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Dynamics of a seafloor-spreading episode at the East Pacific Rise

Nature volume 540, pages 261–265 (2016)Cite this article

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Abstract

Seafloor spreading is largely unobserved because 98 per cent of the global mid-ocean-ridge system is below the ocean surface. Our understanding of the dynamic processes that control seafloor spreading is thus inferred largely from geophysical observations of spreading events on land at Afar in East Africa and Iceland1. However, these are slow-spreading centres1 influenced by mantle plumes2,3. The roles of magma pressure and tectonic stress in the development of seafloor spreading are still unclear. Here we use seismic observations to show that the most recent eruption at the fast-spreading East Pacific Rise just North of the Equator initiated at a melt-rich segment about 5 kilometres long4. The change in static stress then promoted almost-concurrent rupturing along at least 35 kilometres of the ridge axis, where tectonic stress had built up to a critical level, triggering magma movement. The location of impulsive seismic events indicative of lava reaching the seafloor5 suggests that lava subsequently erupted from multiple isolated6,7 magma lenses (reservoir chambers) with variable magma ascent rates, mostly within 48 hours. Therefore, even at magmatically robust fast-spreading ridges, a substantial portion of the spreading may be due to tectonic stress building up to a critical level rather than magma overpressure in the underlying magma lenses.

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Figure 1: Location of various seismic events.
Figure 2: Temporal and along-axis progression of various seismic events.
Figure 3: Lava flow velocities.
Figure 4: Schematic representation of the dynamics of seafloor spreading at the East Pacific Rise.

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Acknowledgements

Y.J.T. thanks W. R. Buck for discussions and reading of the manuscript. Y.J.T. thanks A. H. Barclay and R. S. Matoza for discussions. We thank R. P. Dziak and D. R. Bohnenstiehl for providing the T-wave earthquake catalogue. We thank the captains, crews and science parties of the RV Knorr and RV Atlantis. This work was supported by NSF grant OCE-0961594.

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Authors and Affiliations

  1. Lamont-Doherty Earth Observatory of Columbia University, Palisades, 10964, New York, USA

    Yen Joe Tan, Maya Tolstoy & Felix Waldhauser

  2. School of Oceanography, University of Washington, Seattle, 98195, Washington, USA

    William S. D. Wilcock

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Contributions

Y.J.T., advised by M.T. and F.W., analysed the seismic data and wrote the manuscript. M.T. and F.W. conceived and ran the experiment. W.S.D.W.’s identification of impulsive lava signals at Axial inspired this study. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Yen Joe Tan.

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The authors declare no competing financial interests.

Additional information

Reviewer Information Nature thanks J. Karson and V. Schlindwein for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 Impulsive lava event signals.

a, b, Velocity seismograms of the same event recorded at two different stations. The difference in station relative arrival time of >1 s for the first arrival suggests a waterborne arrival (that is, the seismic wave travelled through the water column rather than through the crust), since these stations are around 2 km apart, with water velocity being 1.5 km s−1 and P-wave crustal velocity being around 4–5 km s−1 (ref. 37). The event also has multiple arrivals: the first arrival represents a signal that has bounced off the sea surface once (B1) and the second arrival represents a signal that has bounced off the sea surface twice (B2) (Extended Data Fig. 2b). c, The signal shows a dominant frequency of about 22 Hz.

Extended Data Figure 2 Impulsive lava events.

a, Location of seismometers (yellow triangles) and impulsive lava events in the first hour (red), first two days (pale red), and last five days (dark red). The AST17 is shown as a thin blue line with green arrow marking its end. The bold black line marks revised 2006 lava flow farther off-axis than originally identified18. AML disruptions (purple arrows) were inferred from seismic reflection studies post-eruption6.b, Histogram of hourly impulsive lava event rate. The red line shows hourly average impulsive lava event magnitude. Mid-ocean-ridge basalt on the East Pacific Rise has been found to have low volatile content40. Therefore, the larger-magnitude impulsive lava events during the first 5 h might be driven by excess degassing of volatiles from a larger reservoir of unerupted magma41, consistent with the estimate that <15% of the available magma in the AML was erupted17. The lava has also been suggested to have degassed from supersaturated conditions owing to a rapid magma ascent rate from depth42. The breaks in the red line are due to hours when there are no events located, and hence no average magnitude data point. Event magnitudes were calculated as the logarithm of the displacement (in micrometres) multiplied by the signal travel distance (in kilometres), which is different from earthquake magnitude. The inset shows the model of the signal travel path used in locating the impulsive lava events (VW is the velocity of the seismic wave travelling through the water column). A direct waterborne arrival is not recorded, probably because of the upward refraction of energy due to increasing velocity with water depth. The lack of crustal arrival indicates a small absolute magnitude.

