Variations in axial magma lens properties along the East Pacific Rise (9°30′N–10°00′N) from swath 3-D seismic imaging and 1-D waveform inversion

Abstract

We use three-dimensional multistreamer seismic reflection data to investigate variations in axial magma lens (AML) physical properties along the East Pacific Rise between 9°30′N and 10°00′N. Using partial-offset stacks of P- and S-converted waves reflecting off the top of the AML, we image four 2–4 km long melt-rich sections spaced 5–10 km from each other. One-dimensional waveform inversion indicates that the AML in a melt-rich section is best modeled with a low Vp (2.95–3.23 km/s) and Vs (0.3–1.5 km/s), indicating >70% melt fraction. In contrast, the AML in a melt-poor section requires higher Vp (4.52–4.82 km/s) and Vs (2.0–3.0 km/s), which indicates <40% melt fraction. The thicknesses of the AML are constrained to be 8–32 m and 8–120 m at the melt-rich and -poor sites, respectively. Based on the AML melt-mush segmentation imaged in the area around the 2005–2006 eruption, we infer that the main source of this eruption was a 5 km long section of the AML between 9°48′N and 51′N. The eruption drained most of the melt in this section of the AML, leaving behind a large fraction of connected crystals. We estimate that during the 2005–2006 eruption, a total magma volume of 9–83 × 106 m3 was extracted from the AML, with a maximum of 71 × 106 m3 left unerupted in the crust as dikes. From this, we conclude that an eruption of similar dimensions to the 2005–2006, one would be needed with a frequency of years to decades in order to sustain the long-term average seafloor spreading rate at this location.