Sismos y geologia

     I have lived through perhaps 40 quakes of varying magnitude since I moved to Mexico some years ago. Every year, mas o menos in the fall, quakes tremble,, mostly at night. I have no idea why there seems to be more from September-November nor why it seems there are more at night. According to the USGS quakes happen any time and in any season. Perhaps I just don't remember "day" quakes very well, although there have been some memorable cockers during the daylight hours. When one is in a car the motion of the auto tends to negate the movement of the earth. Perhaps at night when all is quiet and one is usually in bed,, still,,in a relaxed position, one is more sensitive to movement. The only connection between events on the earth's surface and events far below I have found comes from seismologists at the Bhabha Atomic Research Centre in India. They have found that the earthquake counts go up steadily when the moon comes closer to the Earth (perigee) and also when it is Full Moon. So if there is a season, perhaps perigee is the only one.

     I have experienced all three types,, Vertical plate motion, horizontal, and both motions at once. The vertical one I call the "rumbling turd quake". It's like a hard bowel movement that rolls  down the intestines like a bowling ball and blows out with one great vibrating jolt. The second (horizontal) I call the "drunken quake" because at first one might feel drunk or that one has lost their equilibrium. It sways back and forth sometimes like a baby's cradle but other times with enough force so that you lose your balance.

     The third is the "full monty" because it contains the other two motions. It is the most dangerous and causes the most damage. There was one about a year ago,, its epicenter 300 kilometers away. That was a full monty. However most of the quakes I have experienced have had epicenters far from Tenancingo on the pacific coast.

     I originally came from the northeast, a relatively quiet area seismically,, at least in comparison to Mexico. Eighty percent of all quakes occur on the pacific rim of fire, and Mexico's pacific coast is part of that rim. When there is an earthquake in Pinotepo in Oaxaca we feel it here. The appalachians are for the most part tranquil,, their volcanic days a distant memory. The mountains around Tenancingo remind one of a relatively recent volcanic past,, geologically speaking. On the northwestern horizon the dormant volcano, Nevado de Toluca or Xinantecatl is visible and often covered in snow. 
  Xinantecatl
From the top of Christo Rey in Tenancingo one can see Popocatepetl smoking. }

     The geology of Mexico is volcanic and more recent than the northeast. Rocks here look so different from the granite, shale,and sandstones I was used to. If you drive 9 miles west of Tenancingo there are mountains of white-gray limestone that glisten with crystals. The limestone was deposited by ancient sea then volcanically reborn as something "other". A few years ago the town opened a dump on the side of a small mountain in front of Pueblo Nuevo. When they cut deep into the mountain side to gain a flat area black porous lava rock was exposed like that found at Paricutin, the most recent volcano on earth found in the State of Michoacan.      
     And then there is Paricutin, Hephaestus's sudden outburst. I remember reading its history when I was a little boy just five years after the event. The geology in my zone seemed ancient and predictably stable. I was young and didn't know the geologic nuances under my feet. An earthquake was non-existant. Rocks were usually dark-greenish-gray to black, fine-grained, dense, hard basalt that they broke up for driveways. The most unusual rock I ever found was a piece of coke from the coal burning days that I dug up near the foundation of the house. A decorative brick seemed more unusual than the rocks of the area.
      When I read about Paricutin I tried to imagine a volcano spurting out of the ground one day in the backyard then burying our entire town. Paricutin is the youngest volcano of its kind and is also considered one of the new wonders of the world. One day it revealed itself to Dionisio Pulido, who, while working his land heard the earth rumble at his feet, then open up, and begin to spout steam. Gradually a hill grew into mountain with a height of  more than 2,808 meters. By the time it achieved that height the eruptions were terrifying, some thought eternal, because the volcano spewed lava for 9 years 11 days and 10 hours, then sputtered, and stopped. 
 Paricutin at night
  




During   After

  I was lucky enough to see Paricutin in person a few years back and crawl around the sharp black lava fields and the ruins of the church in the pueblo that doesn't exist anymore called, San Juan Parangaricutiro. One sees the destruction,, the church tower more like a disconnected cruel sculpture, and there is a silence now. Birds sing and the only sizzle is of gorditas on the comal of Doña  "Lite",  the Purepecha that has one of the puestos nestled in the lava field. 

San Juan Parangaricutiro

The Purepecha lady that sells Gorditas "lite" in the midst of the lava field near paraicutin

Her daughter Suzy

Paricutin

    
     
     In 1985 an 8.1 earthquake shook Mexico City leaving vast destruction and killing thousands. On this day in 1985, a magnitude 8.1 earthquake rocked Mexico City and its surrounding environs at 7:17 AM central time.The quake was felt as far away as Guatemala City, Guatemala and Houston, Texas, over an area of about 319,000 square miles 825,000 square kilometers, but the most intense shaking occurred in Mexico City, Ciudad Guzman and the Pacific Coast towns of Lazaro Cardenas, Ixtapa and La Union, according to the U.S. Geological Survey. Tenancingo is 100 kms. from Mexico City and in 1985 buildings were dancing, but there was a more severe quake a few years later that tumbled the parochial school. My wife remembers that one when the buildings swayed so much it seemed they would topple over. I don't ever want to experience one of those. One feels out of control even with a small quake yet respectful of this power to move the earth underneath your feet even if it originated hundreds of kilometers away. 

    




     We live close to Mexico's  trans mexico volcanic belt. Above the Nevado de Toluca is moving east. Below that,, where we are located, is moving west. In 5 million years I will have beachfront on the pacific coast. 

   

      The trans Mexican Volcanic belt is a subduction zone that runs across central Mexico. Volcanoes both active and dead protude like pimples across this swath of Mexico. The volcanic activity over millions of years has been responsible for the great height above sea level in central Mexico. The result of this uplift is the best kept climate secret in the world. Tenancingo, for example, is 18 degrees above the equator. It should be infernally hot but the height, 2020 meters above sea level, gives us warm days and cool nights. I can grow bananas, apples, and oranges and many of the plants are similar to the far north. 

As such, the majority of the Mexican central and northern territories are located at high altitudes, and the highest elevations are found at the Trans-Mexican Volcanic Belt: Pico de Orizaba (5,700 m, 18,701 ft), Popocatépetl (5,462 m, 17,920 ft) and Iztaccíhuatl (5,286 m, 17,343 ft) and the Nevado de Toluca (4,577 m, 15,016 ft). - See more at: http://www.eosnap.com/tag/trans-mexican-volcanic-belt/#sthash.PSXStlzk.dpuf