Seismic and Flood Risk 101

Over the past few years, the Port of San Francisco has been hard at work studying and analyzing seismic and flood risk along the Port's 7.5 mile jurisdiction. This effort has led to some initial findings and strategies for addressing these hazards - view the Measures Explorer and Waterfront Resilience Story Maps to learn more.

If you are learning about earthquakes and flooding due to sea level rise for the first time, or need a refresher on these hazards and related topics, you're in the right place. Read on for a seismic and flood risk primer to help you understand these hazards as they apply to San Francisco's waterfront.

Seismic Risk

What is seismic risk?

Seismic risk refers to the probable building damage and potential harm to people in the event of a likely earthquake on a particular fault. Seismic risk and seismic hazard are occasionally, and incorrectly, used interchangeably.

Seismic hazards include specific effects such as ground motion / shaking, liquefaction, and lateral spreading that can happen during an earthquake. 

What is an earthquake?

An earthquake is a sudden shaking of the ground as a result of movement along a fault line between two tectonic plates or volcanic activity. Seismic activity is another way of talking about earthquakes or related movement of the Earth's crust.

The San Francisco Bay Area is home to many faults, with the two most hazardous (and most well-known) being the Hayward and San Andreas faults. How strongly we feel an earthquake at any specific location within San Francisco is based on which fault line in the Bay Area causes the earthquake, where the movement occurs along the fault, and how large the movement is.

Hazards that can be caused by an earthquake include liquefaction and lateral spreading.

Liquefaction occurs when water-saturated sediment (like sand) temporarily loses strength and acts as a fluid. Imagine jumping on wet sand near the water at the beach until it turns soft and your feet sink in.

Liquefaction is caused by strong ground shaking during an earthquake and is greater in areas with sandy soils. As a result of liquefaction, buildings, roads, and utility lines may lose their foundational support and the likelihood of significant damage increases.

Lateral spreading is a seismic hazard that causes large areas of land to move, typically toward a body of water. This results in potentially large cracks and settlement at the ground surface, resembling a landslide but on relatively flat ground.

How are earthquakes measured?

Earthquakes are commonly rated on the moment magnitude scale. This scale measures the total amount of energy released.

Earthquake magnitudes are also based on a logarithmic scale. This means, for example, that a magnitude 7.0 (M7.0) earthquake would be 10 times stronger than a magnitude 6.0 (M6.0) earthquake.

Oftentimes, assessment of earthquake risk is done using a probabilistic approach. This type of approach takes into consideration how all nearby faults contribute to the likelihood and magnitude of ground shaking at the location of interest.

A probabilistic approach differs from a regional assessment that uses a specific earthquake scenario - such as a M7.0 earthquake on the Hayward Fault. Regional assessments allow for a large range of earthquake scenarios to be evaluated and used for planning and design purposes.

The Port's Multi-Hazard Risk Assessment utilizes four hazard levels to assess seismic consequences. These hazard levels range from a "frequent" earthquake expected to have a 70 percent likelihood of occurring in the next 50 years to a "very rare" earthquake expected to have only a 5 percent likelihood of occurring in the next 50 years. The "frequent" earthquake would be similar to the 1989 Loma Prieta earthquake and the "very rare" earthquake is larger than the 1906 Great San Francisco earthquake.

What's at risk to earthquakes along San Francisco's waterfront?

The Embarcadero Seawall was built over a hundred years ago in earthquake country, without today's seismic standards, atop "weak young bay mud" - a soft, weak mud that can amplify earthquake shaking. With over $100 billion in assets and annual economic activity along the waterfront supported by the Seawall, the Seawall truly is the City's economic backbone.

The Seawall is part of the Embarcadero Historic District and underpins the Embarcadero Promenade, supporting many of the city's iconic destinations, parks, and local businesses, which attract more than 24 million people each year. The Seawall also supports key utility and transportation infrastructure, including BART, Muni, and ferry networks, and serves as a critical emergency response and recovery area. Over 50 key emergency assets depend on the Seawall.

Mission Bay is also at risk. Originally the area was spread over more than 500 acres of salt marsh and lagoon inhabited by the Costanoan people before it was filled in the late 1800s/early 1900s. The area began to transition to industrial in the late 1800s and was filled with debris from the 1906 earthquake.

Similarly, Islais Creek is also at risk to earthquakes in the Southern Waterfront. Islais Creek was once a broad creek and marsh area that was filled for agricultural and industrial uses in the late 1800s. Filled areas, like Mission Creek and Islais Creek, are subject to liquefaction in the event of an earthquake.

