Caspian sea level change impacts regional seismicity

Abstract

The Caspian Sea is the largest internal drainage body of water on Earth. It has a number of unique features, not least the considerable water level fluctuations during the 20th century in response to variations in the volume of water in the sea, affected in part by human activities in the Volga River catchment. Tectonic changes in storage volume of the basin have had negligible effect compared to climatic and human factors. The causes of water level variations have been investigated frequently in the past, but their influences on the seismic regime have not previously been considered. However, the changes involve a huge volume of water and occur in a region with a high intrinsic level of tectonic activity. This is analogous to the problem of Reservoir Induced Seismicity (RIS). We have therefore investigated how changes in the Caspian Sea Level (CSL) affect the seismicity. We find RIS signals in the Caspian Sea region through consideration of anomalous variations in the seismic regime. We support this idea by finding an inverse correlation between the temporal variations in b-value and CSL changes. The b-value is the slope of the Gutenberg–Richter power law relationship by which the size distribution of earthquakes is described. This parameter is commonly used to describe the relative number of large and small earthquakes. Given that changes in the b-value are inversely correlated to changes in stress level we infer that CSL changes have influenced the stress levels in the region leading to changes in the seismic regime.