Insights from synthetic seismogram modelling study of oceanic lower crust and Moho Transition Zone
Résumé
The Moho discontinuity is one of the fundamental boundaries on Earth, separating the crust from the upper mantle. Arrival times of seismic reflection and refraction data are used to estimate the depth of the Moho, but the nature of the Moho remains illusive. We present a synthetic seismogram modelling study for a suite of possible velocity models of the oceanic lower crust and Moho transition zone (MTZ). For modelling, we have used a 45-km long multichannel streamer (MCS) geometry, to get insight about waveforms and amplitudes of seismic phases like the crustal refraction (Pg), Moho reflection (PmP) and mantle refraction (Pn), and the interactions between these phases. We find that the nature of PmP reflection not only depends on the velocity structure of the MTZ but also on the velocity structure of the lower crust and upper mantle. The triplication resulting from these three seismic arrivals and its lateral extent mainly depend on the velocity structure in the lower crust. Apart from the velocity in the crust, the critical distance for the PmP arrival depends on the velocity gradient in the MTZ. The amplitude versus offset analysis of the wide-angle PmP arrivals indicates that there are two broad high amplitude peaks for the MTZ thickness less than or equal to the dominant seismic wavelength and only one large narrow peak for a thicker MTZ. Consequently, strong PmP arrivals for a thick MTZ lie in a narrow offset range around the critical offset whereas they lie over a large offset range for a sharp Moho. We also have modelled the synthetic seismograms for velocity structures from the Oman and the Bay of Islands ophiolites. Our modelling results indicate that the quantitative analysis of amplitudes and waveforms of ultra-long offset MCS data could provide detailed information about the lower crust and MTZ.
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