On February 1, at 7.26 A.M., a magnitude 4.4 earthquake rattled parts of San Diego. It was felt in most, but not all areas of the city and fortunately, damage and injury was virtually non-existent. There was a reason why the quake escaped notice in some areas and the reasons for that irregular impact have meaning for everyone here. San Diego is as likely to have a significant shake anytime; postulating what a quake would do here that was as powerful as the 7.0 event that struck Haiti last month should give all of us something to think about.
San Diego sits on the equivalent of a vast shattered window made of the bedrock lying under the hardened sediments of what was once the bottom of the sea. The bedrock under our feet is fractured in an in-line pattern of lateral faults that parallel the great San Andreas Fault, which is far to our east. These lateral fault lines are connected with myriad crisscrossing faults that can radiate or disseminate the force of a quake along divergent—and little understood—paths. When a quake occurs along one of the faults parallel to the San Andreas, these crisscrossing faults account for why some areas of San Diego shake like a leaf and others feel nothing. But all of the fault lines here have been active in the past with significant force; in some cases with an estimated power of magnitude 6.5 or 7 on the Richter Scale. This magnitude rivals the strength of recent Haitian event.
San Diego is located on the east edge of what is sometimes called the Southern California Continental Borderland—a vast, geographic province extending some 250 kilometers west and south to the edge of the Continental margin. We are at a junction of two major earth plates; the North American, east of the San Andreas Fault, and the Pacific, on which much of coastal California sits. The North American Plate is moving west and south; the Pacific Plate is moving north and west. The collision of the plates result in strike/slip earthquakes as the plates move roughly past east other.Millions of years from now, the split that is known as Baja California will be complete and the west edge of California from north of San Francisco to La Paz in California del Sur will be an island, ultimately bound for Alaska at the rate of about a half a millimeter per year.
Geologists here often complain that it is difficult to study fault lines in San Diego because many of them are effectively covered by vegetation, asphalt, condos and other buildings. Their work tends to receive little recognition locally since the interests of the developers, builders, real estate agents and politicians would be harmed if the value of the land were altered by commonly shared knowledge of active nearby faults. Moreover, scientists are often dependent on builders whose heavy bulldozers sometimes uncover the traces of fault lines they want to study and whose interests do not include waiting for geologists to do their job and harming their investments. Connecting the dots between the faults here is a long, tedious and speculative process that has taken decades and may take decades more to completely understand.
SD FAULTS AT A GLANCE
Starting from the west, San Diego’s most significant faults begin with the Coronado Bank Fault, some ten miles west of Point Loma. This runs roughly north and south and is roughly parallel to the San Andreas. The Spanish Bight Fault, Coronado Fault and Silver Strand Faults are also parallel and lie beneath Point Loma, Coronado’s North Island, the Strand and San Diego Bay, respectively. These faults are somewhat misleadingly named since most of them are now known to continue north directly under much of San Diego. Point Loma is riven with faults that are sometimes referred to as the Point Loma fault zone or the Spanish Bight fault zone.Included in the mix are the Mission Bay Fault, the Country Club Fault, the Mount Soledad Fault and perhaps the most significant of all, the Rose Canyon Fault, which travels beneath I-5 and is part of the Inglewood Fault that ruptured in 1933 and was one of the 20th century’s most significant quakes in the our region.
The Mission Bay Fault is known to run along the Morena district while the others are found in prominent outcroppings in La Jolla. The Rose Canyon Fault almost certainly links up to the Coronado Fault though it is believed to be intersected by the Old Town Fault which runs roughly east-west and fissures extensively throughout much of Mission Hills, Middletown and to the southeast. Faults parallel to the Rose Canyon are known to lie in the bottom of Florida Canyon, which bisects Balboa Park; at Texas Street where it drops into Mission Valley, at Euclid Street where it was mapped at 60th Street and onward to the La Nacion Fault which lies along a path from the border north to the SDSU-Mission Valley area. One of its prominent traces can be seen in Paradise Hills. Lying east of the La Nacion is the San Ysidro Fault which comes up from Mexico and is likely checked by the Mission Valley Fault, another east-west fault running from Mission Bay to somewhere near Cowles Mountain and ending presumably under the Laguna Mountains. Most everywhere in the city has a fault nearby, if not directly underfoot. All of San Diego is affected by their existence. The Coronado Bridge crosses four separate faults collectively known as the Silver Strand. In the North County, the Elsinore Fault cuts west of the Coast Range Mountains and San Jacinto Fault lies east of it, passing through the Salton Sea.
