Title
A Look At Earthquake Risks in San Francisco



Author Information
Jeremy Smith
American River College, Geography 350: Data Aquisition in GIS; Fall 2008
EWC 151


Abstract
There have been many beneficial uses that have been found for geographical information systems; one of which is disaster hazard detection and prevention. Residents of the city of San Francisco are well aware that they live in a highly active seismic zone; however, are they aware that certain areas of the city are at higher risk than others? I intend to explore the various threats that can be posed to a structure based on the properties of the subsoil its foundation is placed of. I will then see how and if these hazards can be applied to the city of San Francisco. Finally, I will conclude with some suggestions as to how these hazards can be avoided or reduced.

Introduction
Ever since the famous 1906 earthquake that occured in San Francisco, California, the city has been well known for seismic activity. The highly active San Andres Fault runs directly through the city and although residents are used to earthquakes, they may not all be fully aware of the risks they take building in the various locations they do. A large part of the damage that occures from an earthquake is directly a result of the type of subsurface material the building foundation is placed on. The composition of the subsoil can cause intensified ground motion and or a phenomenom called liquefaction. I intend to determine how much of a threat the subsoil poses to the structures in San Francisco. By being made aware of the hightened risk that certain areas of the bay may have, residents can better plan building design and location based on the risks associated with the reaction of the subsoil during an earthquake.

Background
To begin to understand the gravity of the problem at hand, one must first understand the processes that can lead to a building’s failure during a seismic event. The most immediate threat, according to the Association of Environmental and Engineering Geologists (2008), is displacement along the fault itself. When this occures, the fault has shifted so that the plates on either side have moved vertically and or horizontally with respect to each other. If a building rests directly on a fault and this occures, it would mean almost certain destruction of the building. An illustation of this effect from the 1906 San Francisco earthquake is presented below.

Northwest of Olema, California 1906

The Association of Environmental and Engineering Geologists also gave valuable information on ground motion and the factors that can increase it. Ground motion is the shaking of the gound as seismic waves propegate away from the earthquake epicenter. This shaking action of the ground is most intense closest to the epicenter and dissipates as the distance from the epicenter is increased. This ground motion, however, is also a function of the type of subsurface soil and rock conditions where the building is placed. An unconsolidated material, such as sand or gravel, will shake more intensely than solid rock. This action of ground motion is measured as a percentage of the gravitational acceleration here on Earth. Denoted as g, 25 percent would be seen as 0.25g. The next hazard associated with the subsoil a building’s foundation is placed on was best described by Jörgen Johansson (Department of Civil Engineering) University of Washington (January 27, 2000); liquefaction is a phenomenom that causes the soil that a building’s foundation is placed on to no longer be capable of supporting the weight of the building during a seismic event. This weakening of the soil is due entirely to the composition of the soil itself. When the particles are loosely packed together such as in sand or clay, water is able to fill the spaces between the individual particles. Under normal conditions, these particle exert a very low water pressure and remain stable; however, during an earthquake the water is forced to the surface by the immense pressures of the quake. This movement of the water causes building foundations to shift and or sink into the soil; in some cases completely destroying the structure. Due to the fact that liquefaction occures only in saturated soils, its effects are most commonly observed in low lying areas near bodies of water; such as rivers, lakes, bays, and oceans. Seeing as how San Francisco is a city surrounded by water, it should also be of concern to the cities residents. Below are two photographs, supplied by the University of Washington, of the damage that can be caused by liquefaction.

Niigata, 1964

San Fernando, 1971

Methods
After having done a few searches on the Internet, I found it was quite simple to find the information I needed for the actual effects an earthquake can have when the subsoil is determined. I was, however, still in need of a good map which would display the various compositions of the subsoils San Francisco is built on. After doing some more research, I discovered that the United States Geological Survey (July 16, 2008), or USGS, provides this service on their website for free. The map provided includes all of the San Francisco Bay area and is capable of detail up to 50 feet. With this data now at my disposal, I was now able to determine how much of a threat the subsoil posed to San Francisco residents.

Results
The following maps and information in the chart were obtained through the USGS website. It is when this information is combined with the background information that it becomes more relevent.

Bay Area Around San Fransisco

San Fransisco

The following table breaks down the various soil types and how sheer waves, or horizontally moving waves,travel through them. The table is catagorized as theUSGS catagorizes them. The shear wave velosity is denoted as ’vs’; where lower vs values indicate a higher intensity of shaking.

Soil Type A	vs > 1500 m/sec	Unweathered intrusive igneous rock
Soil Type B	15oo m/sec >vs > 750 m/sec	Volcanics; mostly Mesozoic with some Franciscan bedrock
Soil Type C	750 m/sec >vs > 350 m/sec	Quaternary sands, sandstones, and mudstones; some Upper Tertiary sandstones, mudstones, and limestones; some Lower Tertiary mudstones and sandstones; Franciscan melange and serpentinite
Soil Type D	350 m/sec >vs > 200 m/sec	Some Quaternary muds, sands, gravels, and silts
Soil Type E	200 m/sec >vs	Water saturated mud and artificial fill

Analysis
It becomes clear when viewing the map of San Francisco’s subsoil composition that nearly half of the city is built on areas of high risk for intensified shaking. The areas which fall under the yellow catagories consist of both residential and business districts; while the areas in red are primarily commercial and business. The red areas consist of largely artificial fill some of which ironically came from the debris from the 1906 earthquake. Unfortunately, when it comes to the problem of liquefaction, the situation only gets worse. While the areas of red and yellow are at a much more elevated risk for liquefaction to occure than the others, the areas that fall under the green catagory also have the potential for this to occure. Due to the fact that they are composed of sands, sandstones, and mudstones; the particles in these subsoils would be loosely packed together. The spaces between the particles allows for them to become saturated, leaving this subsoil type also highly susceptible to liquefaction.

Conclusion
When faced with the fact that nearly all of San Francisco is at risk for intensified ground motion, liquefaction, or both simultaneously; one must wonder what can be done to better prepare for a seismic event when it occures. Building construction plays a large part in how a structure will react to seismic activity. According to the Association of Environmental and Engineering Geologists, more rigid structures tend to do poorly during an earthquake, such as a building made of mostly brick. Buildings constructed of wood tend to do better than brick structures as they are more able to flex and move with the seismic waves as they pass by. Many modern high rise buildings take this in account in their design and materials chosen for their construction. There is, however, little that can be done for the hazard of liquifaction. The best defence against this is to not place structures on subsoils which are likely to have this occure. It is in the best interest of San Francisco residents to know of these dangers as they may effect them in the future. In the areas that fall under the soil type ’C’ catagories, constant measurements should be taken to ensure that the subsoil isn’t becoming saturated; as this could help prepare for the threat of liquifaction. As was previously stated, knowledge of the potential threats provides the best defence as they can be better prepared for.

References
Association of Environmental and Engineering Geologists (2008); http://www.aegweb.org/i4a/pages/index.cfm?pageid=4074
Jörgen Johansson; Department of Civil Engineering, University of Washington (January 27, 2000); http://www.ce.washington.edu/~liquefaction/html/what/what1.html
United States Geological Survey (July 16, 2008); http://earthquake.usgs.gov/regional/nca/soiltype/