Mechanism 1: Impedance Contrast
When seismic waves cross from high impedance (Z = ρv) to low impedance materials, amplitude increases due to energy conservation at the interface.
Transmission Coefficient: T = 2Z₁/(Z₁+Z₂)
Key characteristics:
- Primarily affects high-frequency energy
- Depends on density (ρ) and velocity (v) contrast
- Energy conservation requires amplitude increase when velocity decreases
- More pronounced with larger impedance contrasts
Mechanism 2: Ray Angle Effect
Sensors record wave projections, not true amplitudes. As waves refract to more vertical paths in softer soils, the vertical component (Z) recording increases.
Recorded: Z = A·cos(θ), H = A·sin(θ)
Key characteristics:
- Affects all frequencies based on geometry
- Purely geometric projection effect
- Smaller angles (more vertical) → larger Z recordings
- Can create apparent amplification without energy increase
Combined Effects & Observations
Both mechanisms work together in real earthquakes:
- High-frequency waves: Impedance contrast dominates
- Low-frequency waves: Ray angle effect more important
- Site amplification factors: Range from 2x to 10x+
- Frequency dependent: Different amplification at different frequencies
Total Amplification = Impedance Effect × Geometric Effect
Toggle between modes to see each mechanism separately. Understanding both is crucial for accurate seismic hazard assessment.