by Paul R. Spitzzeri
The recent earthquakes that took place in eastern California earlier this month are potent reminders once again of the geologically unstable nature of much of the Golden State. For those of us further southwest in greater Los Angeles, we occasionally are warned of the potential consequences of “The Big One,” an 8.0 or larger tremor, of which we have not had since 1857. Meanwhile, large, but not enormous, quakes have occurred in recent decades, such as the Sylmar earthquake in 1971, the Whitter Narrows shaker in 1987, and the Northridge tremor in 1994, that can still cause tremendous damage without being on the scale (literally) of “The Big One.”
In the 1920s, there were some quakes that raised questions about what should be done to deal with the future response to major seismic events. In 1920, for example, a tremor largely centered in and near Inglewood caused some significant damage, though the estimate (the Richter scale was not developed until 1935) of 5.0 was relatively mild. Quakes can cause more damage than realized depending on such factors as depth, condition of the soil and bedrock, direction of the force of the event and others.
Santa Barbara was rocked in 1925 by a shake that later was estimated at between 6.5 and 6.8 on the Richter scale, a much larger degree of intensity than Inglewood, and there was major damage done to the historic mission and presidio community. Not as well known, but identified as an estimated 7.3 on the scale, was a quake near Eureka in the northwestern part of the state. Because it was offshore, the damage was not as bad as it could have been if closer to the town.
In July 1928, mining and civil engineer Allan E. Sedgwick, who was also a professor of geology at the University of Southern California, wrote a report, “Earthquakes and Earthquake Insurance in California,” that is interesting to read in light of our recent seismic events ninety years later.
Sedgwick began by noting that the extraordinary diversity of the state’s natural environment is a reflection of its “geologic youthfulness” and “of the great cycles of uplift and subsidence NOW [his capitalization] in progress.” He added that “the grandeur and diversity of California would be impossible without” the phenomenon of earthquakes.
The professor wrote that the accumulation of sediment on the ocean floor creates troughs so that this sinkage means that the rocky subsurface underneath moves both towards land and out to the sea, “setting up enormous compressive stresses in the sub-crustal rocks beneath the land areas” and elevates these blocks of lighter areas of the earth’s crust while heavier sea bottoms subside in what is called “isostasy,” a key element of plate tectonics.
Sedgwick added that “the upward movement of the blocks takes place along the lines of least resistance or the fracture planes. These are called faults. Therefore fault are the loci of movement of the crustal blocks that form the outer covering of the earth.” Earthquakes result from rocks “snapping back” in the alternating stretching and compression of them due to pressures along these fault planes. As he explains, “the vibrations that reach the surface cause the surface to tremble” and causing tremors.
Most of the action occurring regularly are not felt by most people, so it is that
deformed rocks suddenly relieves from stress snap back to their original position because of their elasticity and send out waves of greater intensity which reach the surface and may become disastrous earthquakes.
This push and pull is reflective of “a process which will continue until the balance between the land and the sea areas have been adjusted” and with California’s geologic immaturity, “earthquakes will continue from time to time.” Seismometers set up throughout the state “show that minor movements occur almost daily,” while a catalog of quakes since 1769 show thousands of minor and a large amount of major quakes since then.
Notably, Sedgwick noted that there were a few prevailing scales used to measure quakes before the Richter one was developed and accepted: the Rossi-Forel and the Mercalli with ten degrees of scale and the Sieberg with a dozen. Because the first was more widely utilized in the United States, it was adopted for reference in his report. Levels 1-6 were considered “not damaging;” 6 was considered to cause significant movement but little or no damage; 7 was slightly damaging; 8 would be a “strong shock, causing chimneys to fall and walls of buildings to crack;” 9 was “very strong” with destruction of some structures and the total obliteration of weak ones; and 10 meaning extreme intensity and a “great disaster” causing even major damage to “the better class of buildings.”
For the twelve districts identified in the state, there were 175 reported tremors of levels 7 and 8 and 80 of levels 9 and 10, with the most occurring (59) in the San Francisco Bay area, followed by 42 at Humboldt-Mendocino in the northwest; 29 in the Sacramento Valley; 26 in Riverside-San Jacinto; and 20 in the Imperial Valley in the southeast. For the Los Angeles Basin, the total was 11, with 8 in the Whittier-Elsinore district. Santa Clara-Simi Valley had 13.
The Riverside-San Jacinto was more active than most Southern California districts because of the San Jacinto and San Gorgonio branches of the great San Andreas Fault, which would affect areas from San Bernardino and Colton through Riverside and Hemet and down to El Centro and Calexico.
Whittier-Elsinore involved “a short but active fault [which] passes along the foothills near Whittier [Puente and Chino hills ranges] and intersects the Elsinore fault just northwest of Corona, close to the Prado Dam and east of Yorba Linda and south of Chino and Chino Hills. Quake activity ran down through this region to the Imperial Valley, according to the report. In 1928, he could state “the district through which they pass is not densely populated, except in the northwesterly portion,” meaning Whittier. The “Quaker” (!) town, as well as Fullerton, Corona, Elsinore (and maybe Pomona) would be affected.
