Guest Post from:
PETER T. ERDELYI & ASSOCIATES, INC. • Architectural Engineering & Structural Design since 1974
1975 Sawtelle Blvd Ste 175 • Los Angeles, CA 90025 www.erdelyi.com
Here’s the next serving of our bite-sized engineering stories as they happened at Peter T. Erdelyi & Associates.
SHAKING ALL OVER
In light of the recent earthquakes I thought that sharing a few lesser known facts will help to understand what earthquakes are actually all about.
The earthquakes of California are caused by the movement of huge blocks, or plates, of the earth’s crust – the Pacific and North American plates. The Pacific plate is moving northwest, scraping horizontally past the North American plate at a rate of about 2 inches per year, which is about the rate your fingernails grow. (This maybe explains why my wife has to go to the manicurist so many times a year.:)) You can compare this movement to when you snap your fingers. Before the snap, you push your fingers together and sideways. Because you are pushing them together, friction keeps them from moving to the side. When you push sideways hard enough to overcome this friction, your fingers move suddenly, releasing energy in the form of sound waves that set the air vibrating and travel from your hand to your ear, and you hear the snap. The same process goes on in an earthquake. Slow movements in the earth’s outer layer push the sides of the faults together. The friction across the surface of the fault holds the rocks together so they do not slip immediately when pushed sideways. Eventually enough stress builds up and the rocks slip suddenly, releasing energy in waves that travel through the rock to cause the shaking that we feel during an earthquake.
In Northern California, the last major earthquake was 100 years ago in 1906 in San Francisco. It was believed to be a 7.8 magnitude quake with over 3,000 people were killed and 225,000 people were homeless. This, of course, was much larger than the 6.9 the 1989 shaker. In Southern California, the last major earthquake on the San Andreas Fault was more than 150 years ago in 1857, rupturing the fault from Central California to San Bernardino. Few people lived in the area, so there was very little damage. Further south along the San Andreas Fault, from San Bernardino through the Coachella Valley to the Salton Sea, more than 320 years have passed since the last major earthquake (around 1680). It is still storing and building up more and more energy for some future earthquake. Another major earthquake is likely to happen on this section of the fault within our lifetime, and will shake all of Southern California. How do we know that? There is a stream bed named Carrizo Plain National Monument that crosses the San Andreas Fault north of Los Angeles and is offset almost 300 feet horizontally and growing. The sediments in the abandoned stream bed are about 2,000 years old, more or less. If we assume movement on the San Andreas has cut off that stream bed within the last 2,000 years, then the average slip rate on the fault is more or less 2 inches per year. (Do you remember my wife’s fingernails?) This does not mean the fault slips 2 inches each year. Rather, it stores up about 2 inches of slip each year to be released in infrequent earthquakes. The last earthquake offset the stream bed about 16 feet. If we assume, and it is only an assumption, that all earthquakes have 16 feet of slip, we will have earthquakes on average 100 years: 16 feet divided by 2 inches per year equals about 100 years. This does not mean the earthquakes will be exactly 100 years apart, but since the last major earthquake in Southern California was in 1857 more than 150 years ago, this “Big one” is very overdue. It is only a matter of time before this major earthquake strikes Southern California, which will be large enough to cause extensive damage throughout the entire region.
During the 1994 magnitude 6.7 Northridge Earthquake, intense shaking affected a much smaller area. Northridge was not a major earthquake and very few people have experienced one. In comparison, the Northridge 6.7 earthquake lasted only 7 seconds and ruptured only 9 miles. The theoretical 7.8 “Big One” will last about 110 seconds, almost 2 minutes and rupture the earth for about 250 miles, similar to the San Francisco one in 1906. Imagine the horrible 9.1 earthquake in 2004 in Sumatra, Indonesia, which created the devastating tsunami, killing about 300,000 people, lasting about 500 seconds — more than 8 minutes, rupturing the earth more than 750 miles, twice the distance from Los Angeles to San Francisco.
In this theoretical, but very likely 7.8 earthquake along the San Andreas Fault, the surface will offset the ground by more than 20 feet in places, and bend or break any road, railroad, pipeline, aqueduct, or other lifeline that crosses the fault. In some areas, the ground will shift violently back and forth, moving nearly 6 feet per second – shoving houses off foundations, sending unsecured furniture and objects flying. The overall shaking in this earthquake will be more than 50 times the shaking produced by the Northridge earthquake. Thousands of older buildings will collapse, and another 45-50,000 will be a complete economic loss. How can we greatly minimize this potential damage? The answer is very simple. We have to understand that the “Big one” is coming; it is not a question of if, but when. We have to be prepared. Building codes have to be upgraded regarding building performance in earthquakes. Especially the very vulnerable, older, under reinforced concrete high rise and soft story, or tuck-under apartment buildings, which have to be retrofitted. The intent of the building codes is to protect people during an earthquake, not to keep buildings functional after an earthquake. My intent in this article was not to scare people, but to have a little more understanding of earthquakes and the role of structural engineers to minimize loss of life and property in our earthquake prone area of Southern California.