RESUMO
Surface faulting earthquakes are known to cluster in time from historical and palaeoseismic studies, but the mechanism(s) responsible for clustering, such as fault interaction, strain-storage, and evolving dynamic topography, are poorly quantified, and hence not well understood. We present a quantified replication of observed earthquake clustering in central Italy. Six active normal faults are studied using 36Cl cosmogenic dating, revealing out-of-phase periods of high or low surface slip-rate on neighboring structures that we interpret as earthquake clusters and anticlusters. Our calculations link stress transfer caused by slip averaged over clusters and anti-clusters on coupled fault/shear-zone structures to viscous flow laws. We show that (1) differential stress fluctuates during fault/shear-zone interactions, and (2) these fluctuations are of sufficient magnitude to produce changes in strain-rate on viscous shear zones that explain slip-rate changes on their overlying brittle faults. These results suggest that fault/shear-zone interactions are a plausible explanation for clustering, opening the path towards process-led seismic hazard assessments.
RESUMO
IMPORTANCE: Phthalates are ubiquitous and many are known or suspected human reproductive and endocrine-disrupting toxicants. A data gap exists in reporting on biomonitoring of phthalate biomarkers in college-aged adults. OBJECTIVE: To analyze phthalate exposure in a cross-sectional sample of female college students using urinary phthalate metabolite concentrations and compare to reference populations including college-aged women sampled in the National Health and Nutrition Examination Survey (NHANES). METHODS: Nine monoester phthalate metabolites were analyzed in spot urine collected from 215 female undergraduates (age 18-22, 2016-2017) at a public university in Charleston, SC USA and a subset of participants completed a questionnaire detailing demographics and behaviors including personal care and cosmetic product use (e.g. in the past 6 or 24 hrs). Urine specific gravity was used to assess effect of urine dilution. Phthalate metabolite concentrations were compared to reference populations and the temporal trends of the same age-group in the National Health and Nutrition Examination Survey (NHANES) were analyzed. RESULTS: Total urinary phthalate metabolite concentrations in individuals ranged three orders of magnitude (geometric mean 56.6 ng/mL, IQR 26.6-114 ng/mL). A third of urine samples had relatively high urine specific gravity levels indicating potential dehydration status. All geometric mean concentrations were similar to the U.S. female population in the most recent NHANES cycle (2015-2016) except for MEP and mono-isobutyl phthalate (MiBP). Relatively low MEP and MiBP may be explained by a time trend of declining MEP in the general U.S. population, the sociocultural character of this cohort, and the time of day of spot sampling in evening. NHANES data indicate a significant effect of sample timing on phthalate metabolite concentrations and decline in most, but not all, phthalate metabolites sampled in women aged 18-22 years over the decade (2005-2016). SIGNIFICANCE: This study reports phthalate metabolites in college-aged women, an understudied group, emphasizes the benefit of survey information for interpreting biomonitoring data, and is a useful case study for communicating phthalate chemical exposure risks to college students.
