3.4 Earthquakes & Volcanoes

Seismic Waves: P-Waves and S-Waves

When rock breaks along a fault, the energy released travels outward as seismic waves from the focus (the underground point of rupture). The point on the surface directly above the focus is the epicenter. Two body waves are central to the Regents.

Remember the order with the names: P for primary arrives first because it is faster; S for secondary arrives second. A crucial consequence is that S-waves cannot pass through liquids, so they do not travel through Earth's liquid outer core. This creates an S-wave shadow zone and is direct evidence that the outer core is liquid (see 3.3).

Using the ESRT Travel-Time Graph

The reference tables include a Earthquake P-wave and S-wave Travel Time graph with P and S curves. Because P-waves travel faster, the two curves spread farther apart with distance — and that growing gap is what lets you find epicenter distance.

To find distance to the epicenter from one station:

1. On the seismogram, measure the time difference between the first P-wave arrival and the first S-wave arrival.
2. On the travel-time graph, find where the vertical gap between the S and P curves equals that time difference.
3. Read down to the horizontal axis to get the distance to the epicenter.

The larger the gap between P and S arrivals, the farther the station is from the epicenter. A small gap means a nearby quake; a large gap means a distant one.

To find origin time (when the quake actually happened): use the graph to read the travel time of the P-wave for that distance, then subtract that travel time from the recorded P-wave arrival time. Origin time = P-wave arrival time − P-wave travel time.

Locating the Epicenter: Why Three Stations

One station tells you only the distance to the epicenter, not the direction — the quake could lie anywhere on a circle of that radius. To pin one point you need at least three stations: draw a circle of the measured radius around each station, and the single point where all three circles intersect is the epicenter. This triangulation is why Regents questions stress “at least three seismic stations.”

Magnitude versus Intensity

These two measures are easily confused, and the Regents tests the difference.

• Magnitude measures the energy released at the source. Each earthquake has one magnitude value, reported on a scale (such as the moment-magnitude or Richter scales) where each whole step represents a large jump in energy.
• Intensity measures the shaking and damage experienced at a given place. Intensity varies with location — it is greater near the epicenter and on soft ground, and weaker far away — so one earthquake has many intensity values.

One earthquake, one magnitude, many intensities is the phrase to remember.

Volcanoes: Types and Distribution

Volcano shape reflects lava chemistry. Shield volcanoes are broad and gently sloped, built from runny, low-silica (mafic) lava, as in Hawaii. Composite (stratovolcanoes) are tall and steep, built from alternating layers of thick, high-silica (felsic) lava and ash, and they erupt explosively. Cinder cones are small, steep piles of erupted fragments.

Volcanoes are not randomly scattered — they cluster along plate boundaries (especially convergent subduction zones) and over hot spots. The Pacific Ring of Fire, the subduction-rimmed edge of the Pacific Ocean, hosts most of Earth's active volcanoes and largest earthquakes, tying volcano distribution back to plate tectonics.

Geologic Hazards

Earthquakes and volcanoes generate cascading geologic hazards: ground shaking and building collapse, tsunamis from sea-floor displacement, landslides, ashfall, lahars (volcanic mudflows), and lava flows. On the Human Sustainability strand, the Regents links these hazards to risk reduction — hazard mapping, building codes, and land-use planning — so understanding where and why quakes and eruptions occur is the basis for reducing their human impact.

A Worked Origin-Time Example

Suppose a station records the first P-wave at 10:05:00 and finds, from the travel-time graph, that the epicenter is a certain distance away. You read the graph and see the P-wave needed 4 minutes of travel time to cover that distance. The earthquake's origin time is then 10:05:00 minus 4 minutes, which is 10:01:00. The logic is always the same: the arrival time is when the wave reached the station, and subtracting how long it traveled gives when it started.

If a question instead gives you the origin time and a station distance, you reverse the process: add the P-wave travel time from the graph to the origin time to predict the arrival.

Why Seismic Waves Reveal the Interior

Seismic waves do more than locate quakes — they X-ray the planet. Because waves speed up, slow down, refract, and reflect as they cross boundaries between materials of different density and state, geologists infer the layers in 3.3 from how waves behave. The fact that S-waves disappear beyond a certain angle (the S-wave shadow zone) shows a liquid layer must lie inside Earth, which is the liquid outer core. P-waves bend sharply at the same boundary, leaving a P-wave shadow zone too.

This is the strongest evidence we have for Earth's internal structure, and it is why the Regents pairs seismic-wave behavior with the interior diagram. Remember the throughline: the same waves that triangulate an epicenter at the surface also prove the outer core is liquid deep below.