Magnitudes for local earthquakes calculated with the El Salvador seismic network

Abstract

The seismic network in El Salvador consists of mainly 19 telemetered short-period stations installed between 1983 and 1992. The network has a central digitizing unit installed in 1992; all data is digitally processed. The new digital data from El Salvador and other Central American countries has been used to test and calibrate different magnitude scales based on coda duration and amplitude. Due to the small dimension of the network in El Salvador, no attenuation function significantly different from the original Richter relation could be determined. The coda wave magnitude scale calibrated with Ml is: Me = 2.5 * log(coda) + 0.0016 * dist- 1.7 where coda is the coda duration in sees and dist is the hypocentral distance in km. Some data from other Central American countries have been used in order to get a better distance and magnitude range than provided by the El Salvador data. Using the same data set, moment magnitudes Mw were calculated based on P-wave spectra and compared to Ml and Mb from PDE. Average Ml, Mw and Mb were 4.9, 5.2 and 5.0 with data in the magnitude ranges 3.4-6.4, 4.1-6.2 and 3.6-6.0 respectively. The systematic difference between Ml and Mw for the same data set might indicate that the attenuation functions used are inadequate, that there are instrumental problems or that there is some systematic difference in the way Ml and Mw are determined. Despite the uncertainties, internaily consistent magnitudes in reasonable agreement with externally determined magnitudes in the range 4 to 6 can now be determined using the El Salvador digital network based on both coda duration and P or S-wave amplitudes.