Seismic energy from a major meteorite impact would be expected to disrupt the surface on the opposite side of the Earth.2,12  Computer simulations conducted at Sandia National Laboratories in 1994 showed how the diverging shock wave from an impact reconverges along the axis of symmetry at the opposite point (antipode).2

How much disruption would be caused has been argued among researchers.  Some hold that many volcanic "hotspots" and even flood basalts were triggered by antipodal effects,3 while others find the energy reaching the antipode to be insufficient to cause volcanism.4  Disrupted terrain, and possibly volcanism, are thought to have been found antipodal to large impact craters on the Moon, Mars, and Mercury.12,14  A planet like Earth, with a molten outer core and a more solid mantle will focus seismic waves to maximize antipodal pressure.13  The energy released by the Shock Dynamics impact is estimated to have been at least 200 times that of the Chicxulub impact, and should have affected a large area.

The Shock Dynamics meteorite apparently struck at an oblique angle, probably near 30 degrees, and along an east-southeast (ESE) azimuth (direction).  This is based on three observations.  A huge tektite strewnfield attributed to no other meteorite extends east from the impact site.

The Bonin-Mariana and Tonga-Kermadec Trenches override Pacific crust from west to east, and are interpreted in the Shock Dynamics model to be halves of a frozen crustal wave launched eastward from the front of the oblique impact.

The central peak of the crater is U-shaped (characteristic of an oblique impact) and open (indicating downrange direction)5,9 to the ESE.  Right image is close-up of central peak.

                   
Satellite Altimetry Map. David T. Sandwell, Walter H. F. Smith. 1995-1997.

Impacts between 90 and 45 degrees deliver an essentially hemispherical shock wave.  By 30 degrees the shock wave has shifted somewhat toward the downrange direction.  Thus not only the impact angle (30 degrees) but also the azimuth of the meteorite flight path should influence which way the shock wave propagates.


From - E. Pierazzo, H.J. Melosh. 2000. "Melt production in oblique impacts". Icarus 145, pp. 252-261.

To date, hydrodynamic code (hydrocode) simulations on supercomputers of oblique impacts have not (to my knowledge) studied their antipodal effects.  However, it is possible to replicate the basic result of the hydrocode simulation of energy focusing from a vertical impact (see image at top of page) using an animation program to perform a simple dynamics simulation.  Spherical particles launched in streams from 4 emitters reflect from the inside wall of a cylinder.  Except for the arced trajectories, the result is essentially the same.

Shifting the emitters slightly (4 degrees) to simulate the off-center energy focus of an oblique impact angle around 30 degrees produces this result:

The energy refocuses at a new antipodal axis, as indicated by the triangles, drawing nearer to the crater in the direction the impact angle tilts.  This simulation and the hydrocode simulation both assume a uniform mantle for simplicity. The actual mantle is heterogeneous, with pockets of varying composition and density.6,7,10,11  This should cause the energy to lose some of its focus and affect a broader area.

Seismic tomography allows us to look deep into the Earth, though the images differ according to the methods researchers use to interpret the data.  There are two particularly large regions with slow seismic velocities (considered "hot" or "wet") that led to the proposal of "superplumes" rising from the outer core of the Earth.  One is under eastern and southern Africa; the other (below) is centered roughly on the islands of French Polynesia in the Pacific.  The South Pacific Superswell is the surface feature, and it looks like a good candidate for antipodal focused energy from the Shock Dynamics meteorite impact.  Geologists have not been able to determine its origin.  It now appears to be a cluster of at least 6 smaller plumes.8  They speculate that the southern part "may be related to a rising superplume which has been delayed in the transition zone and then initiated the secondary plumes", whereas the northern part "could also be related to another superplume which is now going down but which was coming up 30-90 million years ago".1  Or maybe not.


QRLW8, 210 km.  Anelastic 3D model of the upper mantle from full seismogram waveform inversion.
Yuancheng Gung and Barbara Romanowicz, "Q tomography of the upper mantle using three
component long period waveforms", Geophys. J. Int., 157, 813-830 (2004).

