Elastic versus acoustic 3-D Full Waveform Inversion at the East Pacific Rise 9°50'N

International audienceIn the field of controlled source seismology, the acoustic 3-D Full Waveform Inversion (FWI) technique has become a common tool for imaging geologically complex structures in land, as well as in marine settings. However, the Earth behaves elastically and, therefore, excluding the elastic effect could have a significant impact on the inversion results, especially for interfaces with large S-wave velocity contrasts. To examine the contribution of the elastic approach, we compare acoustic and elastic 3-D FWI applied to a 3-D seismic data set from the East Pacific Rise (EPR) 9°50' N, where the subsurface is represented by igneous basaltic rocks. To establish an efficient inversion strategy, we first conducted a number of tests, which suggest a simultaneous, multiparameter inversion as the most efficient approach when inverting signals with frequencies below 7 Hz. The reduction in the total misfit for the elastic case is 10-15 per cent lower than that for the acoustic one, suggesting that the elastic approach explains the observed data better than the acoustic approach. Furthermore, the compressional velocity images of the upper-oceanic crust obtained using the two approaches differ significantly, not only in velocity magnitude but also structurally. We argue that the results obtained from the acoustic modelling are geologically less plausible and suggest the elastic model as a more reliable representation of the upper-oceanic crust

Insights into dike nucleation and eruption dynamics from high-resolution seismic imaging of magmatic system at the East Pacific Rise

International audienceModels of magmatic systems suggest that the architecture of crustal magma bodies plays an important role in where volcanic eruptions occur, but detailed field observations are needed to evaluate them. We present ultrahigh-resolution reflection images of magma bodies beneath a region of multiple eruptions along the East Pacific Rise derived from three-dimensional seismic surveying. The observations reveal magma bodies with elongate ridges and troughs vertically aligned with seafloor eruptive fissures that we interpret as remnant dike root zones where repeat dikes nucleate. We document a triangular feeder zone to the axially centered magma body from the off-axis source for a newly forming seamount of the Lamont chain and infer bottom-up eruption triggering due to recharge from this deeper source. The findings indicate that magma bodies are sculpted by both processes of magma recharge from below and magma extraction to the surface, leaving a morphological imprint that contributes to localization of dike nucleation and eruption sites at the East Pacific Rise

Links Between Volcanic Eruptions and Magma Body Geometry Revealed by Seismic Reflection Imaging at the East Pacific Rise

International audienceThe structure of the magmatic system beneath subaerial volcanos, including the architecture and distribution of the bodies where magma is stored and the network of conduits that transport melt between these accumulations and the surface, plays a fundamental role in all aspects of volcano construction and evolution, from igneous differentiation to hazard assessment. However, due to inaccessibility, little is known about the geometry of the magma bodies residing beneath subaerial volcanos. Mid-ocean ridges host the most extensive magmatic system on Earth, with 98% of its length below the ocean surface, which makes them an ideal target to be scanned by controlled-source marine seismic techniques. Beneath some portions of this vast system, the shallowest magma bodies are present and represented by long-linear Axial Magma Lenses (AML). It is at these shallow-most AMLs where dikes nucleate and connect the magma accumulations to the surface to result in an eruption. To explore the magma plumbing systems at mid-ocean ridges, we use 3-D multichannel seismic data across a mid-ocean ridge environment and apply advanced marine seismic techniques to develop the highest resolution reflection images of the AMLs so far. The data were collected across a magmatically dynamic portion of the East Pacific Rise at 9°50’N with documented dike intrusion and eruptions in 1991/1992 and 2005/06.The observations indicate that the magma reservoirs in the shallow crust are not represented by smooth bodies, but show strongly lineated topography that is spatially linked to the distribution of eruptive fissures and erupted lavas above. In the detailed topography, we find evidence for: 1) a dike root zone beneath where a caldera-like axial eruptive fissure zone is present, 2) deep excavation of this root zone within the primary eruption site for the last documented eruption, and 3) dikes rupturing from edges as well as the center of magma lenses. We also demonstrate that the distribution of additional, off-axis crustal magma accumulations further impact the stresses and melt budget at shallow-level magma accumulations leading to more frequent eruptions. Our results show that the mechanism behind eruptions along mid-ocean ridges is predominantly bottom-up and not fundamentally different from the eruptions’ mechanism at subaerial volcanoes. Considering the fine-scale morphology of shallow magma bodies will be critical for future generations of more realistic numerical models to aid in effective global volcanic hazard assessment and mitigation