Estimation of geometrical spreading and investigation of some dynamic parameters for micro-earthquakes in Eastern Alborz using their spectral contents

In this study, the spectra of 14 micro-earthquakes occurred in 2007-2008 were investigated. The waveforms of these events were recorded with the local seismological networks (2007-2008 and 2008) of the Geological Survey of Iran and also of the Institute of Geophysics, University of Tehran. Magnitude (ML) range of these micro-earthquakes was 1.0-4.0. They were recorded from 2007/10/12 to 2008/12/30 and they were well located using 29 stations from the above-mentioned networks. Parameters related to the earthquakes sources, e.g. source dimension, scalar moment, P and S corner frequencies, dynamic parameter of stress drop and moment magnitude, which are applicable in seismology, are calculated and processed using their spectral contents. The exact values of the above mentioned parameters are not reasonably extractable, but changes in these parameters with time or space and comparison with each other could be virtually processed. They could be considered as physical characteristics and properties of the shallow earth crust. In this study, spectra of different events recorded with three components of one station and also the spectra of a specific event recorded in different stations in three components were processed. Processing concluded that there was a little difference in the corner frequency and high-frequency decay between micro earthquakes recorded in different stations. All the spectral diagrams in this study were computed using a Fast Fourier transform program. The relationship between corner frequency and sample duration, corner frequencies of P and S waves, amplitude and epicentral distance, scalar moment and source dimension and finally local magnitude and source slip of the earthquakes were investigated. We tested the effects of the signal truncation on spectra of some events. The geometrical spreading of the wave path medium was inversely proportioned to the hypocentral distance in the Eastern Alborz. The amplitude decay was about f-2 in the frequency (f) spectra. The maximum stress drop of these earthquakes was 126 bar (related to the earthquake of magnitude 4.0 occurred at 2008/7/16 over the most active Chashm thrust fault within the area) which was noticeably greater than the others. Finally, for the first time at the Eastern Alborz, an empirical relationship between the local magnitude of the earthquakes and their source slips was derived.