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This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

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Susan Kieffer can be contacted at s1kieffer at gmail.com


Monday, October 24, 2011

The Chicxulub impact--What happened on the opposite side of the earth?

Artist impression of Chicxulub impact.
Artist unknown.
A number of researchers have asked "what happened on the side of the earth opposite the place where the meteorite hit in the Yucatan (Mexico) 65 million years ago?" This point is called the "antipode." The earth is a sphere, and acts like a lens to focus seismic waves on the opposite side of the planet. Body waves travel through the interior and are focused by reflection and refraction off of boundaries such as the crust-mantle, mantle-outer core, etc. Surface waves converge at the antipode after spreading out from the impact site, which looks like a point source of energy at the scale of the whole earth. Models to date have used  a spherically symmetric earth and did not include subtlties such as the elliptical shape of the earth or continents. These models have suggested that there could have been at least 10 meters (33 feet) of shattered uplifted rocks at the antipode.

 In a new paper in the October 2011 issue of Geophysical Journal International, Meschede et al. have combined a detailed 3-D model of the earth's interior and crustal structure with numerical calculations of the propagation of seismic waves around the earth after an impact.*** They model the impact as a single-force point source from the impact of a stony meteorite 20 km in diameter impacting at 20 km/sec.  They use a Gaussian source-time function to model the duration of the event, and assume that 0.001 to 0.0001 of the meteorite's energy ends up in the seismic waves that propagate away from the point of impact. In their model, the earth has a solid inner core, fluid outer core, ellipticity, topography and bathymetry, oceans, and rotation. The average node spacing is 10 km (this must have been done on a humongous computer!).  Since the continents were in different positions 65 million years ago, they chose an impact position to mimic it's position relative to the Eurasian and American continents at that time. The ancient antipode position was north of Australia. 
They found that it takes about 1.5 hours for the waves to reach the antipode.  The maximum displacement was calculated to be 4 meters, less than half that of the older models.  The structure of the displacement field is not symmetric, but has a starfish rayed shape because of heterogeneities in the crust, such as the thick seismically slow crust of the Andes.  In vertical cross section down to the base of the mantle, there are "chimneys" of peak stress, regions where stresses are concentrated.

A number of researchers have asked "what happened on the side of the earth opposite the place where the meteorite hit in the Yucatan (Mexico) 65 million years ago?" This point is called the "antipode." The earth is a sphere, and acts like a lens to focus seismic waves on the opposite side of the planet. Body waves travel through the interior and are focused by reflection and refraction off of boundaries such as the crust-mantle, mantle-outer core, etc. Surface waves converge at the antipode after spreading out from the impact site, which looks like a point source of energy at the scale of the whole earth. Models to date have used  a spherically symmetric earth and did not include subtlties such as the elliptical shape of the earth or continents. These models have suggested that there could have been at least 10 meters (33 feet) of shattered uplifted rocks at the antipode.

Peak displacements in the impact hemisphere (left) and the antipode (right).
 In a new paper in the October 2011 issue of Geophysical Journal International, Meschede et al. have combined a detailed 3-D model of the earth's interior and crustal structure with numerical calculations of the propagation of seismic waves around the earth after an impact.*** They model the impact as a single-force point source from the impact of a stony meteorite 20 km in diameter impacting at 20 km/sec.  They use a Gaussian source-time function to model the duration of the event, and assume that 0.001 to 0.0001 of the meteorite's energy ends up in the seismic waves that propagate away from the point of impact. In their model, the earth has a solid inner core, fluid outer core, ellipticity, topography and bathymetry, oceans, and rotation. The average node spacing is 10 km (this must have been done on a humongous computer!).  Since the continents were in different positions 65 million years ago, they chose an impact position to mimic it's position relative to the Eurasian and American continents at that time. The ancient antipode position was north of Australia. 
They found that it takes about 1.5 hours for the waves to reach the antipode.  The maximum displacement was calculated to be 4 meters, less than half that of the older models.  The structure of the displacement field is not symmetric, but has a starfish rayed shape because of heterogeneities in the crust, such as the thick seismically slow crust of the Andes.  In vertical cross section down to the base of the mantle, there are "chimneys" of peak stress, regions where stresses are concentrated.

The calculated stresses from the impact are comparable to stress drops observed in moderate to large earthquakes, prompting the authors to speculate that there could have been earthquakes in response to the seismic waves propagating away from the impact.

The calculated stresses from the impact are comparable to stress drops observed in moderate to large earthquakes, prompting the authors to speculate that there could have been earthquakes in response to the seismic waves propagating away from the impact. They say that the stresses are probably large enough to trigger volcanism, and that the seismic waves are large enough over areas of the ocean to induce tsunamis.


***Meschede, M.A., Myhrvold, C.L., and Tromp, J., Antipodal focusing of seismic waves due to large meteorite impacts on Earth, Geophysical Journal International, 187, 529-537, 2011.

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