Earthquakes are the result of a sudden release of energy from the Earth's crust. Earthquakes occur naturally at the fault lines within the Earth's crust. The San Andreas fault in California is a major transform fault formed by the convergence/passing of the North American and Pacific Plate Tectonics. A transform fault is a type of strike-slip fault, where strike-slip faults are known to cause many and very strong earthquakes.
The strength of earthquakes are measured by Richter Scale magnitude or just magnitude. The greater the earthquake, the higher the magnitude values. For instance, the strongest magnitude earthquake ever record along the San Andreas fault was 8.1 on the Richter Scale. This high frequency of strong earthquakes have the capability of causing large amounts of property damage and sometimes deaths within the population.
The United States Geological Survey produces map called ShakeMaps that analyze the ground motion and shaking intensity from earthquakes in near-real time. These ShakeMaps do not analyze the populations risk to earthquakes, which is what was done in this study.
Methodology
For this analysis we downloaded data from the USGS earthquake archive for the whole of California for the years of 2000 and 2010. County level census data for California was also downloaded from the American Factfinder application provided by the U.S. Census Bureau. Both data sets were imported into ArcGIS 10.1 geospatial software for analysis. The geographic locations of earthquakes events were fist displayed and exported as its own file. The information for the earthquakes was then spatially joined with the county level census data.
The first analysis comprised of change in risk of population density to the mean magnitude of the county. Population density was calculated by normalizing population with area. The second analysis performed was on property damage based on magnitude by depth. The magnitude by depth was calculated by dividing individual earthquakes magnitude by the depth of occurrence and then averaging the values over the county.
Results
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Conclusions
The map shows a high concentration of risk in areas where major cities are located in. Overall there seems to be more of an increase in risk from 2000 to 2010 in large part due to population growth.
When completing the study a couple of assumptions were made. For the first analysis the first assumption is that population density would be uniform throughout the county and second that all of the individuals within the county would be affected at the same scale. The second analysis assumed that there is a correlation between how an earthquake affects materials on the surface and its depth.
