To detect the precursors of deep groundwater changes related to anticipated large earthquakes off Miyagi Prefecture, we selected two deep boreholes located on the hanging walls of active reverse faults, and established an observation network with a real-time, automatic data-acquisition system. After taking into account the hydrological conditions and the effects of barometric pressure, the earth s tide, and rainfall, as well as the resolution limits of temperature measurements of groundwater for the two boreholes, we identifi ed plausible earthquake-related groundwater changes during the observation period ( April to December ). One of the events was a preseismic . °C fall in groundwater temperature that began . hours prior to the main shock (M J . ) of the December off-Miyagi Prefecture earthquake. The second event was a preseismic .°C rise in groundwater temperature that started hours before a shallow earthquake (M J . ) located beneath Sendai Bay on April . The third event was a coseismic temperature fall by as much as . °C associated with a shallow earthquake (M J ) on May , located close to one of the observation sites. The resolution limit of our temperature observation system indicates that it has the potential to detect the precursors of future large earthquakes off Miyagi Prefecture if associated with preseismic slip as large as that for a M J . earthquake.
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