When an earthquake occurs, seismic waves travel in all directions from the focus. Seismic waves are important clues for learning about Earth's interior.
Basic Knowledge
Seismic waves are broadly divided into two types.
P waves (primary waves, longitudinal waves, or compressional waves): Vibrate parallel to the direction in which the wave travels. They can pass through solids, liquids, and gases. Their speed is approximately 6〜7 km/s in the crust and approximately 8〜12 km/s in the mantle. They cause the preliminary tremor.
S waves (secondary waves or transverse waves): Vibrate perpendicular to the direction in which the wave travels. They pass through solids only and cannot travel through liquids or gases. Their speed is approximately 0.58 times that of P waves (approximately 3〜4 km/s in the crust). They cause the principal motion.
As seismic waves travel through Earth, they undergo refraction and reflection whenever they pass through layers with different velocities. While P waves can pass through the outer core (liquid), S waves stop there, creating an S-wave shadow across the region on the opposite side of the focus. P waves also undergo a sudden change in velocity in the outer core, producing a P-wave shadow (shadow zone) from 103〜143° from the focus.
P wave（longitudinal wave. Passes through solids, liquids, and gases. Speed approximately 6〜7 km/s (crust)）
S wave（transverse wave. Passes through solids only. Speed approximately 3〜4 km/s (crust)）
Preliminary tremor（small shaking caused by the arrival of P waves）
Principal motion（large shaking caused by the arrival of S waves）
Duration of the preliminary tremor（time difference between the arrivals of P and S waves. Proportional to the distance from the focus）
Shadow zone（range 103〜143° from the focus. P waves have difficulty reaching it）
Further Exploration
The velocity of seismic waves changes with depth, and the density and elastic moduli of Earth's interior can be estimated from these changes. This has developed into a method called seismic tomography, which makes it possible to visualize the mantle's temperature structure in three dimensions, including cold regions of subducted plates and hot plumes at hotspots.
The velocities of P and S waves are expressed as V_P = √((K + 4μ/3) / ρ) and V_S = √(μ/ρ), respectively (K: bulk modulus, μ: shear modulus, ρ: density). Because liquids have zero shear modulus (μ=0), their S-wave velocity is zero; thus, the equation directly explains why S waves cannot pass through the outer core.
- P waves = longitudinal, fast, pass through all materials
- S waves = transverse, slow, pass through solids only
- S waves cannot pass through the outer core → the outer core is liquid
- Duration of the preliminary tremor ∝ distance from the focus
- Shadow zone = 103〜143° (P waves)
- Refraction and reflection of seismic waves → used to analyze internal structure
Points to Note
① It is useful to remember P as “Primary” (arrives first) and S as “Secondary” (arrives later). ② S waves cannot pass through liquids, whereas P waves can pass through liquids. ③ Do not confuse a long duration of the preliminary tremor with a distant focus. ④ Shadow zones exist for both P and S waves, but their ranges differ.
Practice
- State why S waves cannot pass through the outer core.
- The longer the duration of the preliminary tremor, what can be said about the focus?
- From what degree to what degree from the focus is the P-wave shadow zone?