San Diego could go another 150 years without another earthquake. But it’s possible the whole damn thing could blow tomorrow.

A rumble in Rose Canyon

After the Rose Canyon Fault bends around Mt. Soledad, it reaches southward through the western edge of Clairemont.
  • After the Rose Canyon Fault bends around Mt. Soledad, it reaches southward through the western edge of Clairemont.
  • Image by Craig Carlson

San Diego's first and only official earthquake alert was sounded in June of 1985, after swarms of small earthquakes started a five-day rumble beneath San Diego Bay. Three of the biggest shakers, with Richter scale magnitudes of 4.0, 4.2. and 4.0, were detected within four hours of each other on June 17, located near the Thirty-second Street Naval Station. This conjunction of temblors was considered rare and highly significant, given San Diegos generally low level of seismic activity, and resulted in a warning that a sizable quake might follow these “foreshocks." Transmitted to San Diego disaster officials by the Office of Emergency Services in Sacramento, the advisory read, “There is a slight increase in the probability of a potentially damaging earthquake in the San Diego area. There is a five-percent chance of a 5.0 magnitude or greater earthquake in the next five-day period. For internal use only: Not for release to the press or public."

The fault line. In 1985 three more faults were found during excavation for the new police administration building at Fourteenth and Broadway.

The fault line. In 1985 three more faults were found during excavation for the new police administration building at Fourteenth and Broadway.

The press and the public found out about the advisory through the actions of other public agencies in the area, but the bigger quake never arrived. Still, unsettling jitters continued to rattle around beneath San Diego. In the summer of 1986, there were more small quakes in the South Bay. On July 13 of that year, a sharp 5.3 temblor rolled in from an epicenter thirty miles southwest of Oceanside, and thousands of aftershocks have continued from that one, including a magnitude 3.6 on January 5 of this year. Seismologists have identified a mysterious “line of seismicity” extending from the South Bay out toward Catalina Island that does not coincide with any mapped faults. Whether the South Bay and offshore quakes of the last three years are on some unknown fault or are occurring on known faults such as Rose Canyon and the Coronado Banks is a subject of debate among the experts. But it is now reasonable for a layman to ask: Is San Diego awakening from a long seismic slumber?

“The recent number of earthquakes is unusual for the San Diego area,” says Mike Reichle, senior seismologist for the California Division of Mines and Geology, based in Sacramento. “Whether it’s to be expected, we don’t know. Our experience is so short in San Diego. The earth hasn’t told us enough to know. It’s possible that San Diego could go another 150 years without another earthquake. But it’s also possible the whole damn thing could blow tomorrow.”

“The whole damn thing” is represented by a succession of faults that runs roughly north to south, beginning out along the eastern shore of the Salton Sea with the San Andreas Fault, then the San Jacinto Fault bisecting the Imperial Valley, the Elsinore Fault extending from the Los Angeles area southeast through the Jacumba Mountains and down into Mexico, various poorly understood faults beneath the San Diego metropolitan area, and a series of active offshore faults. When epicenters of Southern California and Baja earthquakes occurring in the last twenty years are charted, the areas to the north, south, east, and west of San Diego are shown to be dotted liberally with quakes; San Diego, however, is almost blank by comparison, forming a “seismic gap.” No local faults are officially considered by the city to be “active," a fact that discomfits many local geologists.

It is true that in historic times, the 219 years since Europeans first settled here, the San Diego area probably hasn’t experienced a seriously damaging earthquake. From old weather records and newspaper stories, seismologists have determined that nineteenth-century San Diego experienced stronger ground shaking than the present century, but it is impossible to know which local faults produced earthquakes. One strong quake in particular, which rocked San Diego on May 27, 1862, “could have been on the Rose Canyon Fault,” remarks Duncan Agnew, a seismologist with the Institute of Geophysics and Planetary Physics at the Scripps Institution of Oceanography. “Nineteen Thirty is the beginning of instrumental recording [of earthquakes),” Agnew continues. “Prior to that, about all you’ve got is people saying how much their house shook. So you can’t really say with certainty that there’s been little or no seismic activity on Rose Canyon in historic times; about all you can say is, there’s been no activity on it since the 1930s.”

