A general overview of the catalog of recent seismicity compiled by the Mexican Seismological Survey

Abstract

We have analyzed the catalog of recent (1974-1998) seismicity in Mexico, based on the automatically compiled reports of the Servicio Sismológico Nacional (Mexican Seismological Survey). To this end, we employed various tools developed for the analysis of seismic catalogs as well as for detailed studies of seismicity characteristics. Such tools comprise both newly developed techniques and traditional methods which deal with the subject of artificial and natural variations of seismicity. We found that the time characteristics of reporting suffered a drastic change during 1988. Before 1988 routine procedures involved locating a large number of events and assigning magnitude only to the largest. After 1988 a decrease in the location of small events and a significant increase in the number of magnitude determinations were noted. The minimum magnitude of completeness for the period 1988- 1998 is Md = 4.3. A comparison of the frequency-magnitude distributions for mb (PDE) and Md (SSN) indicates that mb magnitudes are basically equivalent to Md for the range M ² 5.0. These results are important in that they show that when using statistics which do not take into consideration the period of reporting, serious biases could be introduced. Conversely, our results provide means to correct for the unhomogeneity of the catalog. In terms of the most significant seismicity variations in both space and time which could be due to natural causes we found several cases that need to be studied individually.