The 1906 earthquake which devastated San Francisco, on California’s Pacific coast, killed an unknown number of people (estimated to be in the thousands) and was one of the largest recorded on the North American continent. Located on the notorious San Andreas Fault, a major seismic zone at a boundary between two of the earth’s tectonic plates, the rebuilt San Francisco remains at risk of a further major tremor.
The San Andreas Fault: a Seismic Zone
The San Andreas fault zone is a small part of the zone of earthquake and volcanic activity which encircles much of the Pacific Ocean. Known to earth-scientists as the Pacific Ring of Fire, this zone of seismic activity is the result of the earth’s tectonic plates moving against one another and is regularly the site of major earthquakes and volcanic eruptions.
Where it passes through California, the plate margins form what is known as a transform boundary – that is, the two plates (in this case the Pacific and North American plates) move past each other laterally, rather than colliding directly. The boundary is complex and is characterized by a wide range of related fault zones. The best known and most evident of these is the San Andreas Fault itself.
As the plates move, friction builds up and is released in the form of earthquakes. Such events are common and most are relatively small. San Francisco lies directly on top of the fault and was struck by the large 1906 earthquake, estimated to be of magnitude 7.8 on the Richter scale (USGS; other estimates of magnitude vary). The earthquake caused a tear in the landscape which was visible over 300 km and vertical displacement in places was almost a meter.
The Impacts of the 1906 San Francisco Earthquake
Unlike Los Angeles, which lies some way to the east of the main fault zone, San Francisco sits directly above the fault and it was this that explained the extreme devastation of the earthquake. The tremor itself, which struck in the early morning of April 18, lasted for about a minute and caused immense destruction, especially in the areas of reclaimed land around the bay where land effectively became fluid during the event – a process known as liquefaction.
The drama continued after the earthquake. Blazes broke out, as domestic fires, untended and fed by the collapse of many wooden buildings, quickly took hold. Broken water pipes meant the fires could not be contained and many of those trapped were burned to death. Eyewitness reports tell of trapped victims begging to be shot rather than be left to burn (Max Fast, Adolphus Busch, quoted on Eyewitnesses to History).
The extent of the casualties caused by the earthquake remains unknown. The USGS cites extensive references whose estimates range from 664 to over 3,000, directly or indirectly, with the majority in San Francisco itself. Of a population of around 400,000, over half were made homeless; and an estimated 28,000 buildings were destroyed.
Another San Francisco Earthquake?
At magnitude 7.8 (other estimates put it higher), the 1906 earthquake was one of the largest to strike in California: a review of earthquakes in the state shows the only one larger was that which struck further south at Fort Tejon in 1857 (Ellsworth). In San Francisco itself, the earthquake nearest in comparable magnitude was the 1989 Loma Prieta earthquake near to the city, at magnitude 7.1.
Given the local and regional geology, it is almost inevitable that there will be another major earthquake in the area; USGS forecasts indicate a 99.7% chance that California will be struck by a quake of magnitude 6.7 or more within three decades (USGS, quoted by Wagner in Digital Journal). The article including these forecasts was published just days before California was rocked by an earthquake of magnitude 5.4 in July 2010.
Lessons learned from the 1906 San Francisco quake and other seismic events will, it is hoped, mean that any subsequent earthquake event, even in a densely populated area, won’t have as devastating an effect as that of 1906. The earthquakes at Loma Prieta (1989) and Longridge (1994) killed 123 people between them. New building techniques and regulations, along with mapping of local ground techniques and seismic hazards should contribute to earthquake-preparedness and reduction of both damage and casualties.It is, however, virtually impossible to predict when an earthquake will occur and how big it will be. Even large-scale prediction experiments such as that undertaken at Parkfield in California in the 1980s have failed to yield an accurate prediction. Thus, although a large earthquake in California on the scale of San Francisco seems possible or even probable, when or where it will occur is a matter of conjecture.
Sources and Further Information
Ailsa Allaby and Michael Allaby (eds) Oxford Dictionary of Earth Sciences. Oxford University Press. (1999).
Christopher Wager. The Big One: California’s Future Earthquake Probabilities. Digital Journal, 4 July 2010. Accessed 29 August, 2011.
William Ellsworth. The San Andreas Fault System, California. US Geological Survey Professional Paper. Accessed 29 August, 2011.
The San Francisco Earthquake 1906. Eyewitness To History. Accessed 29 August, 2011.
Robert A. Page, Peter H. Stauffer, and James W. Hendley II. Progress Towards a Safer Future Since the 1989 Loma Prieta Earthquake. USGS. Accessed 22 July, 2010.
The Great 1906 San Francisco Earthquake. USGS. Accessed 22 July 2010.
This article was first published on Suite 101