Estimating the magnitude of completeness for assessing the quality of earthquake catalogue of the ENSN, Egypt

Abstract

After the Cairo Earthquake in 1992 with moderate magnitude (Ms 5.9), the government established the Egyptian National Seismological Network (ENSN) with 66 short-period and broadband stations, organized by National Research Institute of Astronomy and Geophysics (NRIAG). Around 55,000 local events were recorded from 1997 to 2014. The aim of this study is to assess the magnitude of completeness (M c) of an instrumental earthquake catalogue. Three different methods named the maximum curvature (MAXC), b value stability (MBS), and entire magnitude range (EMR) were applied to calculate M c and the results are compared together; all of these used methods are catalogue-based methods, in which M c is estimated based on departure from the linear frequency-magnitude relation of the local earthquakes published in the ENSN catalogue. Accurate knowledge of the magnitude of completeness M c is essential for many seismicity-based studies and particularly for mapping out seismicity parameters such as the b value of the Gutenberg-Richter relationship. The difference in M c between the three used methods is 0.9. However, tests performed on the diverse data sets presented here had confirmed that the EMR method is the more accurate method to estimate M c. Estimating M c solely based on the frequency-magnitude distribution (FMD) has some obvious drawbacks like the difficulty of estimating M c in areas of low seismicity. The final results showed that the MAXC technique (M c = 1.6 ± 0.02) needs fewer events to reach a stable result. While the MBS results (M c = 2.5 ± 0.04) gave always higher M c values compared with the other two methods, comparing to the third method, the EMR results (M c = 1.8 ± 0.03) maximize the amount of data available for the M c determination, which should serve to stabilize the M c estimates. This study will play an important role in the future reconfiguration of the ENSN stations.