The two most dangerous faults are the Coronado Bank and the Rose Canyon. The Coronado Bank is thought to run north-south for more than two hundred miles south along the Baja coast; its entire fault zone—fragmental faults that can be triggered by a rupture along its length—runs for more than six hundred miles, and its assumed risk is for an earthquake of up to 7.5 on the Richter Scale—considerably more powerful than the one Haiti experienced and approaching the famous quake that destroyed San Francisco in 1906. Some scientists believe that a rupture along the Coronado Bank will likely diffuse its energy along the other faults under San Diego with appropriately interesting results. The gentle hills of the city anywhere from 20 to 200 feet high are evidence of the kind of vertical displacement that can happen suddenly here; east of Park Boulevard in downtown to past 30th; the long rise east of Florida Canyon to the 805 cut in North Park, the hills north of 8 near SDSU and the sharp, irregular canyons cutting through Mission Hills were likely formed by sudden, upward, vertical jerking quakes. The presumed depth at which the epicenters of quakes triggered by the Coronado Bank Fault would occur are believed to be anywhere from five to twelve kilometers deep. Deeply occurring quakes characteristically spend energy differently than shallow ones; the difference is usually measured in time, severity of shock and sudden displacement of earth—and water. A simple way to imagine the differences between deep quakes and surface ones is to consider the deep, rolling, thunderous kind of booming explosion produced by black powder filled fireworks as a signifier of the energy release of a quake at depth. Contrast this with the sharp, shattering, brissant concussion of a blast made by a modern high explosive. Though the Coronado Bank has its many dangers, depth may help shield San Diego from the worst kinds of sudden, shattering damage. The Rose Canyon Fault is another matter entirely.
Some scientists believe that the Rose Canyon channels its force at relatively shallow depths—or at the surface of—the ground. Significant sudden, vertical energy release accounts for the steep hills of La Jolla, Clairemont Mesa and the Morena and Linda Vista districts and is likely responsible for the slow sinking of Mission Bay and Pacific Beach. The Rose Canyon Fault is presumed to be capable of a magnitude 6.0 quake and is thought to be likely to generate the greatest sudden local ground acceleration when a quake occurs. Soledad Mountain’s upward thrust of some 60 meters in about 120,000 years is evidence of the power the Rose Canyon Fault can unleash. In terms of geologic time, that is a sudden burst of upward energy in the blink of an eye. The Rose Canyon Fault may unleash that kind of sudden, upward shocking force anywhere along its length, through the city and under the bay. Many other faults under San Diego are thought to be able to unleash similar force.
LIKELY EFFECTS OF A MAJOR QUAKE
Though for nearly a century, San Diego has been promoted as a place “relatively” free of quakes, the evidence in the earth says otherwise. Assuming a serious shock along the Rose Canyon Fault of 6.0 magnitude at a shallow depth, San Diego would suffer greatly and its likely that a great loss of life would happen within seconds, especially if we were struck on a busy weekday when many offices and buildings were filled with people and the roads were full. We could expect to lose I-5 and the junction where 8 crosses it. Rail tracks would likely be out of commission. Lindbergh Field would suffer from damage to its runways from liquefaction of the filled earth the airport sits on. It is possible, though difficult to predict that the entire area surrounding San Diego Bay could suffer great damage from sudden water displacement caused by the rupture of any of the fault lines beneath the surface. This would make the Coronado Bridge a very unsafe place to be should a quake along the Rose Canyon and its “tributary” faults let go.
Water, power and sewerage systems would break and both Mission Bay and the Harbor would be fouled with tons of raw sewage from broken pipes for an indefinite time, causing a danger to public health. The communications towers on Mount Soledad would likely be damaged, making the transmission of news and other communications impossible. Landslides would bury many streets and roads and the network of streets would likely be broken everywhere. Broken pipes would cascade water into low lying areas, perhaps drowning those who couldn’t get out of basements or lower floors of damaged buildings. For those who flee, fallen power lines would create a maze of dangers enhanced by the flooded streets.
