Abstract:
In this study, an empirical magnitude calibration method developed by Mayeda et al., (2003) was tested by using both local and regional dista1tce earthquakes that occurred along the North Anatolian Fault Zone as well as throughout the broader region of Turkey. The method is based on source spectra that are derived from time-domain amplitude measurements of coda envelopes for 14 consecutive narrow frequency bands ranging between 0.02-8.0 Hz. Previous application of the methodology to earthquakes in the western United States and Dead Sea Rift regions (Mayeda and Walter, 1996; Mayeda et al., 2003) show that the moment magnitudes, Mw(coda), based on coda envelopes are significantly more stable and unbiased than conventional narrowband regional magnitudes which are obtained from direct phase measurements. Coda envelopes have a number of properties that make it a desirable choice for sparse station monitoring: 1) the coda envelopes are nearly insensitive to the radiation pattern and directivity of the earthquake source, 2) the coda is not as sensitive to lateral crustal heterogeneity because of the crustal averaging due to scattering, 3) clipped data can be used for fitting the envelopes after the clipped portion of the seismogram. During the calibration study, the empirical magnitude calibration method was applied in order to calibrate the three broadband stations, ISP, ISK and MALT in Turkey. Therefore, 182 common events, which mostly occurred along· the North Anatolian Fault zone near the Marmara region and recorded at both stations ISP and ISK, 137 common earthquakes, which are distributed over the broader region of Turkey recorded by stations ISP and MALT, were used. After applying the method to both pairs of datasets, consistent source spectra that were validated by equivalent seismic moment estimations from long period waveform modeling and derived moment magnitudes (Mw) were obtained. This study resulted in amplitude measurements that are a factor of 3 to 4 less variable than distance-corrected direct wave measurements (i.e., Pg, Lg, and surface waves). Upon comparing our coda-derived moment magnitude, Mw(coda)'s with those from long-period waveform modeling, Mw(waveform), a standard deviation of 0.17 for ISP-ISK and 0.14 for ISP-MALT station pair was 'Observed comparable to the results of previous studies (Mayeda and Walter, 1996; Mayeda et al., 2003). After calibrating the stations ISP, ISK and MALT, for some recent earthquakes such as the Piiltimtir earthquake of January 27, 2003, Mw = 6.1, the Urla earthquake of the Apri110, 2003, Mw = 5.7 and the Bingol earthquake of the May 1, 2003, Mw = 6.4, Mw(coda) values were estimated. The successful application of the method is remarkably important considering we are studying a much larger region with significant lateral complexities. With these calibrations we can extend the measurement of stable Mw to significantly smaller events, which could otherwise not be waveform, modeled due to poor signal-to-noise ratio.