Since 1934, when San Diego got its first earthquake-detecting accelerograph, the greatest local ground shaking measured was caused by a magnitude 5.6 quake offshore on December 22, 1964. The overall lack of seismic activity here has resulted in less stringent building codes for San Diego than for almost all other cities in California.

Within the Uniform Building Code, San Diego is a zone III, while most of the rest of California is a zone IV, meaning San Diego builders generally need to design for a twenty-five-percent weaker groundshaking potential than the rest of California. And unlike most other major cities in the state, San Diego employs no staff geologists to review the work of hired geotechnical consultants.

Many local geologists, both in private business and in academia, believe San Diego’s seismic risk is underrated. In other areas of California where similar seismic gaps exist, the potential for earthquakes is thought to be greater — the logic being that the gap is a kind of boil waiting to burst.

Although empirical evidence to support a higher potential for earthquakes here is skimpy, many geologists who have studied the San Diego fault structure harbor a deep suspicion that the earth's local quietude is portentiously too quiet. Charles F. Richter himself expressed this disquieting ambivalence in 1959 when he wrote, “There has been a general impression that earthquake risk does not exist at San Diego, historical records to the contrary being forgotten or ignored. Older structures were erected with no close attention to soundness. During and since World War II, population has increased enormously, and the city area has expanded at a pace hardly consistent with careful construction and inspection. Fortunately, most of the expansion has been over the higher ground ...”

Since Richter wrote those words, urban expansion has become so prolific in the lower ground (closer to the actual fault breaks) as to obscure the visible traces of most local faults — including the one generally believed to be most potentially damaging, the Rose Canyon Fault. It comes onshore through the La Jolla underwater canyon, runs almost beneath the La Jolla Beach and Tennis Club, crosses Torrey Pines Road a few hundred feet west of the La Jolla Shores Boulevard intersection, and extends southward around the eastern flank of Mt. Soledad. It crosses Ardath Road at Interstate 5, passes below the freeway, then continues through Rose Canyon east of the railroad tracks and along Morena Boulevard above Mission Bay and on into downtown San Diego.

From the cross atop Mt. Soledad, looking east, one can observe the trace of one of the three faults that slice across the mountain. The Mt. Soledad Fault, along with the Country Club Fault a few hundred yards to the east, is part of the Rose Canyon Fault zone. Looking eastward from the cross, one can see where the Mt. Soledad Fault creates a ravine and cuts right beneath a tennis court, then snakes southward down toward Interstate 5, where it appears to join the main trace of the Rose Canyon Fault. Geologists aren’t sure exactly what happens to the Country Club Fault (named after the La Jolla Country Club, which it bisects) after it drops over the other side of Mt. Soledad.

After the Rose Canyon Fault bends around Mt. Soledad, it reaches southward through the western edge of Clairemont, a dense neighborhood of modest, 1950s-style homes and apartments, the kinds of structures that received the most damage in last October’s Whittier earthquake. At Ticonderoga Street and Moultrie Avenue, fault maps show it running beneath William P. Toler elementary school. Farther south, the fault crosses Clairemont Drive just above Denver Street, a little east of the Safeway and right beside the new Image Inns motel. Another half-mile south, the fault runs beneath Bay Park elementary school at Erie Street. After crossing the San Diego River channel and bisecting Old Town, the fault splinters down across the Marine Corps Recruit Depot and Lindbergh Field, and then beneath the bay and across Coronado. But the main trace continues along Columbia Street and appears to pass directly beneath the City Operations Center at First and A. In the event of an earthquake, the City Operations Center is expected to be one of the main command posts for disaster relief.

The paving and building along the fault zone has made research on the Rose Canyon Fault extremely difficult, as has the continuing problem of winning research grants to study seismicity in San Diego. Most of the money goes to more active areas like San Francisco and the San Andreas Fault. But in recent years, research activity is increasing on the Rose Canyon Fault, along with disagreements among geologists as to exactly how the fault system works in relieving local seismic strain, where it extends, and which strands of it, if any, are active. One thing is indisputable, however; if the Rose Canyon Fault jerks anywhere close to its potential, the upper limit of which is considered to be about a 6.8, calamity will be visited equally upon some of San Diego’s upper-class and middle-class neighborhoods, its primary tourist havens in Mission Bay and Mission Valley, and the resurgent new downtown.

