IS THE TEMPORAL PATTERN OF SEISMICITY IN THE ALBORZ MOUNTAINS, INFLUENCED BY SEASONAL PATTERN OF PRECIPITATION?

The Alborz Mountains, a major northern topographic highland of Iran, is a significant climatic frontier between the coastal plains of the humid Caspian lowlands and arid- semiarid Central Iranian Plateau. It produces an intense climate variety with a wide range of precipitation rate. The southern slope demonstrates a highly seasonal pattern in precipitation. The west Caspian lowlands with the greatest amount of precipitation are the wettest part of the Alborz, while the least precipitation occurs in the SE slops where access by the moist air masses appears to be severely restricted.

To constrain the climate forcing of seismicity in the Alborz Mountains, synoptics of precipitation in the range has been employed. Applying daily and monthly precipitation and discharge data derived form gauging stations operated by Ministry of Energy of Iran and Iranian Meteorological Organization measured in a period <40 years, the spatial-temporal pattern of annual-seasonal precipitation and discharge of the Alborz has been computed.

The seasonality is defined as the relative variability of seasonal precipitation proportions evolves with the annual total precipitation. When normalized by the annual total precipitation, the variability of seasonal precipitation is greatest where the annual total precipitation is smallest. Spatial distribution of the variability displays a well defined correspondence with annual precipitation.

In this study, we are looking for evidences of the climatic forcing of seismicity in the Alborz Mountains, northern Iran. Whether the seasonal variations of precipitation in the Alborz can be translated into the temporal-spatial distribution of the seismicity in the mountain range or not?

Institute of Geophysics, Tehran University seismic network, has been monitoring the study area since 1996. The network has a threshold of completeness of 2.5 in magnitude. We use all events larger than 2.5 recorded between 1996 and 2010. To remove the possible bias introduced by aftershocks, we decluster our catalog. Statistical analysis of the 15 years cumulative microseismicity demonstrates that the population of earthquakes in arid south flank of the Alborz is seasonal within the collectively same magnitude range. North flank of the Alborz displays a different temporal distribution with less seasonality. The periodicity of the discharge, precipitation and seismicity is estimated and cross correlated using Least Squares Spectral Analysis (LSSA). Also, we explore the lag time between enhanced precipitation and enhanced seismicity demonstrating spatial-temporal pattern of the precipitation and dynamics of vegetation and run off across the Alborz Mountains.