On Monday 2 April 2012, at 12.36 pm local time (5.36 pm GMT) the Oaxaca region of southern Mexico was stuck by an Earthquake recorded by the United States Geological Survey as measuring 6.3 on the Richter Scale at a depth of 12.3 km. There are no reports of any serious damage or casualties at this time, though with a quake of this size there is a strong possibility that there will have been fatalities.
The location of the quake and the areas likely to have been worst affected. USGS.
On 20 March the same region was struck by a quake measured as 7.4 on the Richter Scale at a depth of 20 km. This quake is now known to have caused at least two fatalities. It is quite likely that these two quakes are related, but it is too early to say this for certain.
The south coast of Mexico lies on the boundary between the North American and Cocos Plates. The Cocos Plate is being subducted beneath the North American Plate in the Middle American Trench, to the south of the Mexican coast. The subducting plate then passes under Mexico, generating friction which causes Earthquakes. As the Cocos Plate sinks further into the interior of the planet, it is melted by the Earth's heat, some of the melted material then rising through the overlying North American Plate to form volcanoes.