Earthquakes—Nature's Faults

The chances of a major earthquake hitting South Central Texas are the same as the Dallas Cowboys facing the New England Patriots in this year's Super Bowl.
It's not gonna happen.

Natural fracture caused by erosion in outcropping
of Balcones Fault System. Colloquially known as "caliche,"
its scientific name is calcium carbonate. (Malacara photo)

Despite recent earthquakes in the Dallas/Fort Worth area, probably caused by man-made activity like hydraulic fracking, Texans may be relieved to know the state is one of the most geologically safest spots in North America. Texas shares that distinction with Florida and Alabama.
Abdul Qudus, a former San Antonio College (SAC) geology professor, said San Antonio is stable because it's located far away from plate margins and volcanoes.
Plate margins are the edges of the Earth's crust which move against each other creating fault lines along the boundaries.
Qudus said 20 plates exist throughout the world and move at a rate of 1 inch per year.
The plates responsible for quakes in California are the North American Plate and the Pacific Plate.
When the plates move, they create fractures or faults in the Earth's crust.
Both side of the fault move in opposite directions, sometimes getting stuck along the way.
When a fault line gets stuck, it builds up energy or stress which when released causes the violent shock waves or quakes to travel through the Earth, destroying things along its path.
About 1 million quakes occur in a year, Qudus said.
He added that San Antonio sits on top of an inactive fault system.
The Balcones Fault System divides the Edwards Plateau from the Texas Coastal Plain. Qudus said this fault line formed around 30 million years ago when most of Texas existed under water.

The Balcones Fault System visible from street level
beneath McAllister Fine Arts Center. (Malacara photo)

The formation of this fault is responsible for the creation of the Edwards Aquifer. This fault system, he said, is also responsible for the creation of San Pedro Springs, which made San Pedro Park a perfect camping ground for early Texas settlers.
Evidence of this fault line, Qudus said, can be seen along West Dewey Street where SAC's McAllister Fine Arts Center on the north sits higher than McCreless Hall on the south.

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The KSYM radio tower atop the McAllister Fine Arts Center.
The building sits atop the Balcones Fault System. (Malacara photo)

When continents collide

The Earth's surface is made of about 20 wide sections called tectonic plates. The plates can move with—or against—each other in one of three ways.
Convergent movement: when two plates move toward on another. The Himalayan Mountains were formed when two plates collided.
Divergent movement: when the plates move away from each other. The Great Rift Valley in Africa and the Mid-Atlantic Ridge are divergent faults.
Transform movement: when two plates collide and grind against each other sideways. The San Andreas Fault was caused by a transform fault movement.

Examples of different fault movement within the Earth.
(Archived image courtesy oocities.org)

Transform Faults

When this type of movement occurs, two types of faults can form.
Strike-slip: Sometimes two plates sliding past each other horizontally get caught, building tension. When the energy is released, an earthquake occurs. The San Andreas Fault System is a strike-slip fault.
Dip-slip: are inclined fractures where the blocks have mostly shifted vertically. If the rock mass above an inclined fault moves down, the fault is termed "normal," whereas if the rock above the fault moves up, the fault is termed "reverse." A thrust fault is a reverse fault with a dip of 45 degrees or less. Oblique-slip faults have significant components of different slip styles.

—source: U.S. Geological Survey

©2015 Martin Malacara