Grade 10-11 (age 15-17)

high school Earthquakes and Volcanoes

Where plates collide or slide past each other, stress builds up in the rock until it can no longer hold, and the rock breaks, sending the released energy outward as seismic waves. The underground point where the rupture begins is the focus, and the point on the surface directly above it is the epicenter; the fast P-waves arrive first, and the slower but stronger S-waves arrive later. The gap between their arrival times, the P–S time, grows with distance from the focus, so measuring it tells a station how far away the focus is. In the diagram, letting time run after the quake spreads the two wavefronts through the ground and prints the P-wave and then the S-wave on the station's seismogram.

Why the ground shakes

🌍 Where do earthquakes and volcanoes happen?

①Where plates collide or slide past each other, stress slowly builds up in the rock

②When the stress passes what the rock can bear, the rock suddenly breaks and slips, and the released energy spreads as seismic waves

③Where one plate sinks beneath another, or where plates pull apart at mid-ocean ridges, rock melts into magma, and magma that reaches the surface builds volcanoes

④So earthquakes and volcanoes cluster together in belts along plate boundaries, most clearly in the Ring of Fire around the Pacific

⑤Some volcanoes, such as those of Hawaii, form over hot spots in the middle of a plate

How seismic waves spread

Time since the earthquake16 s

💡 What to look for in the diagram

①P-waves travel at about 6 km/s and S-waves at about 3.5 km/s, so the blue circle (P) always runs ahead of the red one (S)

②At the station 75 km from the focus, the P-wave arrives after about 12.5 s and the S-wave after about 21.4 s, and the seismogram shows small P shaking followed by larger S shaking

③The P–S time is about 8.9 s, and the farther a station is from the focus, the wider the gap between the two waves and the longer the P–S time

④The focus is the underground point where the quake starts, and the epicenter is the surface point directly above it

Types of seismic waves

Seismic waves compared

P-waves and S-waves

Feature	P-wave (longitudinal)	S-wave (transverse)
Direction of vibration	along the direction the wave travels	perpendicular to the direction the wave travels
Speed in the crust	fast (about 6 km/s)	slow (about 3.5 km/s)
Passes through	solids, liquids, and gases	solids only
Arrival and shaking	arrives first, weaker shaking	arrives later, stronger shaking

P–S time and distance to the focus

distance to focus = P–S time × (Vp × Vs) / (Vp − Vs)

With Vp = 6 km/s and Vs = 3.5 km/s, the distance to the focus ≈ P–S time × 8.4 km. Draw circles with these distances around three stations, and the common chords of the circles meet at the epicenter.

Magnitude and intensity

Magnitude vs. intensity

Two ways to describe how big a quake is

Feature	Magnitude	Intensity
Meaning	how much energy the earthquake released	how strongly the shaking was felt or damaged buildings at a given place
Number of values	one per earthquake	different at each place (usually smaller farther from the epicenter)
How it is written	a number to one decimal place (e.g. 5.8)	Roman numerals I to XII (Modified Mercalli scale)

Magnitude and energy

magnitude +1 → about 32 times the energy

Two steps up in magnitude means about 32 × 32 ≈ 1000 times the energy. Korea's 2016 Gyeongju earthquake (magnitude 5.8) released about 32 times the energy of a magnitude 4.8 quake.

Worked Examples and Exam Practice

Example 1

After an earthquake, which seismic wave reaches a station first, and which cannot travel through liquid?

The P-wave is longitudinal, pushing and pulling the material along its direction of travel, and it is fast enough to arrive first.

The S-wave is transverse, shaking the material sideways, so it cannot pass through a liquid, which does not resist sideways shearing.

▸ The P-wave arrives first; the S-wave cannot travel through liquid.

The observation that S-waves cannot cross the outer core is how we know the outer core is liquid. The seismogram in the diagram also records the P-wave first and the S-wave later.

Example 2

At a station, the S-wave arrives 10 s after the P-wave. If P-waves travel at 6 km/s and S-waves at 3.5 km/s in the crust, roughly how far away is the focus?

Call the distance to the focus d. Then the P–S time = d / 3.5 − d / 6 = d × (6 − 3.5) / (6 × 3.5) = d / 8.4.

The P–S time is 10 s, so d = 10 × 8.4 = 84 km.

▸ about 84 km

The station in the diagram has a P–S time of about 8.9 s, so it is 75 km from the focus. The longer the P–S time, the farther the focus.

Mock-test style

Which statement about earthquakes and seismic waves is correct?

①S-waves pass through both solids and liquids

②Magnitude takes a different value at each observation site

③P-waves arrive before S-waves

④Intensity measures the energy an earthquake released

⑤The longer the P–S time, the closer the focus

▸ ③ P-waves arrive before S-waves

P-waves are faster than S-waves, so they arrive first, and the gap between their arrivals (the P–S time) gives the distance to the focus.

S-waves pass through solids only (①), magnitude is one value per quake while intensity varies by place (②, ④), and the P–S time grows longer the farther the focus is (⑤).

Summary

Earthquakes in brief

stress builds at plate boundaries → rock breaks → seismic waves spread (P first, S later)

Earthquakes and volcanoes gather along plate boundaries, most of all around the Pacific Ring of Fire.

🎯 Exam Points

①P-waves are longitudinal, fast, and pass through solids, liquids, and gases; S-waves are transverse, slower, and pass through solids only

②The observation that S-waves cannot cross the outer core is the evidence that the outer core is liquid

③The P–S time grows with distance from the focus, and the distance to the focus is P–S time × Vp Vs / (Vp − Vs)

④Magnitude has one value per earthquake while intensity varies by place, and each step up in magnitude means about 32 times the energy

⑤Earthquake and volcano belts follow plate boundaries, and the Pacific Ring of Fire is the best-known example

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Last updated 2026-10-17

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