AN ULTRA-TRACE-ELEMENT RIVER SIGNATURE OF BEDROCK FRACTURE PROCESSES TRIGGERED BY DISTANT SEISMIC EVENTS

Bedrock fractures exert major influences on the export of water and rock-derived elements from catchments and thus play vital roles in the functioning of watersheds and the critical zone. Fractures expose rock surfaces to groundwater, initiating the chemical weathering reactions that ultimately transform protolith to soil, and provide fluid flow pathways connecting groundwater and surface waters over large ranges in space and time. Fracture processes are thus integral to landscape evolution but our understanding of fracture dynamics is limited due in large part to the inaccessibility of the subsurface. Changes in fracture area and connectivity are expected to affect river chemistry but because rivers integrate myriad individual processes, and because water chemistry is affected by numerous forcings, dynamic signatures of fracture processes have not been identified.

Here we show that long-term, high-frequency measurements of the river concentration of the ultra-trace element thorium can provide a novel signature of bedrock fracture processes spanning neighboring watersheds in Colorado. Comparison of the patterns in Th excursions and seismicity strongly suggest that the formation or motion of stressed, high-permeability fractures can be initiated by ground motion caused by distant seismic events. This finding is the first chemical signature of the phenomenon of dynamic earthquake triggering, previously identified through geophysical methods. The use of high-frequency elemental data will complement emerging geophysical methods to reveal the architecture and impact of the deep fracture hydrology of the critical zone.