Understanding the unique propagation characteristics of Lg waves trapped in the continental crust
What is Lg Wave?
Lg is a regional seismic phase consisting of multiple S-wave reverberations trapped in the continental crust. Key features:
- Velocity: ~3.5 km/s
- Frequency: 0.5-5 Hz (high frequency)
- Distance range: 150-1500 km
- Largest amplitude phase at regional distances
Wave Composition:
• Fundamental mode Love wave
• Higher mode surface waves
• Multiply reflected S-waves
• Guided by velocity contrast at Moho
Propagation Characteristics
| Path Type |
Lg Efficiency |
Q Factor |
| Stable Continental |
Excellent |
800-1200 |
| Active Tectonic |
Moderate |
200-500 |
| Oceanic Path |
Blocked |
N/A |
| Thick Sediments |
Attenuated |
100-300 |
Lg amplitude ∝ Δ-0.5 × e-πfΔ/(QV)
Where Δ is distance, f is frequency, Q is quality factor, V is velocity
Lg Blockage Conditions
Lg waves cannot propagate through:
Complete Blockage:
• Oceanic crust (thin ~7 km)
• Continental margins
• Deep sedimentary basins
• Crustal thickness < 25 km
This property makes Lg useful for:
- Nuclear test monitoring (continental events)
- Crustal structure studies
- Distinguishing continental vs oceanic paths
- Yield estimation for explosions