INDUCED INTRAPLATE EARTHQUAKES IN COLORADO FROM EXTREME SEISMIC WAVES FROM THE END-CRETACEOUS ASTEROID OR COMET IMPACT

The end-Cretaceous impact in the Yucatan/Caribbean region generated extreme seismic waves larger than those produced by ordinary earthquakes. Shaking with periods of tens of seconds with particle velocities of ~3 m/s continued for hundreds of seconds. One would expect that such extreme shaking induced earthquakes and that it caused ground damage. (1) With regard to induced earthquakes, we found two outcrops of syn-impact fault displacement near Trinidad, Colorado. The nearby ~1 km apart localities at Long’s Canyon and Madrid Canyon roadcut may represent the effect of a single event. The normal displacement was ca. 1 m. The faults appear to have slipped just once, cutting strata below the impact horizon but not above it. The event was crudely magnitude 6. (2) The dynamic strains of ~10^-3 were large enough for shallow sedimentary rock to fail nonlinearly, but too small to produce strain markers that survived subsequent sediment compaction. However, grain-scale cracks associated with nonlinear failure likely increased permeability. Sulfate-bearing water likely mixed with sulfate-free barium-bearing water at the swampy land surface. Heavy mineral separations to obtain chromite from the impact horizon also revealed numerous barite grains possibly from this process. (3) We found ca. 30 cm long and ca. 1 cm wide vein entering the impact horizon from below. Local dewatering of the shallowest sediment likely occurred. The vein subsequently buckled from sediment compaction. A third outcrop exhibiting likely K-Pg impact related faulting was found at the Raton, NM K-Pg boundary site.

In general with regard to induced seismicity, extreme shaking tends to induce earthquakes that relax ambient tectonic stress. Intraplate tectonic stresses and interplate earthquakes are ubiquitous, however the ubiquitous plate boundary earthquakes do not produce sufficiently strong waves to trigger intraplate earthquakes thus relieving the intraplate tectonic stresses. The K-Pg impact did produce sufficiently strong seismic waves to trigger such intraplate earthquakes.