Earthquakes

Rock is stronger than it gets credit for, but it is not infinitely strong. As tectonic plates grind against each other, stress builds in the rock along their edges. The rock bends slightly and stores that energy the way a stretched rubber band does, sometimes for centuries. Then, in a few seconds, it snaps. The rock breaks along a fault, the two sides lurch into new positions, and all that stored energy races outward as vibrations. That sudden snap and release, called elastic rebound, is an earthquake.

Two locations matter for every quake, and the Regents exam loves to make sure you know the difference. The focus is the point underground where the rock actually broke, and it can be anywhere from a kilometer down to nearly 700 kilometers deep. The epicenter is the point on the surface directly above the focus. News reports name the epicenter, because that is where a quake appears on a map, but the shaking is born at the focus.

The first shock is rarely the last word. Aftershocks, smaller quakes on the same fault, can continue for days or months as the rock settles into its new position. And when a large quake displaces the seafloor, it can shove the entire column of water above it, launching a tsunami that crosses oceans at jet speed.

The energy released at the focus travels as seismic waves. The two kinds that move through Earth's interior behave very differently. A P-wave, or primary wave, is a compression wave. It squeezes and stretches the rock along the direction it travels, like a push sent down a spring toy. A P-wave moves fast, and it can pass through solids, liquids, and gases. An S-wave, or secondary wave, shakes rock from side to side, at right angles to the direction it travels, like a whipped rope. An S-wave is slower, and it has one important limitation. It cannot travel through a liquid at all, because liquids do not spring back when you try to shear them sideways.

Because P-waves are faster, they always arrive first. Watch the race below. Both waves leave the focus at the same instant, but the gap between them grows the whole way. That growing gap is the most useful fact in this chapter. There are also surface waves, which roll along the ground once the body waves reach the surface. They are the slowest of all, and they cause most of the damage in a large quake.

A seismograph is an instrument that records ground shaking, and the record it draws is a seismogram. Reading one is a Regents skill worth mastering, and there is not much to it. The line wanders gently until the P-wave arrives and the wiggles sharpen. Later the S-wave arrives and the wiggles get dramatically larger. The time gap between those two arrivals is the separation, or lag time, and it is the number everything else depends on.

Here is the logic. Both waves left the focus at the same moment. The P-wave outran the S-wave the entire way. So the farther the station is from the epicenter, the longer the race, and the bigger the gap between arrivals. A station close to the epicenter records the two arrivals nearly together. A distant station records them many minutes apart. Measure the gap, and a travel-time graph converts it directly into distance. One seismogram tells you exactly how far away the quake was.

One station's seismogram gives you a distance, but a distance is not a location. If a station calculates that a quake happened 1,000 kilometers away, the epicenter could be anywhere on a circle with a radius of 1,000 kilometers around that station. North, south, or out over the ocean. The seismogram cannot tell you which.

The fix is more stations. Draw the distance circle for a second station and it will cross the first circle at two points, which narrows the answer to two possibilities. Add a third station and its circle passes through only one of those points. That single point where all three circles meet is the epicenter. This method is called triangulation. It is why three is the number to remember on the exam. The minimum number of seismic stations needed to locate an epicenter is always three.

In practice, modern networks use dozens of stations and a computer does the drawing. That is how the USGS can publish an epicenter within minutes of a quake anywhere on Earth. The logic is exactly the three circles you are about to draw yourself.

An earthquake just happened somewhere on this map. Three stations recorded it, and each one has already converted its lag time into a distance, shown at the right. Drag each station's slider until its circle matches its measured distance. When all three circles agree, they will meet at one point.

Magnitude measures the energy an earthquake releases, calculated from the size of the waves on seismograms. The scale is logarithmic, and that trips people up. Each whole step is not "one more": each step up represents about 32 times more released energy. A magnitude 6 releases roughly 32 times the energy of a magnitude 5, and about 1,000 times the energy of a magnitude 4. That is why a magnitude 7 near a city is a catastrophe while the planet's daily magnitude 3s pass unnoticed.

Magnitude is not the same as intensity, which describes how strong the shaking felt and how much damage it did at a particular place. One quake has exactly one magnitude, but its intensity fades with distance from the epicenter and depends on local ground conditions. Soft, wet sediment shakes far more violently than solid bedrock, which is why buildings on filled land often fare worst.

These are the most significant earthquakes recorded worldwide in the past 30 days, pulled from the U.S. Geological Survey the moment you loaded this page. For each one, notice the depth. A shallow focus usually means a transform fault or a ridge. Anything deeper than about 70 kilometers almost certainly happened inside a subducting slab, which connects straight back to the boundary types from the last chapter.

• Model of Earth's Interior Structure (page 11). The workhorse of this chapter. Practice all three moves: given a distance, read each wave's travel time; given both arrival times, use the gap between the curves to find distance; given a P arrival and a distance, work backward to the origin time of the quake. Keep your two points on the curves directly above one another when measuring separation.
• Model of Earth's Interior Structure (page 11). Shows the liquid outer core that stops S-waves and creates the shadow zone.
• Global Tectonic Activity of the Last One Million Years (page 13). Where the epicenters you locate actually cluster, from the previous chapter.

Download the current tables from NYSED (2026 Revised Edition, used from the 2026-27 school year): Reference Tables for Earth and Space Sciences.

• Earthquakes happen when stressed rock snaps along a fault and releases stored energy (elastic rebound). The focus is the break point underground; the epicenter is the surface point directly above it.
• P-waves are faster, arrive first, and pass through solids and liquids. S-waves are slower and cannot pass through liquids.
• The lag between P and S arrivals grows with distance, so a seismogram plus a travel-time graph gives the distance to the epicenter.
• One distance defines a circle, not a point. Three stations' circles intersect at the epicenter: triangulation.
• Each whole step in magnitude means about 32 times more released energy. Magnitude is one number per quake; intensity varies from place to place.
• The S-wave shadow zone is evidence that Earth's outer core is liquid.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.