Abstract

Large water level changes were observed in six bore wells in the Koyna region in western India following the Great Sumatra earthquake of Mw 9.3 on 2004 December 26. This earthquake occurred at 00:58:50 coordinated universal time (UTC) off the coast of Sumatra located about 3000 km from the Koyna region. The arrival time of the P wave recorded at the Koyna seismic station was 01:04:24.45 (UTC). The anomalous water level changes were observed at 01:15 (UTC) at all of these wells where the sampling rate was 15 min. No abnormal changes were noticed in the preceding sampling time at 01:00 (UTC), clearly indicating that these changes occurred due to the passage of seismic waves generated by the Sumatra earthquake. However, due to the large sampling interval, it is difficult to correlate the water level changes either to shear or surface waves. No local earthquake was recorded by the network during this period. The anomalous water level changes were of the order of 2–65 cm and show either a spikelike or a steplike change. These types of changes are attributed to the dynamic strain induced by the passage of seismic waves, most probably long-period surface waves.

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