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  • Earthquakes  (3)
  • 1995-1999  (3)
  • 1
    ISSN: 1420-9136
    Keywords: Earthquakes ; hydrogeology ; earthquake prediction ; crustal deformation ; aquifers ; earth tides ; strain ; hydraulic diffusivity
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Two of the four wells monitored near Parkfield, California, during 1985 showed water level rises beginning three days before theM w 6.1 Kettleman Hills earthquake. In one of these wells, the 3.0 cm rise was nearly unique in five years of water level data. However, in the other well, which showed a 3.8 cm rise, many other changes of comparable size have been observed. Both wells that did not display pre-earthquake rises tap partially confined aquifers that cannot sustain pressure changes due to tectonic strain having periods longer than several days. We evaluate the effect of partial aquifer confinement on the ability of these four wells to display water level changes in response to aquifer strain. Although the vertical hydraulic diffusivities cannot be determined uniquely, we can find a value of diffusivity for each site that is consistent with the site's tidal and barometric responses as well as with the rate of partial recovery of the coseismic water level drops. Furthermore, the diffusivity for one well is high enough to explain why the preseismic rise could not have been detected there. For the fourth well, the diffusivity is high enough to have reduced the size of the preseismic signal as much as 50%, although it should still have been detectable. Imperfect confinement cannot explain the persistent water level changes in the two partially confined aquifers, but it does show that they were not due to volume strain. The pre-earthquake water level rises may have been precursors to the Kettleman Hills earthquake. If so, they probably were not caused by accelerating slip over the part of the fault plane that ruptured in that earthquake because they are of opposite sign to the observed coseismic water level drops.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1420-9136
    Keywords: Earthquakes ; strange attractor ; deterministic chaos ; reservoir
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract Nurek, Aswan and Koyna reservoirs were affected by moderate earthquakes with continuing seismic activity. Microearthquake data recorded through local networks have been used to determine the strange attractor dimensions, using deterministic chaos which were found as 7,2, 3.8 and 4.8, respectively. This would imply that while 8 parameters are needed to model earthquakes near Nurek reservoir, only 4 to 5 parameters are needed for the Aswan and Koyna regions. The differences in the strange attractor dimension suggest them to be a measure of seismotectonics around such reservoirs.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1420-9136
    Keywords: Earthquakes ; water injection ; oil field
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract In China, the earthquakes induced by water injection have occurred in four oil fields including the Renqiu oil field, and in two mines. Production of oil from the Renqiu oil field began in 1975 and the injection of water into the oil field commenced in July 1976. The induced earthquakes have been occurring in the area for the past 17 years, since December 1976. The controlled experiments of water injection showed the cause and effect relation between water injection and earthquakes. Source parameters such as source dimension, seismic moment and stress drop of a large number of the induced earthquakes, andQ factor for the area have been determined. The results indicate that the stress drop varies from 0.2 to 3.0 bar and theQ factor has an average value of 75.0. The low-stress drop and lowQ factor values imply that the earthquakes are caused by the brittle fracture of weak rocks under low ambient stresses, due to a decrease in their strength because of the injection of water. The induced earthquakes are unevenly distributed in the oil field. The northern part of the oil field, where the reservoir rocks are characterized by low porosity and low permeability, exhibits high seismic activity with the largest earthquake registering a magnitude of 4.5 and about 68% of the total number of induced earthquakes in this part. Whereas, the southern part of the oil field with higher porosity and higher permeability is characterized by low seismic activity with the largest earthquake registering a magnitude of 2.5 and only 4% of the total number of earthquakes which occurred in this part. These features of the focal region suggest that larger earthquakes may not occur in the Renqiu oil field area.
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