Earthquakes and Volcanoes

When plates collide or slide past each other, stress builds until rock fractures and energy spreads as seismic waves. The focus is the underground source and the epicenter is the surface point above it; P-waves arrive before S-waves. Earthquakes and volcanoes concentrate along plate boundaries, and larger magnitudes send waves farther. Here you can change the magnitude slider and watch P- and S-waves spread from the focus.

Why the ground shakes

🌍 Why do earthquakes happen?

①When plates collide or slide past, enormous energy accumulates

②Past a threshold, rock fractures suddenly and releases the energy

③This energy spreads outward as seismic waves

Propagation of seismic waves

Magnitude M5

💡 Key observations

①Larger magnitude → waves spread further

②P-waves (blue) arrive before S-waves (red)

③Focus: actual earthquake source (underground) / Epicenter: surface point directly above

Types of seismic waves

Wave comparison

ChartP-wave vs S-wave

Item	P-wave (longitudinal)	S-wave (transverse)
Vibration direction	parallel to travel	perpendicular to travel
Speed	fast (~6 km/s)	slow (~3.5 km/s)
Medium	solid + liquid + gas	solid only
Arrival order	first	second

P-S time formula

epicentral distance ∝ (S arrival − P arrival)

larger P-S time → farther from epicenter; 3 stations triangulate the epicenter

Magnitude and Intensity

Magnitude vs Intensity

ChartMeasuring earthquake size

Item	Magnitude	Intensity
Definition	absolute energy of the earthquake	shaking felt at a specific location
Number of values	one per earthquake	varies by location
Unit	Richter magnitude (M)	Modified Mercalli (I~XII)

Magnitude–Energy relation

magnitude +1 → energy × ~32

magnitude +2 → energy × ~1,000 (32 × 32)

Worked Examples and Exam Practice

Example 1

When an earthquake occurs, which wave arrives first at a station, and which wave cannot pass through liquid?

The P-wave is a longitudinal wave and is fast, so it arrives first.

The S-wave is transverse and travels only through solids, so it cannot pass the liquid outer core.

▸ Arrives first: P-wave / Cannot pass liquid: S-wave

That S-waves cannot cross the outer core is evidence that the outer core is liquid.

Example 2

If a quake grows from M5 to M7 (an increase of 2), about how many times more energy is released? (each +1 in magnitude is about 32× energy)

Each increase of 1 in magnitude multiplies energy by about 32.

An increase of 2 gives 32 × 32 ≈ 1,000 times.

▸ About 1,000 times

Magnitude is a logarithmic scale, so a small number difference means an enormous energy difference.

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 represents the absolute energy of the earthquake

⑤A longer P-S time means closer to the epicenter

▸ ③ P-waves arrive before S-waves

P-waves are faster and arrive before S-waves; their arrival-time gap (P-S time) gives the epicentral distance.

S-waves pass only through solids (①), magnitude is one value per quake while intensity varies by site (②④), and a longer P-S time means farther from the epicenter (⑤).

Summary

Earthquake essentials

stress at plate boundary → rock fracture → seismic waves (P + S) spread

earthquakes/volcanoes concentrate along plate boundaries (Pacific Ring of Fire)

🎯 Exam Points

①P-wave: longitudinal, fast, passes solids+liquids / S-wave: transverse, slow, solids only

②S-waves cannot cross outer core → evidence outer core is liquid

③Magnitude: 1 absolute value / Intensity: varies by location

④Magnitude +1 → energy × ~32

⑤Pacific Ring of Fire = quakes along plate boundaries

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Plate Tectonics

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