IDENTIFYING MAJOR PALEOSEISMIC EVENTS IN PUERTO RICO BY DATING BROKEN SPELEOTHEM

Two large, ancient seismic events of Puerto Rico were identified, and time of occurrence bracketed, by dating pre-breakage-event stalagmite growth in caves, and post-event regrowth on the same fallen stalagmite. The older earthquake was dated between 62000-77000 yBP, and a second of lower resolution between 13000-47000 yBP.

Puerto Rico is at the plate margin of the Caribbean and Atlantic, has a history of large earthquakes, and contains numerous caves with damaged speleothem. The ground acceleration/ magnitude of seismic activity capable of damaging certain cave deposits (such as stalagmites) generally exceeds M7. Stalagmites often grow for 10⁵years, and are suitable for high resolution U-series dating.

A cave in the extensive karst was chosen for investigation because of scores of broken speleothems, and the absence of other likely causes of breakage [water flow, sediment creep and subsidence, or human and animal activity], that leaves seismic activity as the most likely cause. An analysis of the fall orientations of 39 stalagmites also showed them to be non-random, in a few preferred directions. Stalagmites with regrowth on the original stump were of interest: because the top of a broken stalagmite may have stopped growing at an unknown time before breakage, regrowth was seen as a good indication that the feature was still growing at the time of breakage. Dating the original top of the broken feature, and the bottom of the new stalagmite regrowing on the broken stump, is likely to bracket the seismic event, hopefully within a short time interval.

Drill coring [1/2 inch diameter] was chosen (in preference to complete removal of the specimen) to preserve as much of the original stalagmites as possible for future research. Coring provides sufficient material for dating, but allows for fewer date replications. Four specimens were dated at the University of Minnesota; both pairs of dates were in sequence- the fallen tops were older than the regrowth bottoms- and gave the results cited above. Changes in the procedure of selecting and sampling stalagmites will greatly improve the bracketing precision: some weathering of the original tops of the fallen stalagmites likely contributed to older dates for time of breakage, and the removal of larger samples and/or thicker cores would give better estimates for the stumps beneath regrowth.