National Center for Earthquakes and Volcanoes, Earthquake Magnitude Calculations - 2006, Kingdom of Saudi Arabia
Elliott Endo and Seismic Center Staff
In the earthquake reporting process, an estimate of magnitude is essential information along with origin time, epicenter, and depth of origin. At the Saudi Geological Survey National Center for Earthquakes and Volcanoes (SGSNCEV), earthquake local magnitudes from broadband seismic records are estimated using a Wood-Anderson simulation for equivalent amplitudes and the U.S. Geological Survey (USGS) earthquake location program HYPOINVERSE for local magnitude calculations. The Wood-Anderson simulation is an amplitude measurement option in the Nanometrics earthquake analysis program ATLAS. From April to November 2006, over 300 earthquakes were selected for this magnitude study. Earthquakes ranged from local magnitudes of 2.9 to 5.6 and the closest reporting seismic stations ranged from 121 to 1057 km from the epicenters. Wood-Anderson equivalent local magnitudes were determined for 203 earthquakes. Differences between Euro-Mediterranean Seismic Center (EMSC)-reported magnitudes and ATLAS magnitudes were calculated and 172 or about 85 percent of the differences were ±0.25 or 1 STDEV about the mean of 0. The results show that the SGSNCEV has a good Wood-Anderson equivalent-magnitude calculator for earthquakes recorded with the Nanometrics broadband seismic system that is being used for the National Seismic network. With few exceptions, SGS-calculated local earthquake magnitudes are in close agreement with earthquake magnitudes reported by more experienced seismic networks. Using Wood-Anderson simulation for broadband seismic records we estimate the local magnitude for the Makkah, September 11, 2005 22:56 UT event as 1.9 ±0.25 magnitude unit.
Endo, E. and Seismic Center Staff, 2007, National center for earthquakes and volcanoes, earthquake magnitude calculations - 2006, Kingdom of Saudi Arabia: Saudi Geological Survey Technical Report SGS-TR-2007-1, 19 p., 20 figs., 3 apps.