San Diego’s Faults
Information About Earthquake Safety
By Samantha Gianulis

    Tornadoes, hurricanes, and ice storms don’t concern San Diegans. In fact, very little can get our attention quite like someone mentioning our Golden State falling into the ocean. California becoming an island, Arizona meeting the Pacific are among many earthquake myths that distract us from the truth about Southern California fault lines. The danger is real, but the facts are helpful. Can a tsunami strike without warning? How bad will a major earthquake be? What can we do to protect ourselves? The first thing to do is familiarize ourselves with local fault zones and their basic topography. With that information, there’s still plenty of uncertainty about when an earthquake will strike and the damage it will cause. But like always, if you have a plan, you’ll sleep better.

     Maybe you’ll even sleep through the tremors. A fault line is simply a fracture in the earth. The outer layer of the earth, called the lithosphere, rests on seven tectonic plates. In southern California, we’re on the Pacific plate. The Pacific and North American plates are separated primarily by the San Andreas Fault, which separates from the North American plate at the San Andreas fault. A plate boundary is created where the plates meet. Plate boundaries are categorized based upon the different types of plate movement. Here in earthquake country, we’re on a transform boundary, which means our plates slide past each other. This type of horizontal movement is indicative of a strike-slip fault. San Diego county has three principal fault zones, the San Jacinto, Elsinore, and Rose Canyon; each a strike-slip fault. So, what kind of damage can a strike-slip fault do?

    The San Andreas is also a strike-slip fault, and the San Jacinto fault is the western main branch of the San Andreas. The San Jacinto is one of the most active fault lines in California, according to geology professor Tom Rockwell at San Diego State University. Capable of a magnitude 7.5 quake, the San Jacinto fault line is under the communities of Hemet, Anza, Borrego Springs and Ocotillo Wells. The Elsinore fault is also part of the San Andreas system, however, no quake greater than a magnitude 6.0 has been recorded. As its name suggests, this fault line concerns Lake Elsinore as well as Temecula and Julian. Just as the San Andreas gets the most press in California, the Rose Canyon is notorious in San Diego. Every San Diegan should be aware of the threat this fault poses. The Rose Canyon could generate a quake up to a magnitude 7.3, but that’s not the only troubling fact about our local menace. An earthquake on the Rose Canyon fault could liquefy the soil in some areas, like Mission Bay.

    As if that is not enough to scare you, all coastal cities should recognize the possibility of tsunamis. I have lived in San Diego since 1975 and heard the tests of the Emergency Broadcast System a thousand times, but this past June was the first time I heard a warning containing the term ‘tsunami’. A tsunami, in San Diego? After the earthquake-triggered tsunami in the Indian Ocean (December 26, 2004), most of us have probably wondered if our fault lines could generate a tidal wave? The answer isn’t so easy, and it’s the subject of contention among scientists. “In 1862, a magnitude 5.9 quake hit the San Diego area, possibly originating on the Rose Canyon or other nearby offshore fault, and no tsunami was recorded,” states Rockwell. However, onshore as well as offshore faults can cause tsunamis, and San Diego has both types of fault lines. “There are many tsunamis that occurred from onshore earthquakes triggering offshore slumps. Tsunamis are caused when seawater is displaced, by whatever cause. Fault rupture producing significant vertical motion can produce a tsunami,” says Rockwell. “Most earthquakes in San Diego do not produce tsunamis,” he adds. There’s no unanimous verdict among geologists or geophysicists on the possibility of a destructive tsunami hitting San Diego. The fact that we have active offshore fault lines and that Mother Nature is full of surprises warrants concern, but not panic.

     One fact is certain; there are more places on the globe more likely to be hit by a large-scale tsunami than San Diego because of the difference in offshore topography. The tectonic plates concerning San Diego slide up against each other, not one underneath another, as in a subduction zone. The tsunami of December 26, 2004 in southeast Asia was generated in a subduction zone, and that earthquake was at least a magnitude 9.0, possibly greater. Regarding falling into the ocean, our plates are moving up, not out to sea. At a rate of approximately six centimeters per year, no zoning changes will need to be made for at least, say, 10 million years. Once the myths are dispelled and the facts are clear, what next?

    Don’t be left unprepared, it could mean the difference between life and death. Your chances of survival, or at the least being more comfortable, increase when the supplies you need are accessible.

 Review this Disaster Preparedness List provided by the San Diego/Imperial Chapter of the American Red Cross.
 A more detailed list is at sdarc.org. Disaster Preparedness List

First Aid Kit and essential medications.
 Canned food and can opener
At least three gallons of water per person. (Store one gallon of water per person, per day).
 Protective clothing, rainwear, and bedding or sleeping bags
Battery-powered radio, flashlight, and extra batteries
Special items for infant, elderly, or disabled family members
Written instructions for how to turn off gas, electricity, and water if authorities advise you to do so
(Remember, you’ll need a professional to turn natural-gas service back on.)
Keep essentials, such as flashlight and sturdy shoes, by your bedside
Store kit in a convenient place known to all family members.
Keep a smaller version of the Disaster Supplies Kit in the trunk of your car.
Store items in airtight plastic bags.
Change your stored water supply every six months.
Replace your stored food every six months.
Re-think your kit and family needs at least once a year.
Replace batteries, update clothes, etc.
Ask your physician or pharmacist about storing prescription medications.

Samantha Gianulis is a freelance writer living in San Carlos.
 © Samantha Gianulis 2006
Published in San Diego Family Magazine