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These veins of calcite reveal a record of fluid-driven earthquakes clustered together on a fault typically characterized by less frequent, periodic earthquakes caused by mechanical stress.They helped UW-Madison researchers trace the oldest and longest earthquake record ever documented. U-Th ages obtained by mass spectrometry in corals from Barbados: sea level during the past 130,000 years. Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U-Th ages from Barbados corals. Atmospheric radiocarbon calibration beyond 11,900 cal BP from Lake Suigetsu.

After doing so, the researchers report that earthquakes occurred at fairly regular intervals for most of the timeline they built, as was expected, but they also found that for one short period of time approximately 430,000 years ago, there was a cluster of larger-than-normal earthquakes.Credit: Laurel Goodwin and Randy Williams, UW-Madison The researchers note that their findings are similar to events happening in modern times due to human activities, such as fracking and other operations that involve forcing water underground—the end result is unnatural earthquakes occurring out of sync with a fault's natural cycle.Explore further: Garnet crystal microstructures formed during ancient earthquake provide evidence for seismic slip rates along a fault More information: Randolph T. Reading a 400,000-year record of earthquake frequency for an intraplate fault, Proceedings of the National Academy of Sciences (2017).DOI: 10.1073/pnas.1617945114 Abstract Our understanding of the frequency of large earthquakes at timescales longer than instrumental and historical records is based mostly on paleoseismic studies of fast-moving plate-boundary faults.Similar study of intraplate faults has been limited until now, because intraplate earthquake recurrence intervals are generally long (10s to 100s of thousands of years) relative to conventional paleoseismic records determined by trenching.

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