Low lying areas built on filled land or the San Diego River bottom could expect to be seriously damaged. In a strong quake, it could be likely for a surge effect to push water downriver or suck seawater inland; Mission Bay and the beaches could become a quagmire of liquefied fill.
The damage to the roads and streets, coupled with loss of power, would mean that cars won’t be able to move far or get gas from stations without power to work gas pumps. San Diegans will be instantly back in the 19th century when it comes to transportation, with bicycles and even skateboards at a sudden premium to move people and things. This will mean that emergency medical care, food and water distribution and all other kinds of aid will be slow in coming . The loss of I-5 from La Jolla to perhaps as far south as downtown would make crossing San Diego from north virtually impossible, to say nothing of the damage likely to parallel surface streets and Pacific Highway. West of the five, residents may find themselves cut off from passable surface streets for a long time after the quake.
Since the Rose Canyon almost certainly connects with the Silver Strand Fault, force moving along it would likely damage much of the center of San Diego. The older buildings of the gaslamp and the Coronado Bridge would likely suffer damage if not complete collapse. The Navy has long had a plan in place to blast a channel in the south bay along the Strand to create a second entrance to the harbor if necessary. While perhaps a useful short term fix, it may be likely that any southern opening of the Strand would begin an erosion of coastline that couldn’t be stopped; this may ultimately turn Coronado into an island—or series of islands, if the Spanish Bight Fault liquefies the fill land under the runway at North Island.
Point Loma, riven with parallel faults en echleon with the Coronado Bank Fault, might be subject to great landslides through many of its hills. A 19th century quake was once powerful enough to crack the walls at the Old Lighthouse and cause erosion on the unpopulated hills there. That same kind of power would likely tumble many homes off the slopes and cause both Shelter and Harbor Islands to liquefy and settle into the bay.
Further east, the La Nacion Fault is thought to be capable of producing vertical thrusts of up to 20 feet. Damage to I-805 and I-15 would be very likely south of Mission Valley.
A strong earthquake would not stop at the border and the effects among a million Mexicans in Tijuana—suddenly displaced, injured and without resources or ability to acquire them from their own government—can be imagined. Any American relief effort would have to include these people, if for no other reason than to stave off riots born of desperation and the problems of disease caused by lack of sanitation and medical care.
Besides the certainty of quakes, there is a danger of fire following a significant one. Perhaps the county would be at its unluckiest if a strong shock happened during a Santa Ana, and the wind blew impossible to control fires from the east to the sea.
Perhaps recognizing its own lack of preparedness for disaster following the fiascoes of the 2003 and 2007 wildfires and chronic lack of funds, the County and City of San Diego are tacitly warning residents to look after themselves when disaster strikes. This is couched in the language offered by the County’s Emergency Services website. Presently, the county urges everyone to pack three to five days of drinking water, canned foods, first aid materials and batteries to keep body and soul together until help arrives. This is likely provided that Brown Field, Tijuana’s Rodriguez Field, Montgomery, Miramar and Carlsbad airports are still intact. Except for North Island and Lindbergh which are bisected by faults and on filled earth and vulnerable in a strong quake, the other county airfields may be expected to function. While not officially recommended, getting to one of those airfields in the aftermath of a quake may make all the difference when it happens. In addition to that, tarps, shovels, radios, knives and other weapons, hand operated can openers, bleach, medicines and some thick boots and sox to match would be essential afterward. An old bit of quake lore reminds us to keep shoes beside the bed; if a quake hits at night, it may be wise to find something to wear rather than risk cutting your feet on fallen crockery or glass. In the age of ubiquitous electronics, some form of portable generator will be essential to keep cell phones and computers operable; solar, portable or run from autos, keeping juiced up will be a challenge that needs to be met. It’s also wise to scout a relatively safe place to reassemble family and friends in advance and have one’s circle clear on where to go. Parks, parking lots, malls; anywhere it’s open and relatively unencumbered will do.
Unlike Haiti, where indifferently constructed buildings collapsed and killed many of the 200,000 lost there, San Diego’s building codes have produced much safer structures—for the most part. Older areas of the city where brick buildings are common are at substantially higher risk for damage, as are houses on hillsides, in low areas, on fill land or near water.
Some useful information can be found at www.sdcounty.ca.gov/oes/disaster_preparedness.com.
Something more complete can be had at LAFD_EmergPrepGuide.pdf Other interesting sites for tracking quake progression are: