Simulation to Examine How Lower Crustal Velocity and Moho Depth Trade Off in Low-Frequency Full-Waveform Inversions that use Transverse-Component Seismograms. Strategy: Compare seismograms produced by three models: A reference model, a model where just lower crustal velocity is perturbed, and a model in which both lower crustal velocity and Moho depth are perturbed in a compensating way. Bill Menke, December 2000.
 

• Case A: Reference Model: Icleand-like with 30 km thick crust
• Lowermost crustal layer
• Vp = 7.10;  Vs = 4.03;  H = 5.00
• Case B: Perturbed Model: velocity in lowermost crust slightly higher
• Lowermost Crustal Layer
• Vp = 7.20;  Vs = 4.09;  H = 5.00
• Case C: Compensated Model: thickness increased to have same vertical traveltime as Reference Model
• Lowermost Crustal Layer
• Vp = 7.10;  Vs = 4.09;  H = 5.07
• Compute Full Waveform Synthetics for 2km depth earthquake 175 km range
• Transverse Components: Cases A, B & C
• Comparisons
• Differential Seismograms: Case A, (Case B-CaseA) & (Case B-CaseA)
• 0.03-0.10 Hz Bandpassed Differential Seismograms: Case A, (Case B-Case A) & (Case B-Case A)
• 0.01-0.02 Hz Bandpassed Differential Seismograms: Case A, (Case B-Case A) & (Case B-Case A)
• Spectral of Differential Seismograms: (Case B-Case A)/Case A & (Case B-Case A)/Case B
• 0-0.1 Hz range
• 0-1 Hz range
• Conclusion: at low frequencies lower than about 0.5 Hz, the Compensated seismogram is identical to the Reference Seismogram at the 1% level. Thus lower crustal velocity and Moho depth trade off very strongly.
• Appendix: Velocity Model