Source data

Extended Data Figure 3 Earthquake locations.

The A–A′ depth slice shows a 1-km-thick east-southeast (ESE)-dipping structure towards the ridge axis. The B–B′ depth slice shows that the structure is subvertical in this direction (that is, north-northeast to south-southwest, NNE–SSW) with a width of around 500 m. Earthquakes occurred between 13:30 and 20:00 on 22 January 2006, mostly in the first two hours. The 50 largest-magnitude earthquakes are highlighted with black circles.

Source data

Extended Data Figure 4 Local magnitude of earthquakes.

a, Earthquakes show increasing local magnitude ML with time in a two-hour period leading up to the eruption. Blue dots represent earthquakes during the eruption period analysed in this study in detail while black dots represent earthquakes over a longer period. b, An 8-h zoom-in on panel a, demonstrating a clear trend of increasing magnitude immediately before plate rupture.

Source data

Extended Data Figure 5 Lava flow velocities.

a, Location of seismometers (yellow triangles labelled with station numbers) and impulsive lava events (coloured dots) in the four flow lobes17 (thin black outline enclosing the lava flow). The bold black line marks the revised extent of the 2006 lava flow farther off-axis than originally identified18 (see legend to Fig. 1). b, c, Propagation of impulsive lava events off-axis in two flow lobes.

Extended Data Figure 6 Synthetic test of earthquake locations.

a–c, Actual locations of synthetic sources. d–f, Locations of synthetic sources with normally distributed noise of standard deviation 200 m added. These are used as the initial locations for double difference relocation. g–i, Relocated locations of synthetic sources. Events were relocated using synthetic arrival time with normally distributed noise of standard deviation 0.01 s added.

Extended Data Figure 7 Earthquake relative location errors.

Distribution of least-squares errors for the relative locations in longitudinal distance, latitudinal distance and depth.

Extended Data Figure 8 Long-period event locations.

a, Long-period event locations assuming 0 km depth (black dots) and 1.5 km depth (green stars), assuming a constant crustal velocity of 2.5 km s−1. The red line marks the extent of melt-depleted sub-AML30. Although location depth cannot be definitively determined, the 1.5-h time gap between the initiation of long-period events and melt reaching the surface strongly supports a deeper source. b, Long-period event locations assuming a constant crustal velocity of 2.5 km s−1 (green stars) and 2.9 km s−1 (black dots) assuming 1.5 km depth. Regardless of the depth and velocity used, events generally cluster near the western edge of the melt lens and bound the area of sub-AML depletion. Faint blue dots represent local earthquake locations (Fig. 1c) for reference.

Extended Data Figure 9 Long-period event velocity assumption.

Minimum of standard deviation for different guess (modelled) velocities, assuming a depth of 1.5 km. The overall minimum of the standard deviation occurs at a velocity of 2.5 km s−1.

Extended Data Figure 10 Waveforms of long-period events.

Waveforms of two separate events at all three stations low-pass filtered at 5 Hz. Red arrows mark the first arrivals. Both event signals arrived at stations S204 and S205 at the same time. This constrains the location of these events to the west of the ridge axis, between these two stations. The event to the south arrived first at S204 and S205 before S209. The event to the north arrived first at S209 followed by S204 and S205. This constrains the northward migration of the long-period events.

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Tan, Y., Tolstoy, M., Waldhauser, F. et al. Dynamics of a seafloor-spreading episode at the East Pacific Rise. Nature 540, 261–265 (2016). https://doi.org/10.1038/nature20116

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