Human-swallowing fissures, a backward flowing Mississippi River, and so-called earthquake lights were just a few of the strange phenomena reported during the series of powerful intraplate earthquakes that occurred in the New Madrid Seismic Zone between December 1811 and February 1812.
Those earthquakes, which took place in Arkansas and along the Reelfoot reverse fault in Missouri and Tennessee, made history as some of the strongest to occur east of the Rocky Mountains, but many secrets remain about prior seismic activity in the New Madrid region.
“The earthquake record is just really sparse” in this area, says Jaime Delano, a geologist with the U.S. Geological Survey (USGS) Geologic Hazards Science Center in Golden, Colo. However, high-resolution airborne lidar data from this region have given Delano and her collaborators new insights into past shaking events in the New Madrid Seismic Zone, which they reported in a study in Geophysical Research Letters (GRL).
The lidar data revealed linear ridgetop spreading features like scarps, called sackungen, that hadn’t been previously described, says Ryan Gold, also a geologist with USGS in Golden and a coauthor of the study. The researchers mapped the sackungen on bluffs in the Mississippi River valley in northwestern Tennessee. Those features “likely form or are reactivated during large earthquakes,” the researchers wrote.
The sackungen were concentrated on the hanging wall of the Reelfoot fault, and their preferential orientation indicated ground motion perpendicular to the fault strike. These observations are consistent with the notion that at least one earthquake occurred on the southern portion of this fault since a layer of windblown sediment called the Peoria loess was deposited in this region approximately 11,000–30,000 years ago.
“It would be really cool to trench more of these” sackungen.Additional work by Delano, Gold, and their colleagues not reported in the GRL study agrees with this assessment. At the Geological Society of America’s (GSA) 2018 annual meeting in Indianapolis, Ind., Gold reported that paleoseismic trenching across sackungen revealed four packages of colluvial sediment that indicate past episodes of ground shaking and postdate the deposition of the Peoria loess. (Publication of these findings is forthcoming.)
The trenching site is located approximately 8 kilometers from Samburg, Tenn., a small town on the southeastern shore of Reelfoot Lake (which was formed as a result of the 1812 New Madrid earthquake along the Reelfoot fault, Gold notes).
The researchers performed radiocarbon and luminescence dating on samples collected from the four colluvial packages. They also measured the concentrations of lead and cesium isotopes in the samples.
On the basis of this dating, Gold reported in his GSA presentation a history of four earthquakes (or earthquake sequences) that occurred before 1860, one of which occurred thousands of years ago in the early to middle Holocene and the rest of which occurred after 340 CE.
“It would be really cool to trench more of these” sackungen to determine whether the properties of the sediments underneath them echo those of the collected samples, Delano says. Together, these studies set a precedent, she says.
The orientation of sackungen “could be used to infer the source fault for past earthquakes here and elsewhere,” the researchers wrote in GRL.
“This is another way of looking at the problem that may not have been considered yet,” Delano adds.
“Innovative and Comprehensive” WorkThe team’s work “is innovative and comprehensive, demonstrating yet another exciting application of high-resolution lidar topography for studying faults and ancient earthquakes,” says Samuel Johnson, a geoscientist at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, Calif., who was not involved with the study.
“Sackungen have been almost exclusively described in mountainous terrain, so their recognition and documentation in the far more subtle, low-relief, midcontinent Mississippi River valley region is especially interesting,” Johnson says.
Besides bolstering the region’s known seismic record, the team’s analysis underscores the difficulty of predicting future seismic activity for this region, given the variable frequency of past earthquakes. The “nonperiodic earthquake recurrence has important implications for seismic-hazard and geodynamic modeling in the New Madrid Seismic Zone,” the researchers wrote in the abstract for Gold’s GSA presentation.
—Rachel Crowell (racheljcrowell@gmail.com; @writesRCrowell), Science Writer
from Eos https://eos.org/articles/secrets-from-the-new-madrid-seismic-zones-quaking-past?utm_source=rss&utm_medium=rss&utm_content=secrets-from-the-new-madrid-seismic-zones-quaking-past
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