Main features of hydrogeodynamic earthquake precursors

Abstract

An analysis of available data on hydrogeodynamic precursors of large earthquakes in different regions of the world is presented. These precursors were registered by variations of groundwater level before 31 earthquakes with magnitudes between 4.5 and 8.0. We investigated some features of the precursors, such as the signal amplitude, shape and time, as well as some precursor indices, as functions of magnitude and epicentral distance. In most cases, groundwater level variations before earthquakes have extreme values. A new index, the precursor extremum time, has been defined for such precursors. It has been found that this index, as well as the amplitude of groundwater level variations, correlates well with precursor time. The hydrogeodynamic precursor time and signal amplitude do not show sufficiently close correlative relationships with earthquake magnitude and epicentral distance. At the same time, there is a trend of increasing precursor time and amplitude with the growth of earthquake magnitude, and an inversed relationship for epicentral distance. These properties of hydrogeodynamic precursors seem to be associated with geodynamic processes developing in an extensive area during the preparation of a large earthquake. Manifestations of these precursors also depend on the tensosensitivity of the borehole-aquifer system which controls the transformation of strain into the hydrogeodynamic precursory response.