Potential Earthquake Impact in California, USA
Which areas are at the highest risk?
Introduction
Which areas in California are at the greatest risk of acquiring high negative impact from earthquakes? That's what I wanted to find out. Using elevation, distance, location, and population data I found the cities and general areas in California that would be most impacted by an earthquake from the San Andreas Fault.
Concerning this study, the focus on impact was specific towards levels of human impact (i.e. which cities would receive the highest number of casualties or have the highest number of people effected).
Data Analysis
California DEM
The first thing I did was clip my DEM (Consbio), or Digital Elevation Model, to the boundaries of California (U.S.). On this map, the white/red areas show high elevation and the green areas show low elevation.
Most of California has a relatively low elevation, but there is a mountain range (the Sierra Nevadas) that runs through the Eastern part of the state.
San Andreas Buffer
The next step was creating a buffer around the San Andreas fault (Ferrar) to act as the "high impact" zone. I chose a distance of 20 miles, as that is a distance close enough to feel the worst effects of any earthquake.
After creating the buffer, I turned it into a raster object so that I could reclassify it later and run raster analysis.
Cities Buffer
For this project, I chose to only look at cities within California that had a population greater than 100,000 people. One reason for this was because there would be too many cities to run analysis on, and another reason was cities with higher populations would have more people affected from an earthquake than small towns.
There are 71 cities in California with a population greater than 100,000 people. Los Angeles has the highest population at 3,887,115 people (California) and San Mateo has the lowest population of the set at 100,361 people (California).
After creating the buffer, I transformed the features into a raster so that further analysis could be done.
Hillshade
To make my final map more cartographically pleasing, I decided to make a hillshade of California. I used the DEM that I clipped previously to create it.
Hillshade
Here is a map of the DEM overlaid on the hillshade to show the elevation changes in California.
Elevation Reclassification
I took the DEM of California and reclassified it into three categories. The first class (red) is made up of locations where elevation is between -105 and 637 feet above sea level. I chose this range because the median person lives at an elevation of 637 feet, or 194 meters, above sea level (Cohen). The next range (yellow) is from 638 to 1,274 feet above sea level and the last range (green) is 1,275+ feet above sea level.
As most of the green areas consist of high elevation, mountainous areas, they were given a lower rank because less people would (presumably) be affected by an earthquake there.
Fault Reclassification
I reclassified the fault buffer where a value of 1 (the entire buffer) would be classified as 1, and all NO DATA values would be classified as 0. I did this so that when I did my raster analysis later, the entire area of California would be ranked instead of just an area around the buffer.
Cities Reclassification
The last factor I needed to reclassify was the city buffers. I did the same thing as the fault buffer where I set the city values of 1 to 1, and the NO DATA values to 0.
Raster Calculator/Analysis
In order to rank high and low impact areas, I had to perform some raster analysis. The equation and weights that I used are as follows:
(0.4 * Cities_Reclass) + (0.5 * Fault_Reclass) + (0.2 * Elev_Reclass)
I chose to weight the fault line area the highest as I believe that raw distance from the fault line is the highest indicator of impact. I gave cities the second highest weight because areas with high numbers of people are at greater risk. Lastly, I gave elevation the lowest weight because I thought it had the lowest effect on total impact.
Elevation Classes, San Andreas Fault, and Cities
Elevation & Cities
I was not surprised to see that none of the cities in the study were located in the high elevation areas. Of the 71 cities, only three of them were in the mid-elevation range. The three cities are Lancaster (Pop: 164,664), Palmdale (Pop: 159,649), and Victorville (Pop: 115,903).
Fault Line & Cities
There were a surprising amount of cities within 20 miles of the San Andreas fault line. Of the 71 cities, 15 were within that range. Of those 15 cities, both San Francisco and San Jose were included which boast populations greater than 800,000 people respectively.
Final Outcome
Potential Earthquake Impact in California
Little to No Impact
All of the 'Little to No Impact' areas consist of mountainous, low populated areas. Most of the green sections are also a considerable distance away from the fault line. I would expect to see very minimal impact in these green areas.
Small Impact
The 'Small Impact' section takes up the greatest amount of space. I would expect to see only a small amount of impact in the yellow areas, based on their distance away from the fault line.
Moderate Impact
The 'Moderate Impact' zone is where most of the larger cities reside, which is okay. Both California's capital city (Sacramento) and highest populated city (Los Angeles) are in this zone. I would expect to see moderate impact in the light orange areas, and most of the danger comes from having such large populations in a fairly close proximity to the fault line.
High Impact
The 'High Impact' zone is mostly composed of low elevation areas that are within 20 miles of the fault line. There are no large cities included in this zone because if a city were to be placed within the 20 mile buffer, it would automatically be categorized as 'Severe Impact.'
Severe Impact
Areas labeled 'Severe Impact' are threatened the most by a possible San Andreas earthquake. There are 15 cities that are ranked as 'Severe Impact.' Combined, these cities have a population of around 3,800,000 people (this number does not even include cities with populations less than 100,000). In the event of a major earthquake, these cities would experience the highest amount of impact, damage, and casualties.
Future Work
If I were to continue further analysis into earthquake impact in California, here are some of the things I would consider (in no particular order).
- All fault lines across the state, instead of just the San Andreas Fault.
- All cities in the state, instead of just those with a population greater than 100,000.
- Multiple-Ring buffers around fault lines with decreasing ranks the farther the distance.
- More in depth analysis into what makes an area particularly vulnerable to earthquake damage.
- More variables (Location of previous earthquakes, cities with high number of earthquake casualties, geology & how certain types of rock layers behave during earthquakes, etc.)
I want to personally thank UW-Eau Claire's Geography and Anthropology Department and, more specifically, Peter Strand for allowing me the opportunity to conduct this study and provide me with the knowledge and materials to successfully complete it.
Data Sources
Cities in California:
North American DEM:
Consbio (2014). 30 Arc-Second DEM of North America. H:\Documents\ArcGIS\Packages\d2198be9d2264de19cb93fe6a380b69c_674913\commondata\data0\
San Andreas Fault Line:
Ferrar@FracTracker.org (2013). San Andreas Fault. http://services.arcgis.com/jDGuO8tYggdCCnUJ/arcgis/rest/services/San%20Andreas%20Fault/FeatureServer
California Boundary:
U.S. States (Generalized). H:\Documents\ArcGIS\Packages\USA States_69B4FAE9-93D6-4567-A173-31888E77455B\v10\states.gdb
References
Population & Elevation Data:
Cohen, J. E., & Small, C. (1998, November 24). Hypsographic demography: The distribution of human population by altitude. Retrieved from https://www.pnas.org/content/95/24/14009 .
