Earthquake Basics: P and S Waves, Magnitude vs Intensity
A compact GS Paper I Geography revision sheet on earthquake locations, seismic waves, earthquake measurement and the tsunami connection. Use it to revise the distinctions that commonly cause confusion.
📌 Revision Pointers
Focus: Rupture begins inside Earth; epicentre is the surface point directly above it.
P waves: Compressional, faster; travel through solids and liquids.
S waves: Shear, slower; cannot propagate through liquids.
Magnitude: Size of the earthquake at its source.
Intensity: Shaking and effects at a particular location.
Tsunami: Sudden displacement of water; not an automatic result of every offshore earthquake.
Core Concept
An earthquake releases stored strain energy when rocks slip along a fault. The focus, or hypocentre, is the location inside Earth where rupture starts. The epicentre lies directly above it on the surface.
Seismic Waves: Quick Comparison
P waves — Primary: Particles vibrate parallel to the direction of travel. These compressional body waves arrive first and pass through solids and liquids.
S waves — Secondary: Particles vibrate perpendicular to the direction of travel. These shear body waves arrive after P waves and travel through solids, but not liquids.
Surface waves: Travel along Earth’s surface. They generally arrive later and can cause strong ground motion.
The P–S arrival-time gap helps estimate distance from a seismic station to the earthquake. Records from several stations help locate the event.
Magnitude vs Intensity
Magnitude describes source size. Moment magnitude (Mw) is widely used, especially for large earthquakes. Do not label every reported magnitude as “Richter.” Different methods and preliminary estimates may produce different reported values for the same event.
Intensity describes local effects. Modified Mercalli intensity ranges from I to XII. One earthquake can have different intensities in different places because shaking depends on distance, depth and local ground conditions.
Magnitude scales are logarithmic. A one-unit increase corresponds approximately to 32 times the energy release, rather than twice the energy. Damage also depends on exposure and building vulnerability; magnitude alone does not determine losses.
The Tsunami Connection
A tsunami is a series of waves caused by a large, sudden displacement of water. A shallow offshore earthquake can generate one when it significantly displaces the seabed, often vertically. Earthquake location, depth, rupture and seabed movement all matter. Landslides and volcanic activity can also generate tsunamis.
Memory Hook
“P precedes; S shears; Magnitude measures source; Intensity describes place.”
Two Prelims Checks
1. Consider these statements:
P waves can travel through liquids.
S waves can travel through liquids.
A. 1 only
B. 2 only
C. Both
D. Neither
Answer: A. Liquids transmit compression but cannot sustain the shear motion required for S waves.
2. Consider these statements:
One earthquake can produce different intensities at different locations.
Every undersea earthquake necessarily generates a tsunami.
A. 1 only
B. 2 only
C. Both
D. Neither
Answer: A. Intensity varies spatially; tsunami generation requires sufficient displacement of water.
Sources
USGS: Seismic waves; USGS: Seismographs; USGS: Magnitude and intensity; NOAA: Tsunami science.
💭 Conclusion
Revise earthquakes through three separate questions: where did rupture begin, how did seismic waves travel, and what shaking occurred at a particular place? Keep source magnitude separate from local intensity, and assess the tsunami connection through water displacement.