Rapid moment magnitude estimation for large earthquakes in Iran using time integration of absolute ground accelerations

A total of 324 strong ground-motion records from 26 earthquakes with moment magnitude greater than 6 were used to derive an adequate equation for moment magnitude estimation. A parameter called total effective shaking was used to introduce an empirical equation for determining the near real-time magnitude of the Iranian plateau. This parameter was obtained through time integration of the absolute acceleration values from accelerograms over the strong shaking duration. It can be calculated by a simple mathematical procedure 5 seconds after completion of the waveform by decreasing the amplitudes to less than 20% of the maximum ground acceleration. The total effective shaking has a dimension of velocity and corresponds to moment magnitude and hypocentral distance in an attenuation relationship. The optimum coefficients were calculated through least-square regression analysis. Also, the effect of site conditions was evaluated in the analysis. The average shear-wave velocity to a depth of 30 m beneath each recording station was taken into account as the local site effect for 147 records out of the total number of records. The estimated moment magnitudes are in reasonably good agreement with the Global CMT values. Their differences are mostly less than 0.25 in the magnitude unit.