The use of an outlier detecting method in time series of continuous daily measurements of underground water level and temperature in earthquake prediction investigation

Abstract

Continuous daily measurements of underground water level and temperature have been monitored in four selected shallow wells in a seismically active area close to Thessaloniki, Greece. The aim was to study the changes in the underground water level and temperature with respect to earthquakes occurring in the same area during 1 January 1988 to 31 December 1989. A graphical method has been applied to analyze the time series in order to detect outliers. Our results reveal that a strong correlation exists the earthquakes occurred in the area, and the water level time series. The correlation was found to be weak in the case of the water temperature time series. Most of the outliers found correspond to a time interval varying between 1 and 7 days before or after the earthquakes. The magnitude of the underground water level change varies from 3 to 10 cm, and that of temperature from 0.3 to 0.8°C. The strongest outliers correlate to earthquakes of magnitude greater than 3.0. The software developed to detect outliers, use immediate preceding observations in the time series of continuous daily measurements, at the time of their occurrence. This is very important in studying precursory phenomena of impending earthquakes.