Stress Field and Seismicity in the Basin of Mexico

Mexico City is located in the basin of Mexico, inside the so called Trans-Mexican Volcanic Belt. The region in general and the basin in particular, is characterized by local low magnitude seismicity (Mc < 4.0) that may represent a risk to the city due of the nearness from epicenters. We can distinguish three main areas of local activity: 1).- surrounding the old basin of Texcoco lake, 2)- Chalco and 3)- Juchitepec - Milpa Alta outside Mexico City; the rest of the basin presents lower seismic activity. We recorded and located 336 earthquakes with digital seismograms between 1996 and 2007. From them, just 23 focal mechanisms could be evaluated because of low magnitude that creates recording problems in the seismological networks and high frequency background noise. The focal mechanisms are mainly strike-slip and dip-slip (normal) faulting. We used three different techniques (when possible) to calculate the focal mechanisms: simple and composite first motion focal mechanism, Hash's S/P amplitude rate focal mechanism and time domain moment tensor inversion using broadband three components seismograms. The final goal is to find the local and regional stress field for the whole basin.