Meteorite Impact “Earthquake” Features (Rock Liquefaction, Surface
Wave Deformations, Seismites) from Ground Penetrating Radar (GPR) and
Geoelectric Complex Resistivity/Induced Polarization (IP) Measurements,
Chiemgau (Alpine Foreland, Southeast Germany)
- Kord Ernstson,
- Poßekel Jens
Abstract
Densely spaced GPR and complex resistivity measurements on a 30,000
square meters site in a region of enigmatic sinkhole occurrences in
unconsolidated Quaternary sediments have featured unexpected and
highlighting results from both a meteorite impact research and an
engineering geology point of view. The GPR measurements and a complex
resistivity/IP electrical imaging revealed extended subrosion
depressions related with a uniformly but in various degrees of intensity
deformed loamy and gravelly ground down to at least 10 m depth. Two
principle observations could be made from both the GPR high-resolution
measurements and the more integrating resistivity and IP soundings with
both petrophysical evidences in good complement. Subrosion can be shown
to be the result of prominent sandy-gravelly intrusions and extrusions
typical of rock liquefaction processes well known to occur during strong
earthquakes. Funnel-shaped structures with diameters up to 25 m near the
surface and reaching down to the floating ground water level at 10 m
depth were measured. GPR radargrams could trace prominent
gravelly-material transport bottom-up within the funnels. Seen in both
GPR tomography and resistivity/IP sections more or less the whole
investigated area is overprinted by wavy deformations of the
unconsolidated sediments with wavelengths of the order of 5 - 10 m and
amplitudes up to half a meter, likewise down to 10 m depth. Substantial
earthquakes are not known in this region. Hence, the observed heavy
underground disorder is considered the result of the prominent
earthquake shattering that must have occurred during the Holocene
(Bronze Age/Celtic era) Chiemgau meteorite impact event that produced a
60 km x 30 km sized crater strewn field directly hosting the
investigated site. Depending on depth and size of floating aquifers
local concentrations of rock liquefaction and seismic surface waves
(probably LOVE waves) to produce the wavy deformations could develop,
when the big disintegrated meteoroid (a loosely bound asteroid or a
comet of roughly estimated 1 km size) hit the ground. The observations
in the Chiemgau area emphasize that studied paleoliquefaction features
and wavy deformations (e.g. seismites) need not necessarily have
originated solely from paleoseismicity but can provide a recognizable
regional impact signature.