Tunguska and repeated impacts

[Rothiemurchus]

When the forest was flattened in Tunguska witnesses said the ground shook at widely spaced intervals.Was this because of repeated impacts from space or because a shock wave inside the Earth was bouncing around in the mantle.Did some impact debris stay inside the Earth? Does the fact that the impact happened in the north suggest that the Earth's magnetic field guided an object that could respond to magnetism? Has a similar impact happened in the south around or on Antarctica in the past?

 

[billiards]

I don't know much about this particular impact, and I don't know the details of repeated seismicity. What I do know however is that ground shaking is associated with surface waves which don't bounce around in the mantle (although they are composed of waves which do (multiples)), so the short answer is no it's not because a shock wave was bouncing around the mantle. I also doubt that it was due to repeated impacts, unless of course something caused the impactor to break up into lots of smaller impactors before it hit the earth. Most likely it can be explained by the same mechanism which causes aftershocks associated with typical seismic events, after an earthquake occurs the stress field is altered in the vicinity, this alteration can often trigger failure or slip which generates a new earthquake.

Yes the impact debris probably stayed on/in the earth, it certainly wouldn't have escaped the earth/moon system (when you say impact debris I take it you mean the actual bolide (i.e. impactor) material, although the same does apply for excavated Earth material).

No, the Earth's magnetic field is actually quite weak, at least it's certainly not strong enough to guide an impactor.

Yes, impacts have almost certainly occurred 'in the south or on Antarctica in the past'. Most meteorites are found at the poles, although this is only really because they are much easier to find on a 'blank canvas' so to speak (i.e. the ice). In fact, Finland has the most recorded impact sites, however this is only because they set up a special team to identify all the potential meteorite sites in the country because they realized that they were often excellent places for mining - no other country has bothered. If you took the statistics at face value then you could conclude that meteorites are repelled by sea water, there have been no impacts successfully identified in the oceans.

 

[Rothiemurchus]

BILLIARDS wrote:
"the Earth's magnetic field is actually quite weak, at least it's certainly not strong enough to guide an impactor"

The force the Earth's magnetic field could exert on an impactor would be greater if the impactor had a large electric charge or magnetic field of its own and also would be greater if the impactor was traveling very fast.
Also if the impactor had a high charge/mass ratio it would be affected by the Earth's magnetic field more and would be displaced a bigger distance.


Do shockwaves travel to the Earth's core and bounce off it back to the mantle and crust?