In search of earthquake-related hydrologic and chemical changes along Hayward Fault |
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| Affiliation: | 1. Département de radiothérapie, centre Jean-Perrin, 58, rue Montalembert, 63000 Clermont-Ferrand, France;2. Département d’oncologie médicale, centre Jean-Perrin, 58, rue Montalembert, 63000 Clermont-Ferrand, France;3. Département de chirurgie, centre Jean-Perrin, 58, rue Montalembert, 63000 Clermont-Ferrand, France;1. School of Optoelectronics Engineering, Xi’an Technological University, Xi’an, China;2. Key Laboratory of Spectral Imaging Technology, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi’an, China;1. Hôpital Sud, CHU de Rennes, 16, boulevard de Bulgarie, 35200 Rennes, France;2. CH Yves-Le-Foll, 10, rue Marcel-Proust, 22000 Saint-Brieuc, France;3. Lactalis Nutrition Santé, Parc d’activités secteur Est, 35370 Torcé, France;4. Lactalis Recherche et Développement, 8, Fromy, 35240 Retiers, France;1. School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK;2. Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, GR 15784, Panepistimiopolis, Athens, Greece;3. Geochronology and Tracers Facility, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK;1. Limnological Institute, Siberian Branch of the Russian Academy of Sciences, Ulan-Batorskaya Str., 3, Irkutsk, 664033, Russia;2. Institute of the Earth''s Crust, Siberian Branch of the Russian Academy of Sciences, Lermontov Str., 128, Irkutsk, 664033, Russia;3. Renard Centre of Marine Geology, Department of Geology, Ghent University, Krijgslaan 281 S8, B-9000 Gent, Belgium;4. Baikal Division of the Geophysical Survey, Federal Research Center of the Russian Academy of Sciences, Lermontov Str., 128, Irkutsk, 664033, Russia;6. Institut des Sciences de la Terre de Paris (ISTeP), Université Pierre et Marie Curie (UPMC/CNRS), Case Courrier 129 - 4, Place Jussieu, 75252, Paris Cedex 05, France;7. Institute of Strength Physics and Materials Science, Siberian Branch of the Russian Academy of Sciences, pr. Akademicheskii, 2/4, Tomsk, 634055, Russia |
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| Abstract: | Flow and chemical measurements have been made about once a month, and more frequently when required, since 1976 at two springs in Alum Rock Park in eastern San Jose, California, and since 1980 at two shallow wells in eastern Oakland in search of earthquake-related changes. All sites are on or near the Hayward Fault and are about 55 km apart. Temperature, electric conductivity, and water level or flow rate were measured in situ with portable instruments. Water samples were collected for later chemical and isotopic analyses in the laboratory. The measured flow rate at one of the springs showed a long-term decrease of about 40% since 1987, when a multi-year drought began in California. It also showed several increases that lasted a few days to a few months with amplitudes of 2.4 to 8.6 times the standard deviations above the background rate. Five of these increases were recorded shortly after nearby earthquakes of magnitude 5.0 or larger, and may have resulted from unclogging of the flow path and increase of permeability caused by strong seismic shaking. Two other flow increases were possibly induced by exceptionally heavy rainfalls. The water in both wells showed seasonal temperature and chemical variations, largely in response to rainfall. In 1980 the water also showed some clear chemical changes unrelated to rainfall that lasted a few months; these changes were followed by a magnitude 4 earthquake 37 km away. The chemical composition at one of the wells and at the springs also showed some longer-term variations that were not correlated with rainfall but possibly correlated with the five earthquakes mentioned above. These correlations suggest a common tectonic origin for the earthquakes and the anomalies. The last variation at the affected well occurred abruptly in 1989, shortly before a magnitude 5.0 earthquake 54 km away. |
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