THE DOUBLE DIFFERENCE RELOCATION OF AFTERSHOCK SEQUENCE OF 2017 EZGELEH EARTHQUAKE

On November 12, 2017, an earthquake with moment magnitude of 7.3 occurred in vicinity of Sarpol-e Zahab town,Kermanshah, Iran. The main earthquake of November 12 has been proceeded by a large number of aftershocks; from that date until January 2019, 5078 earthquakes have been recorded by Iranian Seismological Center (IRSC). This earthquake has been the largest event in this region in the last hundred years, the previous largest event in the area had occurred in 1909 AD in Silakhor with magnitude of 7.4. In this study in order to relocate 2017-2019 earthquakes in the region, double difference earthquake location method of Waldhauser and Ellsworth (2000) is applied. The double-difference algorithm minimizes errors due to unmodeled velocity structure without the use of station corrections. The method incorporates ordinary absolute travel-time measurements and/or cross-correlation P-and S-wave differential travel-time measurements. Two waveforms recorded at the same station are considered similar when half of the squared coherency value, in a specified frequency range and time window is exceeding the chosen correlation coefficient (Waldhauser and Ellsworth, 2002). Residuals between observed and theoretical travel-time differences are minimized for pairs of earthquakes at each station while linking together all observed event-station pairs. Using the cross correlation method on similar seismograms to obtain accurate phase arrivals, could reduce the location uncertainty that will lead to more efficient detection of waveforms that are more similar to already known events (Ottemȍ ller and Thomas, 2007). In this study, In order to improve the image of the seismogenic features, relocation has been done with both catalog data (P and S phases) and cross correlations data (P phases) combined, for simultaneous relocation for all events. Using Cross Correlation has improved the accuracy of the phase picking (Figure 1).

Keywords:

Earthquake relocation , Double difference method , Waveform cross correlation method , Ezgeleh earthquake , Aftershock distribution