Today in Geological History – April 18th; San Fran Earthquake


The Great San Francisco Earthquake of 1906 is often know as one of the United States worst natural disatsers along side Hurrican Katrina. The Mg 7.9 earthquake dwarfed the 1989 Loma Prieta earthquake in every way.

Figure 1. Fires raged through the Bay area.

At 5.12am residents of the San Francisco Bay Area were awoken by a strong foreshocks. Just 25 seconds later a rupture over 470 kilometres long tore through the northern end of the San Andreas fault. The intense shaking demolished buildings and burst through gas mains causing multiple fires.  Early estimates put the death toll at 700 directly from the quake, but it is thought as many as 3000 people lost their lives in all.

Figure 2. Buildings crumbled under the pressure of the waves.

Poorly constructed buildings just crumbled under the force of the tremor while many others were destroyed in the widespread fires. The situation was made worse by the fact insurers had no policies which covered earthquake damage at the time, so people who had already lost everything deliberately started fires claiming they were caused by the gas leaks. Over 28,000 people were left homeless. It took over five days to put out fires across the county and the damage was estimated at over $400 million. Being in a wealthy stay in San Francisco recovered relatively quickly although some people remained in tempory shelters for over a year. Homes and businesses were rebuilt with stronger building codes put in place; old, wooden Victorian homes were replaced with brick and concrete so that future fires can’t ravage the area as they had. 

Figure 3. Tectonic setting of the San Andreas Fault and its last three major earthquakes.

An important outcome of the earthquake was awareness was raised to the danger posed by the San Andreas Fault. The highly active fault line runs throughout the state of California through many metropolitan areas, and although the residents were used to the odd tremor, few thought it could release something so violent. The transform fault runs roughly 1300 km down the west coat of the United States where the Pacific plate is sliding northwestwards in relation to the North American Plate. Although at the time the process of plate tectonics was not know, the magnitude of the quake and size of the displacement incorraged people to question more the Earth’dynamics. 

The San Andreas fault experiences hundreds of earthquakes yearly, although few much over a magnitude 4. At least one over a magnitude 6 appears to occur ever 20 or so years. In 2008 Uniform California Earthquake Rupture Forecast (UCERF) estimated that the probability of an M ≥ 6.7 earthquake within the next 30 years on the northern and southern segments of the San Andreas fault is somewhere between 21% and 59%, respectively. There seems to be no question of ‘if’ this will occur again but ‘when’ and ‘where’. 

California’s population has grown tremulously in recent decades and the fault cuts through several major cities including San Fransico, Los Angeles and San Diego. If the next ‘Big One’ were to hit such a densely populated area the consequences could be dier. However awareness is the state is high, Hollywood film “San Andreas” even hits screens next month. For now all we can do is be prepared and hope for the best.

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