In and around the city of San Francisco
This was the first devastating Californian earthquake to destroy a major city. Three thousand lost their lives because of it
Early in the morning of April 18, 1906, while most people were still in their beds, a 7.8 strength earthquake hit San Francisco. The shock lasted for less than a minute but that seemed like a year to those who were rudely awakened and had to rush out into the streets with whatever clothing they could lay hands on. Aftershocks soon followed and the destruction they could see in every direction convinced most people to stay away from their crumbling homes. The worst horror came later in the morning with fires all over the city, sixty in all. A firestorm erupted to add to the terror. The fires raged for three days with a total destructive power twenty times that of the earthquake, one of the most devastating in the history of California. There were three thousand deaths.
It took this event, the first major assault on a big city by an earthquake, to set in motion a serious quest for the cause of the earthquake. About five hundred city blocks had been devastated. Masonry buildings collapsed but wood frame homes and skyscrapers withstood the shock. One exception was the landfill areas in Marina District near the water. Wood frame homes there simply disintegrated. There is a special reason for the damage in this area. This district, south of Market Street, was a filled area; that is, it had been constructed by pouring unconsolidated materials, sand and rocks, on stream beds and other places that were too close to the water line to allow for construction. Buildings were then erected on this artificial foundation, and when the shaking from an earthquake occurred, liquefaction took place. Water seeped from below and changed land that formerly seemed quite solid into a watery mess, quite incapable of supporting buildings. Because this was the industrial part of the city people felt they could risk the possibility of a disaster from an earthquake. The cost of building on filled land was much lower than anywhere else in the city and industrialists felt that this lower cost would offset the price of reconstruction after a quake.
Unfortunately, as so often happens in human-induced disasters like this one, the lessons learned are not remembered when there is a different kind of event involving filled land. Over the years, people forgot the dangers of filled land and the area that had been an industrial site was rebuilt again, this time as a fashionable residential subdivision. When the Loma Prieta earthquake struck in 1989, this area was totally demolished and many lives were lost. San Francisco was not the only city to forget the past. Tokyo did the same thing, or rather allowed the same thing to happen. Many years before the 1923 earthquake, one area of the city had been built up on filled land and the authorities in the city knew that this was the case. They also knew about the San Francisco earthquake because Japan experiences more earthquakes than almost any other place on earth and there are always ongoing studies of earthquakes in other parts of the world. Nevertheless, nothing was done about the filled land and new homes were built on it after 1906. When the 1923 event hit Tokyo the buildings on filled land collapsed immediately and many people died. One part of the San Francisco filled land that did not collapse was the Palace Hotel, although it did catch fire.
Most of the buildings in the city were built of wood and, as such, they would normally withstand earthquakes because of the ability of wood to expand and contract under shaking. Most of them were lost because of fire and even those that survived had trouble because their chimneys toppled, ripping plaster off ceilings and walls and breaking floors as they fell. Some places at considerable distances from the city suffered severe damage, far more than other locations at equivalent distances from San Francisco, simply because they happened to be on one of the many faults that stretch out from or run parallel to the main fault. Santa Rosa was one of those places. It is nineteen miles away from the source of the earthquake and it experienced extreme damage. Fifty people were killed there. It was a similar story in several places west of the San Joaquin Valley even though they were thirty miles south of the earthquake's epicenter. Because of their proximity to water courses these areas experienced liquefaction, rift fissures, avalanches, and earth slumps.
Electrical power lines, water mains, and all the other normal services were cut off. When the first fire broke out nothing could be done about it because there was no water. Then a firestorm erupted. These fires raged for three days and the whole city was incinerated. All attempts to create firebreaks failed. There were probably three or more thousand killed, 1 percent of the total population. When the fires finally subsided people searched for their homes or what might be left of them. It was a difficult task. All the familiar landmarks had vanished. The quake was felt over an area close to 200,000 square miles, all the way from Oregon to south of Los Angeles and eastward to Nevada. Cities closer in proximity suffered varying amounts of damage. Stanford University was one of the worst affected. Several buildings were completely destroyed there. One person's observation on the morning of the earthquake sums up the experience of most: the street was undulating as if it were the ocean with waves sweeping toward me. I was terrified.
Earth waves rolled across the state with clear depressions between the swells. When they finally broke open there were parallel fissures with lengths of six hundred feet or more. Another type of fissure took the form of a rectangular-shaped block that dropped, leaving a trench with vertical sides. Landslides occurred wherever the banks of rivers were steep and where there were steep bluffs. Frequently forests were carried down or were overthrown by the slides. Several sections of land were raised as much as twenty feet above the highest flood level for the area concerned. Other places dropped by as much as fifteen feet although the majority of these were of the order of seven feet. The forests of different areas, that altogether added up to 150,000 acres, were completely destroyed. About forty miles to the south of San Francisco, near the limit of the earthquake's destructive power, the Spanish mission of San Juan Bautista, built a hundred years earlier, was severely damaged.