Lost on everyone if the big one hits will be the irony that the same fault system that created much of San Diego’s natural assets could cause the city’s undoing. The Rose Canyon Fault divides two plates of land moving in opposite directions. The western plate is moving to the north, the eastern plate is pulling south. The bend in the fault in the area of Ardath Road has crimped the western plate and created Mt. Soledad, blocking San Clemente Canyon from emptying into the ocean. In reaction to this uplifting mountain, the ground to the south buckled downward, creating the watery flats of Mission Bay and contributing to the drop in elevation that forms San Diego Bay.

They’ve already built on the fault, so what are we supposed to do now? Just to zone it to scare people is the wrong reason.

A secondary fault, known as the La Na-cion, runs from Mission Valley at about Montezuma Road southward into Mexico and appears to be helping to accommodate the spreading of San Diego Bay.

Some believe the Rose Canyon Fault zone connects with other fault systems in Mexico that are definitely active, but strong evidence refuting this southern connection was presented in 1981 hearings concerning the operation of the nuclear power plant at San Onofre. More geologists accept the supposition that Rose Canyon does link up (in yet-to-be understood ways) to the Newport-Inglewood Fault, which goes onshore near Costa Mesa and continues northward through Newport Beach to Culver City. Newport-Inglewood is an active fault that produced the destructive 6.3 Long Beach earthquake of 1933, which killed 117 people. One line of thinking follows the logic that if Rose Canyon connects to other active faults to the north and south, then Rose Canyon must also be active. And the longer a fault system is, the greater its potential for producing large-magnitude earthquakes.

In San Diego, the academic geologists and the geologists working for private consulting firms, whose clients are developers, have generally held distinctly differing views about Rose Canyon. So far, the City of San Diego has officially accepted the view of the consultants —namely, that Rose Canyon is only “potentially active.” But some current studies on the fault may soon force the city to re-evaluate its position and eventually its building codes.

A joint paper just completed by professors Tom Rockwell of San Diego State University and John G. Anderson and Duncan Carr Agnew of the Scripps Institution of Oceanography (which is now undergoing peer review prior to publication) states unequivocally that the Rose Canyon Fault is active. The paper also points out that San Diego faces potential damage from quakes emanating from active offshore faults, as well as from the Elsinore Fault.

San Diego State is considered to be the strongest voice in declaring Rose Canyon active. “It is active, most definitely,” comments Pat Abbott, chairman of the geology department at SDSU. “Every three or four hundred years, it may jump two or three feet, causing gigantic destruction. The weight of physical evidence supports that, but people in the engineering and development community remain unconvinced.” This physical evidence includes aerial photographs, shot in 1929, which appear to show fairly recent (within the last 11,000 years) displacements along the fault, displacements that are now covered by civilization. Geologists also have demonstrated that the creation of Mt. Soledad and the faults responsible for it are very young in geologic terms, and reason dictates that geologic processes don’t cease just because humanity has arrived. They have calculated slip rates along local fault systems that support the contention that the Rose Canyon Fault moves in periodic fits and starts. Evidence of recent fault movement offshore along the trace of the Rose Canyon Fault zone north of La Jolla Canyon is also used as an argument that Rose Canyon is alive. And finally, many geologists believe that the official definition of an active fault — that it has produced an earthquake within the last 11,000 years — doesn’t square with reality.

Abbott says that Rose Canyon should not be viewed as a criminal suspect who is held to be innocent as long as there is the slightest doubt about its ability to inflict harm. He and other professors at SDSU believe there’s enough evidence to declare the fault active and treat construction in the fault zone accordingly. He points out that many recent damaging earthquakes, such as the Whittier quake last October and the 6.6 San Fernando Valley earthquake in 1971 that killed sixty-five people, are associated with faults that were previously considered much less significant than Rose Canyon. The San Fernando Valley quake moved a series of minor faults that hadn’t shown evidence of earthquakes for the last several hundred thousand years.

