface waves are the slowest but the most destructive to buildings. Love waves shake the ground horizontally, side to side, while Rayleigh waves roll in an elliptical motion like an ocean swell.

Because S-waves cannot pass through the molten liquid outer core, they are blocked over a broad S-wave shadow zone on the far side of the planet. This absence is the primary evidence that the outer core is liquid, while refraction of P-waves and a faint inner arrival help show that the inner core is solid.

Seismic waves reveal the whole of Earth's layering because they refract and reflect wherever velocity changes. A sharp increase in speed at the Mohorovicic discontinuity (the Moho) marks the crust-mantle boundary, and a strong drop at the Gutenberg discontinuity marks the mantle-core boundary. By timing waves along these paths, seismologists mapped the crust, mantle, and core long before any borehole came close. Deep-focus earthquakes, reaching nearly 700 kilometers down, occur only within subducting slabs, whereas shallow-focus quakes release their energy near the surface and cause the most damage for a given magnitude.

Recording and Locating Earthquakes

A seismograph records ground motion as a seismogram, on which the successive arrivals of P-waves, S-waves, and surface waves are clearly separated. The instrument works by inertia: a suspended mass tends to stay still while the frame and the ground move around it, and that relative motion is traced onto the record.

Because P-waves outrun S-waves, the gap between their arrivals — the S-minus-P interval — grows with distance from the epicenter. A seismologist reads this interval and uses a travel-time curve to convert it into distance. One station fixes only a circle of possible locations, so three stations are required: drawing a circle of the correct radius around each station, the single point where all three circles intersect marks the epicenter. This method is called triangulation.

Magnitude Versus Intensity

Two very different scales describe an earthquake, and confusing them is a classic exam pitfall.

• Magnitude measures the energy released at the source and yields a single number per earthquake. The older Richter (local) magnitude was based on maximum wave amplitude but saturates for very large events. The moment magnitude (Mw) scale is now standard: it is calculated from seismic moment (rupture area times slip times rock rigidity) and stays accurate even for the greatest earthquakes. Each whole-number step is a tenfold rise in wave amplitude and roughly a 32-fold rise in energy.
• Intensity measures the effects of shaking at a specific place — how strongly it was felt and how much damage occurred. The Modified Mercalli Intensity (MMI) scale runs from I (not felt) to XII (total destruction). A single earthquake therefore has one magnitude but many intensity values, highest near the epicenter and diminishing outward, and it also depends on focal depth, distance, and local ground conditions.

Soft, water-saturated sediments can amplify shaking and even undergo liquefaction, in which saturated sand momentarily loses strength and behaves like a fluid — an applied seismic hazard the ASBOG exams tie directly to local site geology.

Beyond direct shaking, large earthquakes spawn secondary hazards: landslides on steep slopes, fires from ruptured gas and power lines, and tsunamis generated when a submarine fault abruptly displaces the seafloor and the water column above it. Because strain accumulates steadily between events, stretches of a fault that have not slipped in a long time — known as seismic gaps — are treated as likely sites for the next large earthquake, a central idea in seismic hazard assessment.