Synthetic seismograms from coupled free oscillations: Effects of lateral structure and rotation

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Abstract
Synthetic seismograms from coupled free oscillations are calculated for a regionalized continent‐ocean model of the upper mantle and a recently proposed spherical harmonic lateral structure model. Spectral correlation analysis of these synthetic records, containing Rayleigh waves with periodT≳ 160 s, reveals the size of departures from the assumption that the waveform perturbation can be explained by just the structure under the great circle between source and receiver. Arrival azimuth deflections of up to 10° are observed on horizontal synthetics using a 2.3% variation in shear velocity between continents and oceans. Among the M84A synthetics, the great circle travel timesR3–R1,R4–R2,R5–R3vary on average by 3–5 s for 3 ≲f≲ 6 mHz surface wave motion. Comparison between synthetics, calculated to be sensitive to even‐order structure or to both even‐ and odd‐order structure, gives a 5‐s average time delay attributable to odd‐order structure. Significant Rayleigh wave packet amplitude anomalies are common in the synthetic records and are nearly as common in synthetics calculated for the even‐order structure as in those sensitive to all structure. Examples suggest that model M84A does not consistently reproduce the amplitude anomalies observed in International Deployment of Accelerometers (IDA) and Global Digital Seismographic Network (GDSN) data. Lastly, we demonstrate the effect of Coriolis coupling in the time domain. Visible precursors to Rayleigh wave packets are predicted in cases of favorable source‐receiver geometry. These precursors are caused by the spheroidal component of the hybrid, predominantly toroidal, low‐frequency (f≲ 4.2 mHz) Love wave, whose arrival precedes that of the Rayleigh wave. We present examples of this anomalous waveform in IDA data from a large strike‐slip earthquake in New Guinea, showing a qualitative fit to the coupled‐mode prediction.