But geological consultants such as Ernie Artim, who works for Owen Geotechnical and who directed the updating in 1983 of the City of San Diego’s Seismic Safety Study, say they can’t call Rose Canyon active until they see irrefutable evidence. “Consultants have an obligation to the public to be objective in our findings,” Artim remarks. “We just can’t go out and speculate. In the local environment, some people have lost their objectivity.” Artim says certain academic researchers from SDSU have made up their minds that Rose Canyon is active, and they are setting out to prove it, rather than keeping the question open until more facts are available.

Artim believes the Rose Canyon Fault, if it is moving at all, is a secondary feature reacting to movement along the Coronado Banks Fault, which runs parallel to San Diego five miles off Pt. Loma. The Seismic Safety Study he authored reflects his belief that since there is no direct evidence of ground rupture onshore along the fault in the last 11,000 years, it cannot justifiably be termed active. Artim is also not convinced that Rose Canyon is connected to Newport-Inglewood or any active faults to the south. And the Seismic Safety Study, which represents the city’s official approach to building in the fault zone, doesn’t even mention faults that may underlie downtown San Diego.

In 1975 geologist Michael Kennedy, who now works in the marine geophysical laboratory at Scripps, drew the most detailed maps of the Rose Canyon Fault and postulated three fault strands knifing under downtown. Many consulting geologists refused to accept this postulation until 1980, when a fault was found to run between Broadway and E Street, in the block bounded by Front and First Avenue, beneath what is now the Wells Fargo highrise. This one was dubbed the San Diego Fault and did not show evidence of displacement in at least the last 75,000 years, according to Dorian Elder Mills, a close associate of Artim’s, who performed the geotechnical studies of the site.

In 1985 three more faults were found during excavation for the new police administration building at Fourteenth and Broadway. The faults could be clearly observed in the excavation of the site, running north-northwest to south. The middle fault, which bisects the block bounded by Broadway and E, Fourteenth and Fifteenth, was considered extremely significant because it appeared to show repeated displacements within the last 11,000 years. This was the first such evidence onshore of relatively recent "Holocene Era” activity ever discovered in the Rose Canyon fault zone. In reaction to the finding, the police department was obliged to move its building from the center of the lot, where it had been planned, to the northeast corner. Structural changes in the building were also made.

Dave Schug, the geologist with Woodward-Clyde Consultants who performed the geotechnical work beneath the police station, called in many other geologists and recognized soils experts to look at the fault’s displacement. He says that although there was no datable carbon material in the soils, almost everyone who saw it was convinced that it represented a geologically recent movement of the fault.

One geologist who disagreed was Dorian Elder Mills, a consulting geologist who now works for Geocon, Inc., who had done the major work on the other fault located on lower Broadway. She was with Leighton and Associates in 1985, the firm Ernie Artim was also working for then, and she says the City of San Diego asked Leighton to review Woodward-Clyde’s interpretation of the faults beneath the police administration building. "Leighton disagreed that it was an active fault,” she says. “Several of the soils looked the same to me as the ones we encountered on the San Diego Fault [on lower Broadway) ”

Mills says that calling the Rose Canyon Fault active “is pretty extreme. Until we find better evidence of that, we can’t tell our clients it’s active.” Leighton advised the city that it was unnecessary to update the seismic safety study to reflect the presence of active faults downtown.

