A rapid estimation of moment magnitude mw for the October 23, 2011 Van earthquake using strong-motion records

It is well known that the Richter scale (local magnitude scale, ML) based on the measurement of the amplitude in a narrow- band time domain saturates for large crustal earthquakes, and the possibility of observing clipping on seismograms is high. Therefore it is insufficient to use the local magnitude for reliable magnitude estimation. A typical example of this situation is confronted in case of the October 23, 2011 Van earthquake ( ML 6.6 vs Mw 7.2) . Here Mw is the moment magnitude obtained from the far-field displacement spectrum of body waves. On the other hand, a better match is expected in the range of 3 < ML < 7 because in this range the empirical relation between seismic moment M0 and ML is almost the same with the relation between M0 and Mw (Thatcher and Hanks, 1973; Hanks and Kanamori, 1979). Hence, there is a need to find a way for the determination of Mw from near-field records too.The aim of this study is to determine moment magnitude of earthquakes very soon after occurrence, before any source or focal mechanism inversions have been performed. In this respect (Delouis et al.,2009) proposed the MWSYNTH method. In this method, observed displacement spectra are compared with synthetic spectra computed for a variety of finite dimension source models scaled with Mw. In this study we have tried to obtain a rapid estimation of Mw for the October 23, 2011 Van earthquake from the strong-motion records in the epicentral distances about 120km using MWSYNTH method. Using strong motion records of only two stations we obtained an agreeable estimation of moment magnitude Mw=6.9. The result may be improved (approach more seriously to the reference moment magnitude Mw ref=7.15) by doing computation for more stations. And it is foreseen that a correct recovery of Mw for the Van Earthquake within about 100 sec after origin time (recording+computing time) will be possible if the procedure is automated.