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Seismic reflections from axial magma chambers (AMCs) are commonly observed along many mid-ocean ridges, and are thought to arise from the negative impedance contrast between a solid, high-velocity lid and the underlying low-velocity, partially molten sill. The polarity of the AMC reflection (PAMCP) at vertical incidence is often used as a diagnostic tool for the nature of such low-velocity sill, and the contrasting amplitude versus offset (AVO) behaviors of the AMC reflections, e.g., PAMcP and Sconverted PAMCS waves, for the melt and mush cases are exploited to investigate the physical property of the AMC. Here we use time-domain finite difference calculations of the 2D laterally homogeneous models to study the effect of structure features in controlling waveform polarities at near-offset and in AVO behaviors of the AMC reflections. For a thin AMC (thickness depends on frequency content of the signal), the waveform polarity of the top PAMCP reflection may be a poor indicator for the nature of the AMC sill because of the interference of the bottom PAMcP reflection, and the AVO behaviors of the top PAMcP and PAMCS reflections are also disrupted by the bottom AMC reflections. For a thick AMC, the bottom PAMcP reflection disrupts the AVO behavior of the PAMCS reflection. Any of these two scenarios would make the qualitative analysis of melt content from partial-offset stacks of Pand S-waves less clear and more difficult to interpret. Laterally heterogeneous models are calculated to investigate the effects of flat/rough seafloor topography and the finite-width of the AMC. The finite-width AMC causes diffractions at its edges, reducing the amplitude of the AMC reflections. Rough seafloor and/or rough AMC surface can also greatly reduce the amplitude of peg-leg multiples because of the scattering and destructive interference. Sub-AMC events are observed in the 3D multi-channel seismic dataset acquired over the RIDGE-2000 Integrated Study Site at East Pacific Rise (cruise MGLO812). The seismic phases identified from the synthetic modeling of a realistic model suggest that the sub-AMC reflections are unlikely to arise from peg-leg multiples of the AMC reflections, P-to-S converted phases, or scattering due to rough topography, but could potentially arise from deeper multiple magma sills. 2.

Advanced geophysical studies of accretion of oceanic lithosphere in Mid-Ocean Ridges characterized by contrasting tectono-magmatic settings