Magnitude determination of Mexican earthquakes

Abstract

We explore two altemative magnitude scales, MA and ME, for quasi-real time estimate of magnitude of moderate and large Mexican earthquakes using broadband recordings at Ciudad Universitaria (CU), Mexico D.F., Mexico. MA and Me are based on amplitude of band-pass filtered (between 15 and 30 sec) velocity traces and estimate of radiated seismic energy, respectively. Both scales are tied to the moment magnitude, MW. MA is adequate for shallow, moderate, and large earthquakes, but appears to saturate for major and great earthquakes. ME, on the other hand, does not suffer from saturation and should be valid for events of all depths. In as much as MA measures long-period characteristics of an event while the contribution to ME mostly comes from frequencies near the comer frequency of the event, the magnitude on the two scales may differ for the same earthquake, even if MA has not saturated. Since a large disparity in the two magnitudes may be indicative of anomalous nature of an earthquake, we recommend the use of both scales whenever possible.