How has the Port been studying earthquake risk along the waterfront?

Over the past two years, the Port's Waterfront Resilience Program team has conducted extensive studies, including the Multi-Hazard Risk Assessment (MHRA). The MHRA provides new information and insight into how earthquake damage and impacts from different flood events might impact people, the environment, and the economy. The MHRA also provides insight into which assets and services are most at risk along the Embarcadero waterfront.

How have past earthquakes shaped our waterfront?

The 1906 earthquake was centered three miles from the Embarcadero Seawall, and the 1989 Loma Prieta earthquake was centered about 60 miles from the Embarcadero Seawall. Both were based on seismic activity along the San Andreas Fault.

In 1906, much of the City burned following the earthquake, which ruptured water pipes, leaving firefighters little choice but to create fire breaks by demolishing buildings. Afterward, the City installed the largest dedicated high-pressure water system ever built with pump stations, fireboats, and waterfront manifolds as supply sources. Today's waterfront is a vital area in coordinating and leading the City's emergency response.

As the City rebuilt itself, buildings were constructed to better withstand earthquakes and additional measures for fire protection were also installed. San Francisco continues to make updates to its building requirements to improve seismic safety.

  • Since 1906, the Bay Area has enjoyed a historically quiet period of seismic activity. The U.S. Geological Survey estimates a 72% chance of a major earthquake happening between now and 2043.

The 1989 earthquake prompted the transformation of the elevated Embarcadero Freeway into today's street-level Roadway and Promenade, helping shape the waterfront we know and love today.

The magnitude of the 1989 Loma Prieta earthquake is estimated to be about M6.9, which is less severe than the 1906 earthquake, estimated at M7.9. Additionally, the Loma Prieta earthquake was located approximately 60 miles south of the city which influenced the ground shaking felt in San Francisco. Our waterfront's history has shown that San Franciscans have and remain committed to responding effectively to disasters and prioritizing safety and community resilience.

Flood Risk and Sea Level Rise 

What is sea level rise?

Sea level rise occurs when polar and glacial ice melts and global sea levels rise in response to additional water. Polar ice melt has been occurring at an exponential rate due to increased carbon dioxide levels in the Earth's atmosphere, making sea level rise a symptom of global climate change.

Flood risk due to sea level rise examines to what degree flooding is likely to occur at a particular future water level. Flood risk also takes into consideration such factors as storm events, precipitation, and groundwater levels.

  • In an ever-changing climate, sea level rise and flood risk are becoming increasingly interconnected.
  • For example, a sudden downpour during a larger-than-normal rain storm, such as recent atmospheric rivers, causes intermittent flooding on city streets now and in the future could cause more severe and lasting flooding with a rising San Francisco Bay.

Are there different types of flooding?

Yes! There are different types of flooding, and they are already affecting San Francisco's neighborhoods. These impacts will continue to grow in the future.

  • Future tidal flooding is flooding from the bayside and is caused by rising global temperatures that are melting polar and glacial ice that in turn causes global sea levels to increase, making high tide higher.
  • Storm surge is flooding that occurs during a storm event and creates higher water levels and waves for a period of time.
  • Groundwater flooding is when the land absorbs an excess amount of rainfall and/or excess bayside water from rising sea levels.
  • Larger than normal waves can also impact the waterfront and cause flooding.
  • Erosion can be caused by flooding. It refers to the wearing away of the waterfront due to wave impacts.

What are some ways that future flooding could impact San Franciscans?

Flooding on the Embarcadero

Sea level rise is a major, and increasing, threat to San Francisco's safety. Some ways that sea level rise is already impacting the city include:

  • Intermittent flooding of the Embarcadero Promenade. As sea levels continue to rise in the northern waterfront, there will be additional flood risks to the BART Transbay Tube, Muni light rail, key utility infrastructure, and waterfront businesses and neighborhoods.
  • Stormwaters flooding the Mission Creek and Islais Creek areas. As sea levels continue to rise, there will be increased flood risks near Mission Creek and Islais Creek, particularly along the shoreline, which includes Muni and key utility infrastructure. Flooding could directly impact homes and could disrupt local commercial corridors, commutes and transit options, parks and open spaces, emergency services, and hospitals and healthcare centers. These potential disruptions pose serious challenges to the Mission Creek / Mission Bay and Islais Creek / Bayview communities and to the city as a whole.

