Earthquakes, San Andreas, and the inevitable
Learn measures that can be taken to minimize loss of life in a major earthquake
An aerial view of the San Andreas fault in central California where the Pacific and Continental tectonic plates converge. Photo: U.S. Geological Survey.
Seismologists say it’s not a matter of if, but when the San Andreas Fault will rupture; prudent thing is to be prepared to minimize risk
NEWS PERSPECTIVE — It is only a matter of time before people will be blaming the infamous San Andreas Fault for another catastrophic seismic event. The fault hasn’t lurched for 111 years.
Seismologists will readily tell you, it is beyond the time she awoke from her slumber. And it’s got them worried, because, San Andreas, the world’s longest fault line stretching 2,500 miles from Mexico to the San Francisco Bay area, is forecast upon rupture to wreak epic catastrophe in California.
Earthquake faults and why they rupture
Earthquake faults are the space or dividing line between two converging tectonic plates. The San Andreas Fault is a continentaltransform fault that extends roughly 750 miles through California. It forms the tectonic boundary between the Pacific Plate and theNorth American Plate, and its motion is right-lateral strike-slip(horizontal). The fault divides into three segments, each with different characteristics and a different degree of earthquake risk, the most significant being the Southern segment, which passes within about 35 miles of Los Angeles. The slip rate along the fault ranges from 0.79 to 1.38 inches per year.
The fault was first identified in 1895 by Professor Andrew Lawson of UC Berkeley, who discovered the northern zone. It is often described as having been named after San Andreas Lake, a small body of water that was formed in a valley between the two plates.
However, according to some of his reports from 1895 and 1908, Lawson actually named it after the surrounding San Andreas Valley. Following the 1906 San Francisco earthquake, the last year of considerable movement along the fault line, Lawson concluded that the fault extended all the way into Southern California. It is divided into three zones — the Northern, Central, and Southern.
The northern segment of the fault runs from Hollister, through theSanta Cruz Mountains, the epicenter of the 1989 Loma Prieta earthquake, then up the San Francisco Peninsula.
The central segment of the San Andreas Fault runs in a northwestern direction from Parkfield to Hollister.
The southern segment (also known as the Mojave segment) begins near Bombay Beach, Calif. Box Canyon, near the Salton Sea, contains upturned strata associated with that section of the fault. The fault then runs along the southern base of the San Bernardino Mountains, crosses through the Cajon Pass and continues northwest along the northern base of the San Gabriel Mountains. These mountains are a result of movement along the San Andreas Fault and are commonly called the Transverse Range.
In Palmdale, a portion of the fault is easily examined at a road cut for the Antelope Valley Freeway. The fault continues northwest alongside the Elizabeth Lake Road to the town of Elizabeth Lake. As it passes the towns of Gorman, Tejon Pass, and Frazier Park, the fault begins to bend northward, forming the “Big Bend.” Thisrestraining bend is thought to be where the fault locks up inSouthern California, with an earthquake-recurrence interval of roughly 140–160 years. Northwest of Frazier Park, the fault runs through the Carrizo Plain, a long, treeless plain where much of the fault is plainly visible. The Elkhorn Scarp defines the fault trace along much of its length within the plain.
The southern segment, which stretches from Parkfield in Monterey County all the way to the Salton Sea, is capable of an 8.1-magnitude earthquake. At its closest, this fault passes about 35 miles to the northeast of Los Angeles. Such a large earthquake on this southern segment would most likely kill and injure thousands of people in Los Angeles, San Bernardino, Riverside, and surrounding areas, and cause hundreds of billions of dollars in damage.
A study published in 2006 in the journal Nature found that the San Andreas Fault has reached a sufficient stress level for an earthquake of magnitude greater than 8.0 on the moment magnitude scale to occur. According to this study, a massive earthquake on that southern section of the San Andreas fault would result in major damage to the Palm Springs–Indiometropolitan area and other cities in San Bernardino, Riverside, and Imperial counties in California, and Mexicali Municipality inBaja California. It would be strongly felt throughout much ofSouthern California, including densely populated areas of L.A. County, Ventura County, Orange County, San Diego County,Ensenada Municipality, and Tijuana Municipality, Baja California; San Luis Rio Colorado in Sonora and Yuma, Ariz.
Older buildings would be especially prone to damage or collapse, as would buildings built on unconsolidated gravel or in coastal areas where water tables are high (and thus subject to soil liquefaction). The study concluded the following:
“The information available suggests that the fault is ready for the next big earthquake but exactly when the triggering will happen and when the earthquake will occur we cannot tell […] It could be tomorrow or it could be 10 years or more from now.
Nevertheless, in the 10 years since that publication, there has not been a substantial quake in the Los Angeles area, and two major reports issued by the U.S. Geological Survey (USGS) have made variable predictions as to the risk of future seismic events. The ability to predict major earthquakes with sufficient precision to warrant increased precautions has remained elusive.”
Minimizing quake damage, saving lives
While predicting major seismic events is still beyond the range of seismology, measures can be taken to minimize loss of life in a major earthquake. The American Red Cross encourages the following:
- Find safe places in every room, such as under a desk or against an inside wall. During an earthquake, these are places to take cover from falling objects.
- Practice Drop, Cover and Hold On! at least twice a year. Drop under a nearby table or desk and hold on to it. Cover your head with your free arm. If you are not near a table or desk, sit against an interior wall away from anything that might break or fall on you and Drop, Cover and Hold On! Don’t forget, teach your children to Drop, Cover and Hold On!
- Designate an out of town contact for your family. Phone lines are apt to go down during a quake. When they come back up, it will be easier to call out of town or even out of state than locally. During an emergency, each person in your family should contact that person. The out of town contact will be able to tell each of you where the other is when you may not able to call each other.
- Experts are available and very willing to help you find additional ways to protect your home, such as bolting your house to its foundation or training to use a fire extinguisher.
- Inform others, like babysitters or caregivers, of your emergency plan.
- Secure your furniture. Unsecured furniture may fall on you during an earthquake.
- Prepare a Disaster Supplies Kit for your home, work, and car.
- Know what to do when the shaking begins.
Persons in bed should hold on and remain there, protecting the head with a pillow. If outdoors, find a clear spot away from buildings, trees, and power lines. Drop to the ground. If in a car, slow down and drive to a clear place. Remain in the car until the shaking stops.
What to do after the shaking stops
- Check for injuries. Protect from further danger by putting on long pants, a long-sleeved shirt, sturdy shoes, and work gloves. Give aid according to the level of training.
- Look for and extinguish small fires. Eliminate fire hazards. Turn off the gas. (Remember, only a professional should turn it back on.)
- Listen to the radio for instructions. Expect aftershocks. Each time you feel one, DROP, COVER, AND HOLD ON!
- Inspect your home for damage. Get everyone out, if your home is unsafe.
- Expect phone systems to be overloaded during an emergency. Use the telephone only to report life-threatening emergencies.
Content from Wikipedia contributed to this post