Thursday, February 12, 2015

Geo 1095: February 12, Day 773: Quake-Cracked Columns

Standing and zooming closer to the middle of yesterday's photo, we can see the answer to that post's "dance floor" puzzle. The stalagmites and columns are dancing a stately waltz of tectonics. We appear to be looking at a very low-angle normal fault, dipping to the left, with the top offset to the left. Speleothems are notoriously slow-growth features, and highly variable in different locations within the same cave. The fracture across the columns here has clearly been resutured, but not heavily enough to obscure it or its offset. This suggests fairly recent seismicity. I'd hazard a guess that this quake was in the neighborhood of hundreds to a few thousands of years ago, not recent in human terms, but recently enough that I wouldn't rule out more such quakes in the future. In terms of hazard? This is probably a safer environment than many in which to experience an earthquake, and the offset is quite small, which suggests a tiny quake. I'd bet it'd be startling to tourists, and terrifying to few, but the likelihood of injuries or fatalities is really pretty minimal (for a quake of this sort, anyway), in my non-professional estimation.

Photo unmodified. May 9, 2013. FlashEarth Location. (Since we're underground, I have only a vague idea where this is with respect to the surface.)

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