The fault beneath the police station has become very significant locally if for no other reason than it convinced one man, Earl Hart, that further investigation into San Diego’s faults was merited. Hart is the manager of the fault evaluation program for the State of California, and he came down to look at the fault displacement beneath the police station. Although he’s aware that different geologists interpret the break differently, he decided last year (after some prodding by SDSU’s Pat Abbott) to reopen an investigation into whether any of San Diego’s faults should be zoned under legislation passed in 1972, called the Alquist-Priolo bill. Zoning under that bill, passed in the wake of the San Fernando Valley earthquake, requires any new subdivision and most other proposed structures within a certain distance of a known fault to undergo detailed geological investigations prior to construction, which may then require structural changes in the buildings themselves. The state began zoning major faults first, and today there are 358 zone maps throughout California, with sixty more due out soon. “We looked at San Diego in 1978 and decided the faults didn’t meet the criteria for zoning," Hart says from his Sacramento office. The two criteria are that a fault be reasonably well defined as a surface feature, and there be activity during the last 11,000 years, during the Holocene era. In 1978 the Rose Canyon Fault zone was almost completely obscured by surface construction, and no Holocene displacements had been found. But the fault under the police station caused Hart to reconsider, and a new investigation to determine whether San Diego should be zoned under Alquist-Priola as an active area is now under way.

Hart calls San Diego a "big, wide, messy zone" of faults, some of which are apparently active. "But I haven’t changed my opinion on San Diego,” Hart cautions. “It's historically been seismically quiet. But there have been some recent quakes that suggest that maybe we were just looking at a quiet period." Then again. Hart is aware that most of the fault zone is already heavily urbanized, making the zoning of it almost superfluous. "They’ve already built on the fault,” he says, “so what are we supposed to do now? Just to zone it to scare people is the wrong reason." He expects to have the investigation completed within the next couple of months.

What seems most exciting to local geologists now is the prospect of a major earthquake hazard study to be funded by the navy. Glenn Roquemore, a geologist in the office of Applied Geoscience Research in the Naval Weapons Center at China Lake, has proposed a three-year, $1.9 million investigation to determine the earthquake hazards surrounding San Diego navy bases. "The product of this project will help answer the question as to how the navy’s position in relation to a potential enemy may be altered when an important installation or that enemy is destroyed by natural calamity,” reads the proposal. “An example could be the partial destruction by natural calamity of the Soviet seaport of Vladivostok.” Local geologists believe that if the navy funds Roquemore’s project (he is awaiting word on funding any day now), all of San Diego will benefit. Detailed fault and soil mapping, exploratory trenching, the installation of sophisticated sensors, and new computer software will be used to establish an earthquake monitoring and prediction system for the navy but which will be built and utilized by local geologists. A major part of the investigation will focus on the Rose Canyon Fault zone and will attempt to determine once and for all, through trenching studies, the location and magnitude of earthquakes on the fault within the last 11,000 years. “I would say that Rose Canyon is now being put into a category of faults that give it a higher potential of rupture than before," Roquemore remarks. The fault is of particular interest to the navy because of its proximity to Miramar Naval Air Station, the Naval Training Center and Marine Corps Recruit Depot, naval supply facilities downtown, naval bases and docks on Coronado and Pt. Loma, and the Thirty-second Street Naval Station.

In viewing the Rose Canyon Fault from the perspective of national security, the implications of a major San Diego earthquake extend far beyond the damage to civilian buildings and infrastructure. “The collapse of a bridge into a major shipping channel or the underwater slump of material into navigation channels could prevent ships from entering or leaving a harbor,” Roquemore’s proposal states. "The phenomenon of ‘draw down* from a retreating tsunami [tidal wave] could cause moored ships to strike bottom... If a nuclear submarine were severely damaged by collision as a tsunami swept the crowded harbor, the specter of massive nuclear contamination could be added to an already catastrophic picture... ”


What effect would a major earthquake (magnitude six to seven) in the Rose Canyon fault zone have on San Diego? Various scenarios have been assembled in which seismologists and disaster planners have tried to imagine the aftermath of a big quake here, and the picture that emerges is painfully detailed. An early scenario relating to a quake’s effects on navy bases in the bay, written in 1972 by a naval weapons center geologist, looked at tht absolute worst possibilities and came up with a nightmarish vision of mass destruction. It pictured the aircraft carrier Constellation breaking its moorings and ramming the nuclear-powered submarine Sunfish, resulting in major nuclear contamination of the bay. This scenario also envisioned the Coronado bridge collapsing, runways at North Island and Lindbergh Field cracked and submerged, fires raging out of control due to disruption. water supplies, hundreds of casualties among the ruins of the Naval Training Center and the Marine Corps Recruit Depot, and a massive Ft. Loma landslide blocking the ship channel. In short, catastrophe on a major scale.