For the Bay Area, a rising San Francisco Bay means that flooding from future sea level rise is more likely to occur along bay fill, historic creek sheds and waterways, and other places where we have built along the shoreline.

How has the Port been studying flood risk from future sea level rise along the waterfront?

The U.S. Army Corps of Engineers (USACE) and the Port of San Francisco (Port) have partnered to study flood risk along San Francisco's bayside shoreline. The USACE San Francisco Waterfront Coastal Study is one of several coordinated waterfront resilience activities being undertaken in partnership with federal, state, and local agencies to plan for anticipated seismic activity, flooding, and sea level rise.

The approximately three- to five-year Waterfront Coastal Study will identify vulnerabilities and recommend strategies to reduce current and future flood risks for consideration by the Assistant Secretary of the Army and the U.S. Congress for federal investment and implementation.

Why can't we predict exactly how high sea levels will rise and when in the future?

Waves crashing on the Embarcadero during King Tides

Future sea level rise directly depends on global carbon emissions and global climate action. An increasingly warm planet means less polar ice and more water in our oceans.

In 2018, the State of California issued updated guidance on a variety of scenarios predicting how rising sea levels may impact California's coastline. The Port and City and County of San Francisco are using this data to inform the work of the Waterfront Resilience Program and other City efforts to address climate change.

Sea level rise "scenario planning" means the Port uses projections and the best available science to predict where and how flooding will occur along San Francisco's waterfront. Because sea level rise depends so much on how global carbon emissions are curbed, it is useful to prepare for a variety of different "futures."

One way to help visualize future sea level rise is to track King Tides, which are the year's highest tides and will only become more extreme in the future due to sea level rise.

King Tides occur when the sun and moon align along the same gravitational pull and both are at their closest point to the Earth. The Embarcadero already floods during King Tides.

What are some of the sea level rise scenarios that the Port has been using to prepare for future sea level rise?

Over the next few decades, climate and sea level rise projections have a relatively high degree of certainty. After mid-century, the changes are harder to predict and depend on the amount of greenhouse gases (GHGs) emitted globally as well as the sensitivity of Earth's climate to those emissions.

To prepare for future sea level rise, the Port is using the best available science, pulling from city, regional, state, and international guidance. Intergovernmental Panel on Climate Change (IPCC), a United Nations body formed in 1988 and made up of governments and scientists from around the world.

The IPCC has developed and agreed upon a set of four scenarios for greenhouse gas concentration trajectories that assume different levels for human-caused greenhouse gas emissions and different efforts to limit or reduce emissions. These scenarios are known as Representative Concentration Pathways (RCPs).

  • The current State of California Sea Level Rise Guidance (2018) relies primarily on the RCP that assumes human-caused global greenhouse gas emissions will continue to rise over the next century. In other words, there are no significant efforts to limit or reduce emissions. This is RCP 8.5.
  • However, California's ambitious statewide greenhouse gas reduction efforts are aligned with a RCP that assumes strict emission reductions that would curtail human-caused (or anthropogenic) global emissions by 70 percent between 2015 and 2050, to zero by 2080, and to below zero after. Below-zero emissions would mean that humans absorb more greenhouse gases than they emit. This is RCP 2.6.
  • To date, the City and County of San Francisco has selected a RCP that assumes human-caused global greenhouse gas emissions will peak in 2040 and then decline as its guide. This RCP is considered more realistic as a potential lower bound for planning for sea level rise based on current voluntary greenhouse gas emission reduction goals agreed upon by participating nations in the Paris Climate Agreement. Slow action, or lack of action, since the Paris Climate Agreement greatly impacts the likelihood of a more aggressive RCP being achieved. This is RCP 4.5.

Sea level rise projections, or "curves," for the San Francisco Bay are based on conditions from these RCPs rather than moments in time.

In addition to RCPs, the State Guidance also includes an extreme scenario (referred to as H++). This scenario represents a future with rapid Antarctic ice sheet mass loss. It follows the premise that overall warmer global temperatures expected at mid-century will change the physics governing ice sheet mass loss. These changes could cause further impacts to the West Coast. At present, the H++ scenario is highly uncertain and a topic of ongoing scientific research.

Due to the many factors that contribute to sea level rise projections, it is difficult to predict with certainty when and where exactly water will flow.

For this reason, the State Guidance uses the categories "Likely" and "1 in 200 chance" when describing probability that a particular water level will occur.