San Francisco: A City Subject to Natural Hazards

The city of San Francisco is in the State of California, on the west coast of the United States. The city is located in a bay called the San Francisco Bay. The San Francisco Bay Area refers to a group of cities located in this geographic location, such as San Jose and Oakland.

​​​​​In 2021, San Francisco’s population was 815 201 inhabitants, making it the fourth most populous city in the State of California^[3]. The area of the city is 120 km^2[4], therefore its population density is 6793 inhabitants/km².

In comparison, the city of Montreal has a population density of 912 inhabitants/km^2[5].

The high population density of the city of San Francisco puts it at greater risk in the event of a natural disaster, with greater human and material consequences.

San Francisco is located in the United States, a developed country. The gross domestic product per inhabitant (GDP/inhabitant) in the United States is $69,287.54, one of the highest GDP/inhabitant in the world^[6]. The population generally has a good standard of living.

Consequently, San Francisco has the resources and financial means needed to put in place natural hazard preventive measures. These measures do not stop natural hazards, but they can reduce their negative effects. To find out more about preventive measures, check out the preventive measures section.

San Francisco’s geographic location exposes it to many earthquakes, due to the contact between two tectonic plates (the North American and Pacific plates) and the presence of several faults, including the San Andreas fault.

There are several earthquakes in San Francisco every year, but most of them are minor and cannot be felt. However, the city has had some very high-magnitude earthquakes in the past.

There is a very high risk that the city will be hit by another high-magnitude earthquake in the coming decades. This future earthquake is referred to as the ‘Big One’. Since the two plates are continuously moving closer to each other, the pressure on the plates is increasing. When the accumulated pressure releases, it causes a high-magnitude earthquake. This release occurs approximately every 150 to 200 years^[7]. That's why experts know that an earthquake of this magnitude will occur in the next few decades. However, it is not possible to predict the exact moment.

The city of San Francisco is located in a tsunami risk zone since it is on the coast, in an area with strong earthquake activity. Since 1850, scientists have observed and recorded approximately 51 tsunamis^[12].

The biggest tsunami to hit the San Francisco Bay Area was caused by a 9.2 magnitude earthquake in Alaska in 1964. The waves that reached the coast were approximately 1.2 m high^[13].

The tsunamis that could affect San Francisco are classified into three different groups. These groups are based on the distance from the trigger event.

Floods in San Francisco are mostly caused by heavy rainfall. The city is not able to drain the water fast enough and water accumulates, causing a number of problems.

The city of San Francisco is at high risk of landslides, mostly caused by heavy rainfall. Landslides are also caused by earthquakes and can be deep or shallow. Both can have serious consequences for the population.

In general, the city of San Francisco is well prepared for the various natural disasters that can occur on its territory. Here are some examples of the disaster prevention measures put in place.

The city of San Francisco is at a very high risk of earthquakes. Experts predict that the Big One has a 62% chance of occurring by 2032^[7]. As a result, San Francisco has already put in place several measures to reduce the consequences of future earthquakes. Here are a few examples of these measures.

Even though San Francisco is taking numerous measures to strengthen infrastructure, such as schools, hospitals and highways, certain buildings are sometimes less prioritized than others in prevention plans. This is the case of skyscrapers.

In the last two decades, the city of San Francisco has permitted the construction of more and more skyscrapers. These buildings are often built very close to each other and are getting taller in order to try to resolve San Francisco’s serious housing crisis. In 2018, the city already had 160 skyscrapers that were more that 73 metres high, with over a dozen others planned or in construction^[21]. Building experts are increasingly realizing that skyscrapers are not as safe and solid as was once believed.

The discoveries made about the Millennium Tower led to demands for a reassessment of earthquake risks related to skyscrapers. Attention was focused on improving building standards. Here are a few examples of the standards that could be improved.

• Standards for solidity are the same for a 5-storey building and a 50-storey building.

• It is important to accurately calculate the real resistance of skyscrapers to earthquakes, not only because earthquakes are unpredictable, but also considering that buildings in large cities are not often tested by high-magnitude earthquakes.

• Past earthquakes have revealed defects in previous construction techniques:

  • The Northridge earthquake in 1994 in Los Angeles revealed that a very common welding technique does not resist earthquakes. Several buildings in San Francisco that were built using this technique have not yet been updated to implement the new standards, meaning there is additional recognized risk.

• Construction standards require buildings to be collapse-proof. However, they do not require that they be inhabitable after an earthquake. Therefore, even if skyscrapers do not collapse, thousands of people could find themselves on the street after an earthquake because they couldn’t return to their home.

San Francisco’s skyscrapers are not as solid as they could be and are often built very close to each other, which significantly increases the risk of damage during earthquakes.

In addition to inadequate building standards, the location of several skyscrapers puts them in danger. More than 100 of them are located in a zone where the soil is at high risk of liquefying during an earthquake.

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