In reality, the aftermath of a major quake would probably be much less catastrophic than that worst-case scenario. According to Caltrans bridge engineer Joe Borik, the Coronado bridge was strengthened in 1975 to ensure that its five main girder sections would react as one piece in an earthquake and would not break apart. The hinges between these girders were reinforced, and a cafe system underneath the roadway now links the whole bridge together, the logic being that as long as the bridge doesn’t break up, it won’t fall down. As for the columns supporting the bridge, Borik says their foundations were sunk deep enough to reach solid ground underneath the bay sediments, so liquefaction would pose no threat to the columns.

Liquefaction of soils is a process in which sandy sediments become liquefied by ground shaking. This is expected to be a major problem in certain areas of San Diego, such as Mission Bay, Mission Valley, and along the waterfront in San Diego Bay. The ground may sink beneath structures and pipelines in these areas, causing tilting of buildings and the possibility of collapsing walls and broken pipes. In addition to liquefaction, landslides are also expected to be widespread and damaging to roads, pipelines, and canyon neighborhoods.

The State of California is currently writing an earthquake scenario for San Diego, a preliminary summary of which is now available. Local disaster relief officials use such projections as planning guides. But in the event of a large earthquake, even if the disaster relief efforts are carried out according to plan, county disaster specialists say citizens must be able to rely solely on themselves for at least seventy-two hours.

Between the state’s scenario and another detailed earthquake projection assembled in 1986 for the local Office of Disaster Preparedness, the following picture of San Diego after a major quake on the Rose Canyon fault emerges as plausible:

• Casualties — The 6.8 magnitude earthquake is presumed to have occurred at 9:00 a.m. on a weekday. Office buildings, factories, and freeways are full. First reports indicate 1400 to 2000 dead, many injured people trapped in collapsed buildings, and between 5500 and 8000 people requiring hospitalization.

• Hospitals — The San Diego area contains about 7000 acute-care hospital beds. Severe road damage due to landslides and liquefaction will limit access to the hospitals in the first hours after the quake. In addition, Mercy and University hospitals in Hillcrest may be compromised due to their location in steep canyon areas, which may render them inaccessible.

• Highways — In a quake centered near Mt. Soledad, ground rupture will close I-5 from Ardath Road south to Mission Bay. If the quake is centered to the south, on a strand of the fault crossing Coronado, it is projected that 1-5 will be closed all the way from Balboa Avenue in Pacific Beach to Palm Avenue near the Mexican border. The other major highways are expected to be passable, except where they join I-5. The Coronado bridge will be closed due to liquefaction damage to its entry ramps on both sides. The road along the Silver Strand will be flooded. Ardath Road will closed due to landslides from Mt. Soledad.

• Airports — Lindbergh Field will be closed for two weeks to all but emergency operations due to liquefaction affecting runways, access, electrical power, and the East Terminal building. Brown Field and NAS Miramar will suffer little damage.

• Railroads — San Diego’s one set of tracks leading north is expected to be severly damaged due to liquefaction and landslides. Planners project three weeks without rail service.

• Port Facilities — Liquefaction damage is expected to be heavy throughout the bay front if the epicenter is near downtown, preventing access to docks. Planners expect not to be able to count on any port facilities for several weeks after an earthquake at the southern end of the Rose Canyon fault zone.

• Utilities — Electrical lines will be out of operation for between twenty-four and seventy-two hours. Telephone service will be interrupted for about the same amount of time. Some neighborhoods could be without water for several weeks, including the most vulnerable areas bordering Mission Bay, San Diego Bay, coastal areas, western Mission Valley, and the Tijuana River Valley. Fire control will be a problem due to lack of water pressure. Limited sewage disposal capacity is expected to be available to only about half the metropolitan area. Natural gas supplies may be interrupted for up to seventy-two hours downtown and along the beaches, but Coronado could be without natural gas for up to four months, until a new pipe is installed across the bay.

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