Bruce F. Barber
The rash of earthquakes San Felipe experiences occasionally give rise to the question, “What is an earthquake”? The answer is both simple and complex. Simply stated: An earthquake is a shaking of one area of the earth’s surface.
A more complex description involves a sudden release of progressively stored energy in underground rocks, causing movement along a “fault”. To understand a fault, it is essential to understand at least a little about the theory of Plate Tectonics, which states that the surface of the earth is divided into individual PLATES.
Each plate is made up of a portion of the Earth’s CRUST and a portion of the uppermost part of the subsurface MANTLE …and has an average thickness of 100 kilometers. These plates, which can be envisioned as segments of a cracked shell on a boiled egg, are in motion relative to one another, sliding over the lower part of the mantle.
A TRANSFORM BOUNDARY occurs where two plates slide past one another. The San Andreas fault, in Alta and Baja California, is regarded as this type of boundary and the earthquakes along the fault are a by-product of each plate’s motion.
The vertical edges of adjacent plates are anything but smooth and can best be described as looking something like a saw blade. These saw-like edges of rock snag on each other and, although each plate continues its relative motion, the snagged areas cannot move, creating a significant build-up of energy.
Earthquakes occur when built up energy overcomes the resistance between two snagged plates. If you bend a stick of wood, your hands put stress (the energy) on the stick. Like a bending stick, rock can deform only so far and then it breaks.
When a rock breaks under the stress of plate tectonics, waves of released energy are sent out through the earth. These waves of energy are called SEISMIC WAVES. It is these seismic waves that cause the ground to tremble and shake during a temblor.
Because of the continual motion of adjacent plates, temblors occur many times each day along a transform boundary like the San Andreas fault. However, only those with a magnitude in excess of 3.0 on the Richter Scale are felt by humans.
The two quakes felt on 23 and 24 November were centered 150 miles from San Felipe, at Westmoreland, California, at the southern tip of the Salton Sea, and measured 6.0 and 6.3 on the Richter Scale. Last year, we felt a quake here which was centered at Cerro Prieto south of Mexicali.
Each of these quakes occurred along the San Andreas fault system and are constant reminders of the fact that we are continually on the move; between 2 and 6 centimeters per year.
Some day (measured at two inches per year!), San Felipe will cross the international border into the United States. When that occurs, it will probably be during an earthquake thereby causing me to ask, “Will we be taking the Sea of Cortez with us”?