No abstract

Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

We present a new approach to identifying the source and age of paleofluids associated with low-temperature deformation in the brittle crust, using hydrogen isotopic compositions (dD) and 40 Ar/ 39 Ar geochronology of authigenic illite in clay gouge-bearing fault zones. The procedure involves grain-size separation, polytype modeling, and isotopic analysis, creating a mixing line that is used to extrapolate to dD and age of pure authigenic and detrital material. We use this method on samples collected along the surface trace of today's North Anatolian Fault (NAF). dD values of the authigenic illite population, obtained by extrapolation, are 289 6 3&, 290 6 2&, and 297 6 2& (VSMOW) for samples KSL, RES4-1, and G1G2, respectively. These correspond to dD fluid values of 262& to 285& for the temperature range of 1258C 6 258, indistinguishable from present-day precipitation values. dD values of the detrital illite population are 245 6 13&, 260 6 6&, and 264 6 6& for samples KSL, G1G2, and RES4-1, respectively. Corresponding dD fluid values at 3008C are 226& to 245& and match values from adjacent metamorphic terranes. Corresponding clay gouge ages are 41.4 6 3.4 Ma (authigenic) and 95.8 6 7.7 Ma (detrital) for sample G2 and 24.6 6 1.6 Ma (authigenic) and 96.5 6 3.8 Ma (detrital) for sample RES4-1, demonstrating a long history of meteoric fluid infiltration in the area. We conclude that today's NAF incorporated preexisting, weak clay-rich rocks that represent earlier mineralizing fluid events. The samples preserve at least three fluid flow pulses since the Eocene and indicate that meteoric fluid has been circulating in the upper crust in the North Anatolian Keirogen since that time.

Both the sources and pathways of fluid circulation are key factors to understanding the evolution of low-angle normal fault (LANF) systems and the distribution of mineral deposits in the upper crust. In recent years, several reports have shown the presence of meteoric waters in mylonitic LANF systems at mid-crustal conditions. However, a mechanism for meteoric water infiltration to these mid-crustal depths is not well understood. Here we report paired d 18 O and d 2 H isotopic values from dated, neoformed clays in fault gouge in major detachments of the southwest United States. These isotopic values demonstrate that brittle fault rocks formed from exchange with pristine to weakly evolved meteoric waters at multiple depths along the detachment. 40 Ar/ 39 Ar dating of these same neoformed clays constrains the Pliocene ages of fault-gouge formation in the Death Valley area. The infiltration of ancient meteoric fluids to multiple depths in LANFs indicates that crustalscale normal fault systems are highly permeable on geologic timescales and that they are conduits for efficient, coupled flow of surface fluids to depths of the brittle-plastic transition.

Illite polytypes are used to elucidate the geological record of formations, such as the timing and provenance of deformations in geological structures and fluids, so the ability to characterize and identify them quantitatively is key. The purpose of the present study was to compare three X-ray powder diffraction (Q-XRPD) methods for illite polytype quantification for practical application to directly date clay-rich fault rocks and constrain the provenance of deformation-related fluids in clay-rich brittle rocks of the upper crust. The methods compared were WILDFIRE© (WF) modeling, End-member Standards Matching (STD), and Rietveld whole-pattern matching (BGMN®). Each technique was applied to a suite of synthetic mixtures of known composition as well as to a sample of natural clay gouge (i.e. the soft material between a vein wall and the solid vein). The analytical uncertainties achieved for these synthetic samples using WF modeling, STD, and Rietveld methods were ±4–5%, ±1%, and ±6%, respectively, with the caveat that the end-member clay mineral used for matching was the same mineral sample used in the test mixture. Various particle size fractions of the gouge were additionally investigated using transmission electron microscopy (TEM) to determine polytypes and laser particle size analysis to determine grain size distributions. The three analytical techniques produced similar 40Ar/39Ar authigenesis ages after unmixing, which indicated that any of the methods can be used to directly date the formation of fault-related authigenic illite. Descriptions were included for pre-calculated WF illite polytype diffractogram libraries, model endmembers were fitted to experimental data using a least-squares algorithm, and mixing spreadsheet programs were used to match end-member natural reference samples.