Title Seismic Activity in California
Author Joel Boros American River College, Geography 26: Data Acquisition in GIS; Fall 2013 Contact Information
Abstract California's coast occurs on the divide of two tectonic plates, North America and Pacific plates. The place where they meet is known as the San Andreas Fault. Fault lines are a source of seismic events that brings to question which cities are most at risk for earthquake events. Based on distance from the San Andreas Fault, the distance from fault source points, and distance from historic earthquake events of magnitude 5.0 and greater, I queried which cities of population 100,000 and greater are at greatest for earthquake events. Three cities resulted: Daly City, Rancho Cucamonga, and Sunnyvale
Introduction California has a history of seismic activity, some of which have resulted in catastrophic results. The United States Geological Survey (USGS) has multiple stations scattered across California which records daily seismic activity. The most notable source of the activity is the San Andreas fault. I have gathered data for points of seismic activity source points and episodes of seismic magnitude of 5.0 or greater. A good number of those points fall within the area of major cities. I would like to know what cities are at the greatest risk for earthquakes within California. To do this I asked several questions. Which cities are within 20 miles of the San Andreas Fault? Which cities are within 5 miles of a fault source point? Which cities are within 30 miles of historic earthquake events of magnitude 5.0 or greater?
Background Seismic activity is the result of the sudden release of energy stored in the Earth's tectonic plates. California rests on the North America Plate and its coast lies on the Pacific Plate. The plates are sliding past one another in opposite directions. This is a continental transform fault; it occurs as a result of when two tectonic plates slide past one another. This is also known as the San Andreas Fault.
As a result of continuous seismic activity the United States Geological Survey (USGS) has more than 50 seismograph stations that record real-time ground motion across California. The information is recorded using the Richter scale; based on a magnitudes ranging from 1-10 (USGS, 2013). Most of the seismic stations occur along the San Andreas Fault. The influx of continuous seismic data allow for better response and mitigation of those areas affected by earthquakes. The areas most susceptible to seismic damage occur along the fault. Major cities such as San Francisco and San Bernardino have a history of earthquake events. One such event occurred on October 17, 1989, the Loma Prieta Earthquake (Nakata, J. et al). It was a 6.9 magnitude earthquake which caused untold damage to the city and its inhabitants.
Methods I acquired the earthquake fault source points and the list of magnitude 5.0 and greater seismic events from the California Natural Resources Agency (CEIC). Both of the data sets are published by the California Geological Survey. I also acquired a line feature of the San Andreas Fault from within the online databases provided by ESRI; the data set was produced by Ferrar@FracTracker.org. My city population and county boundaries were both acquired from Data & Maps for ArcGIS® via the online ESRI database.
I used my collected data to make spatial queries to determine what cities and counties are most at risk from the San Andreas Fault, are in range of fault source points and those cities that have been affected by 5.0 and greater seismic events. The measurement of the radius of an earthquake takes into account where the location of the epicenter, the magnitude and the amplitude (University of Regina). I do not have amplitude data and therefore I am unable to do any precise calculations. Instead I chose a radius of 30 miles from my earthquakes of 5.0 or greater to query. In addition I chose a radius of 10 miles to see what cities lie within range of fault source points. To determine which of the cities are most at most risk I did a select by location query to see which cities overlapped across the results.
Results First to narrow the scope of my project I narrowed the selection of my cities to those with a population of 100,000 of greater. This resulted with 64 cities to work with.
	I then did a query to see which cities were 20 miles from the San Andreas Fault; this reduced my selection down to 16 cities; the most notable being San Francisco. Another query was done to determine the cities within a 5 mile range of fault source points which resulted in 30 cities out of the 64. Of those 64 cities I did another query to see which have been exposed to a 5.0 or greater magnitude earthquake within the last 100 year, which results in 58 cities. My last query was looking at which counties contain fault source points with; 43 out of 58 counties contain them.
Figures and Maps
	Cities that are within 20 miles of the San Andreas Fault Line.
