Application of a Magnitude Calibration Methodology based on the moment rate spectra of coda waves for local and regional earthquakes  in Turkey.

Project participants:
KOERI, Bogazici University : Prof. Niyazi Turkelli (P.I^*), Msc. Tuna Eken
Lawrence Livermore National : Dr. Kevin Mayeda (P.I^*), Dr. Rengin Gök, Jennifer O’Boyle Labarotory,
                                 USA
Geophysical Insitute of Israel, (GII) : Dr. Abraham Hofstetter  (P.I^*).

A Brief Description

During the present project, an empirical magnitude calibration method developed by Mayeda et al. (2003) was tested by using both local and regional distance 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 (Figure 1). 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 (Figure 2) 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 (Figure 3) (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 (Figure 4) 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 Pulumur earthquake of January 27, 2003, Mw =6.1, the Urla earthquake of the April 10, 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 remarkable considering the significant lateral complexity in Turkey and the simple assumptions used in the coda method.

^*: Principle Investigator

Publications and proceedings released during/after project

·        Eken T., K. Mayeda, A. Hofstetter, R. Gok, G. Orgulu, and N. Turkelli (2004), An Application of the Coda Methodology for Moment-Rate Spectra Using Broadband Stations in Turkey, Geophysical Research Lettters, Geophysical Research Lettters, Vol. 31, No.11, doi; 10.1029/2004GL019627.

·        Eken, T, 2003, An Application of the coda methodology for moment-rate spectra using Turkish broadband stations, Master Thesis, Bogazici University Library, Turkey, 2003.

·        Mayeda K., T. Eken, A. Hofstetter,N. Turkelli, J. O’Boyle, G. Orgulu, and R. Gok (2003), Moment Magnitude Calibration for the Eastern Mediterranen Region From Broadband Regional Coda Envelopes, Proceedings of the 25th Seismic Research Review: Nuclear Explosion Monitoring: Building the Knowledge Base Vol.1, pages between 420-427.

·        Mayeda K., T. Eken, A. Hofstetter, J. O’Boyle, N. Turkelli, and W. Walter (2002), Stable and Transportable Regional Magnitudes Based on Coda-Derived Moment-Rate Spectra, Proceedings of the 24th Seismic Research Review: Nuclear Explosion Monitoring: Innovation and Integration, Vol.1, pages between 517-521.
 

Figure 1. Example coda envelopes at f = 0.1–0.2 Hz demonstrating that the coda energy is homogeneously distributed in the crust. The two largest envelopes correspond to an event that is close to ISP (50 km) but far from ISKB (450 km), yet their envelope levels are the same, in contrast to the direct waves. Likewise, the two smaller envelopes are for another event, in this case located 60 km from ISKB and  400 km from ISP.

Figure 2. Map showing the epicentral distribution of events and stations used in this study. Gray circles indicate earthquakes used in the coda calibration procedure while reed squares represent ground-truth earthquakes for which we have independent moment magnitudes and blue stars represent recent earthquakes used for further validation of the method.

 Figure 3. Comparison between distance corrected direct and coda waves amplitudes at two different stations. a) Distance-corrected direct waves for common events at station ISKB and ISP for the 0.1–0.2 Hz band using the formulation of Street et al. [1975] where critical distance is 100 km, h = 0.5 and Q = 100; b) Distance-corrected coda amplitudes for roughly the same events as in (a) where the standard deviation is a factor of 4 times smaller. The total number of events is denoted as Num and σ is the data standard deviation. Note that the small DC offset is a result of relative site effect between the two stations.
 

Figure 4. a) Examples of coda-derived source spectra computed at stations ISP, ISKB, and MALT. We note that for those events with multiple station recordings, their source spectra are quite similar between the stations; b) Single-station coda-derived moment magnitudes agree with the long-period waveform modeled estimates with a data standard deviation of 0.17.
 

References

Mayeda, K., and W. R. Walter (1996).  Moment, energy, stress drop, and source spectra of western United States earthquakes from regional coda envelopes, J. Geophys. Res., 101, 11195-11208.

Mayeda, K., A. Hofstetter, J. O’Boyle, W. R. Walter (2003).  Stable and transportable regional
magnitudes based on coda-derived moment-rate Spectra, Bull. Seism. Soc. Am., 93, 224-239.