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.

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Quinones permeate our biotic environment, contributing to both homeostasis and cytotoxicity. All quinones generate reactive oxygen species through redox cycling, while partially substituted quinones also undergo arylation (Michael adduct formation) yielding covalent bonds with nucleophiles such as cysteinyl thiols. In contrast to reactive oxygen species, the role of arylation in quinone cytotoxicity is not well understood. We found that the arylating quinones, including unsubstituted 1,4-benzoquinone (1,4-BzQ) and partially substituted vitamin E congener ␥-tocopherol quinone (␥-TQ), were cytotoxic, with ␥-TQ > 1,4-BzQ, whereas the fully substituted nonarylating vitamin E congener ␣-tocopherol quinone was not. In vitro, both arylating quinones formed Michael adducts with the thiol nucleophile N-acetylcysteine (NAC) at rates where 1,4-BzQ > ␥-TQ. In cultured cells, concurrent addition of NAC eliminated 1,4-BzQ caused toxicity, but preincubation was required for the same NAC detoxification effect on ␥-TQ. These data clearly established the role of arylation in quinone toxicity and revealed that arylating quinone structure affects cytotoxicity by governing detoxification through the rate of adduct formation. Furthermore, arylating quinones induced endoplasmic reticulum (ER) stress by activating the pancreatic ER kinase (PERK) signaling pathway including elF2␣, ATF4, and C͞EBP homologous protein (CHOP). Detoxification by NAC greatly attenuates CHOP induction in arylating quinone-treated cells, suggesting that ER stress is a cellular mechanism for arylating quinone cytotoxicity.quinone adduction ͉ thiol nucleophiles ͉ tocopherols ͉ CHOP ͉ cytotoxicity Q uinones and their phenolic precursors are present throughout the biotic environment and include polyphenols and tocopherols in the diet, drugs in medicine, environmental pollutants such as polycyclic aromatic hydrocarbons, and their metabolic products (1-8). They are involved in a wide variety of biological and chemical processes, including electron transport in animals and plants, photosynthesis, posttranslational modification of proteins, metabolism of cellular signaling molecules such as estrogens and catecholamines, metabolism of antioxidant and signaling tocopherol congeners (vitamin E), and the elimination of polycyclic aromatic hydrocarbons introduced by combustion processes associated with our petroleum-based chemical environment.Quinones are a class of highly reactive compounds. Although all quinones are redox cycling agents that generate reactive oxygen species (ROS), partially substituted quinones also function as arylating agents (1-3, 5, 6). The arylating quinones react with cellular nucleophiles such as thiols on cysteine residues of proteins, glutathione (GSH), and detoxifying agents such as N-acetylcysteine (NAC), forming covalently linked quinonethiol Michael adducts (1-3, 5, 6) that retain the ability to function as redox cycling agents (4, 9). In contrast to well studied ROS generation and consequent oxidative stress in living cells (1-3), the role o...

[1] Lower crustal earthquakes are commonly observed in continental rifts at depths where temperatures should be too high for brittle failure to occur. Here we present accurately located earthquakes in central Ethiopia, covering an incipient oceanic plate boundary in the Main Ethiopian Rift. Seismicity is evaluated using the combination of exceptionally well resolved seismic structure of the crust and upper mantle, electromagnetic properties of the crust, rock geochemistry, and geological data. The combined data sets provide evidence that lower crustal earthquakes are focused in mafic lower crust containing pockets of the largest fraction of partial melt. The pattern of seismicity and distribution of crustal melt also correlates closely with presence of partial melt in the upper mantle, suggesting lower crustal earthquakes are induced by ongoing crustal modification through magma emplacement that is driven by partial melting of the mantle. Our results show that magmatic processes control not only the distribution of shallow seismicity and volcanic activity along the axis of the rift valley but also anomalous earthquakes in the lower crust away from these zones of localized strain.

We present analysis of new gravity data to produce a 2D crustal and upper mantle density model across the northern Main Ethiopian Rift (NMER). The magmatic NMER is believed to represent the transitional stage between continental and oceanic tiffing. We conclude that beneath our profile, magma emplacement into the upper crust occurs in the form of a 20 kmwide body beneath the axis of the rift, and a 12 km-wide off-axis body beneath the NW margin of the rift. These are coincident with Quaternary volcanic chains, anomalies in seismic velocity and conductivity identified by the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) along the same profile. We also identify a shallow, high-density body beneath the axial Boset volcano interpreted as either a dyke zone or a magma reservoir that may have fed Quaternary felsic volcanism. Our results provide supporting evidence for a c. 15 km-thick mafic underplate layer beneath the northwestern rift flank, imaged by the EAGLE controlledand passive-source seismic data. A relatively low-density upper mantle is required beneath the underplate and the rift to produce the long wavelength features of the gravity anomaly. The resulting model suggests that the lithosphere to the SE of the rift is unaffected by rifting processes. Our results combined with those from other EAGLE studies show that magmatic processes dominate rifting in the NMER.