With the Los Angeles Basin, an “area from the mountains to the sea west of the San Gabriel River,” it was noted that the metropolis of Los Angeles “occupies almost the entire northerly half of this area.” At the time, it was indicated that two fault systems were within the city, the Hollywood fault running along the Santa Monica Mountains and connected with a smaller San Gabriel fault emanating from Arcadia to the ocean.
The Los Cerritos or Inglewood fault was said to run from Escondido to San Onofre, move into the Pacific and the reemerge at Newport Beach to go along the shore to Seal Beach. From there it moves to Signal and Dominguez hills and then branched off. Part went through Gardena and met the Hollywood fault in Santa Monica Canyon. Another went east of Inglewood through the Baldwin Hills and was lost in the depths there “so that its course cannot be definitely determined.”
There was also a San Pedro fault that crossed the Baldwin Hills though it was difficult to trace at both sides of that range, while “a possible continuation of the Whittier-Elsinore Fault crosses the Los Angeles River north of Elysian Park and went along the north side of the Santa Monicas into the San Fernando Fault.” Sedgwick observed “this is sometimes called the Puente Fault.” A Downey fault was said to have formed the course of San Gabriel Canyon, but tracing through the Whittier Narrows and to the South Bay was difficult, though “there is reason to believe that it extends to Wilmington and possibly through the San Pedro Hills [Palos Verdes Peninsula.]”
In referring to the Inglewood quake of 21 June 1920 (covered in this blog previously), the Rossi-Forel scale was about 9 “judged only by local damage within a restricted area” in downtown Inglewood with “a few veneered and poorly constructed buildings.”
In discussing the “authenticity of early records,” Sedgwick noted that “the three disastrous earthquakes assigned to Los Angeles [and others north and south within Southern California] undoubtedly had their origin along some portion of the San Andreas Fault.” He went to say, though that “a close study . . gives reason to doubt that more than one disastrous earthquake ever visited the City of Los Angeles.”
He noted that the Portolá Expedition of 1769-1770 experienced heavy shaking on the way north through what is now Orange County and then aftershocks in subsequent days in summer 1769, something we’ll discuss briefly at the Homestead’s commemoration of the expedition on the 28th. Sedgwick observed that “these reports are evidence of a very heavy shock but they are not proof that the shock was of sufficient intensity to cause great disaster to a large city like Los Angeles” had it existed then.
In 1812, a major tremor destroyed all or most of the missions at La Purisima in Lompoc, Santa Inez near Solvang, Santa Barbara, and San Juan Capistrano, while major damage was done to San Buenaventura (Ventura), San Fernando and San Gabriel. On 9 January 1857, a San Andreas Fault generated shaker near Fort Tejon caused destruction of some houses in San Fernando and San Diego, displaced water in the Los Angeles River and opened up large cracks in the San Gabriel and San Bernardino valleys. This latter was rated a 9 or 10 on the Rossi-Forel scale (estimated 7.9 on the Richter).
To Sedgwick, a 10 level Rossi-Forel earthquake in Los Angeles “would be disastrous,” while he continued that “Los Angeles is in an area of relatively low seismicity, subject to occasional shocks of moderate intensity and the possibility of a recurrence of major shocks.”
The professor went on to discuss earthquake insurance, noting that Class I (the best-built structures) rates leapt over 300% from 1925 to 1928, while Class II jumped 400%, Class III some 600%, while frame structures over 400 square feet and buildings of any size made of tile, concrete block and adobe in Class IV skyrocketed 1347%. Yet in May 1928 there was a substantial reduction for small buildings in the various classes, except those with tile, concrete and adobe construction.
He observed that policies often had deductibles of 10%, though some recently were reduced to 5%, but he also stated that heavy requirements on earthquake insurance were a problem for modern and well-built concrete and steel commercial structures. In other words, Sedgwick observed,
it is very doubtful if an earthquake with the intensity of the one which occurred in 1857 would cause anything like $100,000 (the required deductible) damage to a well-constructed million-dollar steel building.
Therefore, he called for a 3% premium for these examples.
The state insurance commission reported that, in 1927, there was just shy of $3 million paid in premiums for earthquake insurance, with about $360,000 in payouts. There was, however, no information available on how much insurance was taken out, though it was noted that the average price was 50 cents per $100 of value, meaning about $580 million of insurance was estimated. Yet, Sedgwick noted, those taking out insurance had “no voice in the schedule of rates” and that premiums were not commensurate with losses reported to insurance companies, especially compared to “other lines of insurance.”
In fact, he wrote that premiums for earthquake insurance were 400-500% more than in other types of insurance and he called for changes. He added that companies were concerned that 95% of all earthquake insurance was from California and that high deductibles were imposed because of “a desire to spread the risk more evenly over the rest of the United States.”
Sedgwick closed by asserting that companies could better protect themselves and the public “if they will base their rates on a study of the earthquake tables and then differentiate the rates according to the relative seismicity of the different districts,” noting that “rates apply only where building codes recognize earthquake risks” and mandated better construction. He called for a commission of insurance experts, seismologists and engineers to conduct further study and apply rates more equitably based on what he discussed in terms of risk.
Earthquakes and earthquake insurance happen to be on many Californians’ minds at the moment, so this report, while its findings in terms of faults and earthquake geology are of the time, it is particularly useful in thinking about the role of insurance for a quake-prone, geologically young state.