 

In the next picture, the white dot is the exact antipode of the Shock Dynamics crater north of Madagascar.  The gray shape is the South Pacific Superswell.1  The highest terrain, up to 680 meters, is in the dark gray part of the lower portion.  The Superswell is slightly closer to the crater, consistent with the oblique simulation.

The arrow shows how the Superswell is also in line with the meteorite's ESE azimuth.

So the South Pacific Superswell is not only the right size but in the right place for antipodal disruption associated with the Shock Dynamics event, and explains yet another enigmatic feature of the Earth.

* * * * * * * *

References

  1. Adam, Claudia, Alain Bonneville. 2005. Extent of the South Pacific Superswell. Journal of Geophysical Research, Vol. 110, B09408, 14 pages.
  2. Boslough, M.B., E.P. Chael, T.G. Trucano, D.A. Crawford, D.L. Campbell. 1996. Axial Focusing of Impact Energy in the Earth's Interior: a Possible Link to Flood Basalts and Hotspots. in The Cretaceous-Tertiary Event and Other Catastrophes in Earth History, G. Ryder, S. Gartner, D. Fastovsky (eds.), GSA Special Paper 307, pp. 541-550.
  3. Hagstrum, Jonathan T., 2005. Antipodal hotspots and bipolar catastrophes: Were oceanic large-body impacts the cause? Earth and Planetary Science Letters, 236, pp. 13-27.
  4. Melosh, H.J. 2000. Can impacts induce volcanic eruptions? International Conference on Catastrophic Events and Mass Extinctions: Impacts and Beyond, Lunar and Planetary Science Institute Contribution 1053, 2 pages.
  5. Pierazzo, E., H.J. Melosh. 2000. Understanding Oblique Impacts from Experiments, Observations, and Modeling. Annual Review of Earth and Planetary Science, Vol. 28, pp. 141-167, (Figure 2 - p. 152).
  6. Poupinet, Georges, Nicholas Arndt, Pierre Vacher. 2003. Seismic tomography beneath stable tectonic regions and the origin and composition of the continental lithospheric mantle. Earth and Planetary Science Letters, 212, pp. 89-101.
  7. Rhodes, Mark, J. Huw Davies. 31 March 2004. Constructing a Geodynamic A Priori Seismic (GAPS) velocity model of upper mantle heterogeneity. Geochemistry Geophysics Geosystems, Vol. 5, No. 3, 12 pages.
  8. Schubert, G., G. Masters, P. Olson, P. Tackley. 2004. Superplumes or plume clusters? Physics of the Earth and Planetary Interiors, 146, pp. 147-162.
  9. Schultz, Peter H., Raymond R. Anderson. 1996. Asymmetry of the Manson impact structure: Evidence for impact angle and direction. GSA Special Paper 302, pp. 397-417.
  10. Trampert, Jeannot, Frederic Deschamps, Joseph Resovsky, Dave Yuen. 29 October 2004. Probabilistic Tomography Maps Chemical Heterogeneities Throughout the Lower Mantle. Science, Vol. 306, pp. 853-856.
  11. van der Hilst, Robert D. 29 October 2004. Changing Views on Earth's Deep Mantle. Science, Vol. 306, pp. 817-818.
  12. Watts, Alison Weatherly. 1992. The formation of terrains antipodal to major impacts. Master's Thesis, Abstract, Arizona State University, Tempe, AZ.
  13. Watts, Alison W., Ronald Greeley, H.J. Melosh. September 1991. The formation of terrains antipodal to major impacts. Icarus, Vol. 93, pp. 159-168.
  14. Williams, David A., Ronald Greeley. August 1994. Assessment of Antipodal-Impact Terrains on Mars. Icarus, Vol. 110, Issue 2, pp. 196-202.

 John Michael Fischer, 2006
www.newgeology.us