	Cities that are within 5 miles of a fault source point.
	Cities that are within 30 miles of historic earthquakes of magnitude 5.0 or greater.
Analysis As a state lies on the boarder of two major tectonic plates, California is susceptible to seismic activity. The San Andreas Fault is the result of a transform fault line between the Pacific and North American Plate. As a result of the fault and many smaller faults California has many sources of potential earthquakes. Of California's 58 counties 43 contain a fault source point. This gives cause reason as to why the USGS has many seismograph stations throughout California, especially along the San Andreas Fault. The cities I looked at are those within 20 miles of the San Andreas Fault. There are 16 cities within 20 miles that have a population of 100,000 or greater. Two notable cities are San Francisco and San Jose. San Francisco has a history of seismic activity. I preformed another query looking at which cities were within 5 miles of a fault source point. Thirty of the 64 cities are close to these points which are potential sources of seismic activity. One thing to note is that 3 cities that are within 20 miles of the San Andreas Fault are also within 5 miles of a fault source point: Daly City, Rancho Cucamonga, and Sunnyvale. This is a cause for concern for these cities. Nearly all of the queried cities are within 5 miles of historic earthquake events of magnitude 5.0 or greater. Once again overlap occurred between the other queries. All of the cities within 20 miles of the San Andreas Fault fall into this historic seismic activity range. This once again includes the three cities mentioned above.
Conclusions Earthquakes are a part of the ever changing world. As the tectonic plates of the earth shift and move energy is released in the form of seismic energy. Two of Earth's tectonic plates, the North America Plate and the Pacific Plate lie right on the northern coast of California and proceed inland the further in the south. This gives rise to the occasional earthquake. A little more than a 1,000 earthquakes of magnitude 5.0 occur a year globally; they are considered moderate class of seismic activity (USGS). Typically earthquakes less than 5.0 cause little to no damage (UPSeis). But anything greater than 5.5 is a cause for concern. That is why the USGS has 50+ stations in California that record seismic activity to not only gather data and predict future activity but also as an early warning system. The San Andreas Fault is is the dividing line between the two primary tectonic plates beneath California. California has more than 60 cities that have a population of more than 100,000 peoples. Of those cities, 16 are within 20 miles of the San Andreas Fault. The biggest of them being San Francisco and San Jose. This brings to question which cities along the fault are at the greatest risk for seismic damage. To further narrow my results I looked at fault source points within the state. They are the points at which the stress between the adjacent plates and faults is the greatest which in turn are the source of most potential epicenters for seismic activity. Of the most populated cities, 30 of them are within 5 miles of a fault source point. This leads to the notion that those cities are at the higher risk for earthquake events. My final question looked at those cities which are 30 miles from historic earthquakes of magnitudes of 5.0 or greater. I chose the radius of 30 miles because that is the conservative estimate of the radius of effect caused by a magnitude 5.0 earthquakes. This resulted in nearly all of the cities of a population of 100,000 or greater being selected. This meant virtually all cities within range of the San Andreas Fault have experienced at one point or another, the effects of seismic events of moderate damage over the course of 100 years (1900-2000). Of those cities three repeatedly showed up: Daly City, Rancho Cucamonga, and Sunnyvale. Based on my analysis those cities are the greatest risk for seismic events. However, California as a whole is continuously experiencing and feeling the effects of earthquakes. The USGS can provide early warning systems, but cities such as Daly City must consider building a foundation that can width stand such events.
References United States Geological Survey (USGS). http://www.usgs.gov/. Accessed December 16, 2013. University of Regina. http://mathcentral.uregina.ca/index.php. Accessed December 16, 2013. Michigan Technical University, UPSeis. http://www.geo.mtu.edu/UPSeis/. Accessed December 16, 2013. The October 17, 1989, Loma Prieta, California, Earthquake—Selected Photographs. John K. Nakata et al. http://pubs.usgs.gov/dds/dds-29/. Accessed December 16, 2013.