San Francisco was a flourishing city in 1906. It had sprung into fame as the premier city of the West Coast during the gold rushes of fifty-five years earlier and at the time of the earthquake it had a population of 400,000. San Francisco's prosperity was due to new mining developments in western Nevada, to truck farming from surrounding agricultural areas and, most of all, as a seaport for Asian trade. Fires destroyed large parts of the city six times in the course of its short history and their frequency persuaded builders to switch from wood to brick and stone. They also inspired the leaders of the city to create a brand new fire department. It earned the name of being the best in the world. Sadly, the one consideration that ought to have been uppermost in the minds of the city officials was missing—the provision of emergency supplies of water in the event of the city's mains being severed by an earthquake. As had happened with filled land, so here too, San Francisco's lack of emergency water supplies was a mistake that was repeated in subsequent urban earthquakes, as in Tokyo in 1923. A plan to pump emergency water supplies from San Francisco Bay had been laid out but never implemented.
The fear of earthquakes was never evident in San Francisco in spite of the fact that the city had experienced the tremors of three quakes, those of 1836, 1838, and 1868. Memories seemed to be short. In the immediate aftermath of the quake and fires, with all the awareness of their neglect of adequate preparation for emergencies, and with a quarter of a million of the city's population, more than a half of the total, homeless, everyone took responsibility for the task of reconstruction. Money contributions, practical help, and the provision of military units from the federal government all helped to speed up the recovery. Every individual could, and did, participate in the clean up and in the practical work of either building homes or carrying supplies to building sites. Within three years San Francisco was back to a near normal level of operation and was growing at a fast rate.
San Francisco has the highest density of underground faults of any urban area in the United States. Furthermore, the break in the San Andreas Fault that caused the disaster was six miles below ground and the amount of lateral displacement was as much as twenty feet in places. Costs in 1906 dollars were close to $500 million. That would be about $7 billion today, a figure close to the cost of the 1989 Loma Prieta earthquake. Earthquakes had struck California before. In 1812, an area south of Los Angeles was hit and more than thirty people were killed by it. In 1857, another quake caused considerable damage over an area northeast of Los Angeles. Some decades later a geologist from the University of California who had observed a fault line south of San Francisco, decided to trace its extension north and south. He and his students found, to their surprise, that this was no ordinary fault. It was the San Andreas Fault and it ran almost the full length of the state, close to the coast for the most part but veering inland in the south.
This discovery was made in the 1890s, long before there was any understanding of plate tectonics, so little was done with the new information. Now, with hindsight and present knowledge of tectonic plates, the story of the San Andreas Fault is the key to understanding most of California's earthquakes. First attempts to understand its behavior began within a year of the 1906 disaster, first by a geologist who had lived through the quake. He recognized that the cause of the earthquake was slippage on part of the San Andreas Fault, later identified as a segment that stretched for 250 miles from Monterey Bay northwards. Pipelines and roads that crossed the fault line had been broken and displaced by an average of twelve feet with the western side always moving northwards with respect to the eastern side. It was evidently a strike-slip fault but until the era of plate tectonics everyone regarded it as an anomaly, a one of a kind event unique to California. With the increasing public concern today about the potential for destructive earthquakes in California since the great Alaska earthquake of 1964, and the general acceptance of the concept of plate tectonics and sea-floor spreading in the late 1960's, the San Andreas Fault has received new attention. It is closely related to such recent earthquakes as the Loma Prieta of 1989 as well as the much earlier quake in Fort Tejon in 1857.
At the northwest end of the fault system, the Mendocino triple junction represents an intriguing structural knot where the North American, Pacific, and Gorda plates join. A fourth block at depth, made up of material below the North American Plate but east of the San Andreas Fault and south of the Gorda Plate, is juxtaposed with these three named plates. The San Andreas Fault is the one that dominates in the interaction between the huge Pacific and the North American plates and some of its effects are felt far inland across the western part of the country. It is a rare situation to find these two plates, the Pacific and the North American, meeting on land as they do here. As a strike-slip one it moves as much as two inches a year and it has been doing this for more than fifty million years with a total displacement of hundreds of miles. Massive earthquakes are associated with it. Both the Pacific and North American plates are moving relative to the deeper parts of the earth, so the San Andreas Fault boundary is also moving, changing its shape in the process as the adjacent plates deform. In southern California, the sector of the fault from north of Los Angeles to east of San Bernardino has been rotating slowly counterclockwise. We tend to think of the margins between these plates as narrow lines because this is the way they are depicted on maps and we also tend to imagine the rest of the Cordillera as being static. The reality is quite different. The plates are moving on a sphere, not on a flat surface. In some places the plate margin may be hundreds of miles wide and the whole of California may be in motion at different rates in different places.
The San Andreas Fault in California represents the main boundary between the Pacific and the North American plates ( see illustration p.20, and...
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The San Francisco earthquake of 1906, which occurred on April 18, was characterized by a very high magnitude of 8.3 on the Richter scale. It...