| Title Shaking Earth of California | |
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Author Mateo Yanes American River College, Geography 350: Data Acquisition in GIS; Fall 2005 | |
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Abstract Earthquakes are a way of life in California. Earthquake data along with GIS can be used to show frequency of earthquakes along fault lines but it can only show so much. Currently, earthquakes are only given a probability to happen over many decades. This is a long way from the ability to predict with a high probability. Earthquakes are still being understood. A lot of scientists are gathering information at the scene of where earthquakes take place, fault zones, to better understand them. One day this unpredictable natural disaster may be better understood to give us more warning when the next big one will strike. | |
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Introduction California is a great place to live. People from all over the world come to California to enjoy its coast, mountain ranges, and wonderful forests. But the thing they forget about is right under their feet. California's silent destructive force in waiting is the earthquake. No one knows when the next big one will be. Earthquake faults clutter California's landscape. Many Californians are blissfully unaware of the continued activity of earthquakes. Earthquakes happen almost every day. Many earthquakes are small and most people will not feel the earth movement from them but when big earthquakes hit they can take out entire cities. It is my intention to study the California earthquake data over the last years beginning in 2000 and what it means for California's future. How close are we to the next big earthquake and will we ever know? With the use of GIS software and earthquake data we will explore these questions as we look at the Shaking Earth of California. | |
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Background First, we must understand how earthquakes take place. Earthquakes take place when the plates in the crust of the Earth shift. The theory of this is called Plate Tectonics.  [Source: PBS.org - A Science Odyssey: Intro to Plate Tectonic Theory]  [Source: Hawaii Natural History Association] In plate tectonics, the upper part of the mantle is cooler and more rigid than the deep mantle and it behaves like the overlying crust. Together with the Earth's crust they form a rigid layer of rock called the lithosphere. The lithosphere averages about 80km thick over much of the Earth's crust while it tends to be thinnest under oceans around 5km thick and thickest on the continents around 100km or more. The lithosphere has been broken up into the moving plates that contain the world's continents and oceans. Below the lithosphere it is to believed to be composed of hot, semi-solid material, which can soften and flow after being subjected to high temperature and pressure. This mobile zone in the mantle right underneath the lithosphere is called the asthenosphere. The rigid and brittle lithosphere is thought to float on top of or move about the slowly flowing asthenosphere.  [Source: This Dynamic Earth, USGS] The irregular shaped plates in the lithosphere either move away from, toward, or slide past each other. These are called divergent, convergent, and transform plate boundaries, respectively. Most earthquakes are created when the boundaries of the plates grind past each other. Other earthquakes can be created by erupting volcanos or landslides.  [Source: Hawaii Natural History Association] When plates are divergent, move away from each other, new crust is created by magma being pushed up from the mantle. Divergent earthquakes tend to be smaller than magnitude 8.0.  [Source: Hawaii Natural History Association] When plates are Convergent, move towards one another, usually one plate moves underneath the other. Convergent plates create the largest earthquakes with some earthquakes in Alaska and Chile having exceeded a 9.0 magnitude.  [Source: Hawaii Natural History Association] Transform plates which slide past each other are also shallow using a strike-slip mechanic. Transforms tend to be smaller than magnitude 8.5. In California, the San Andreas fault is an example of a transform.  Aerial view of the San Andreas fault slicing through the Carrizo Plain in the Temblor Range east of the city of San Luis Obispo. [Source: Photograph by Robert E. Wallace, USGS] The San Andreas fault runs the length of California and makes up the core fault area with many branches off of it.  [Source: This Dynamic Earth, USGS] Normal movement between the two plates along the San Andreas Fault Zone is about 1.7 inches per year or as the USGS puts it, "about as fast as your fingernails grow." (USGS Earthquake FAQ) The Pacific and North American Plates are continuously grinding past each other and a lot of tension can build up between the two plates. This built up energy can be stored up as elastic stress that is released suddenly during an earthquake. When a release happens in a strike slip motion an earthquake occurs and depending on the size of the slip the ground can move a large distance compared to normal movement.  Earthquake Damage in San Francisco, California, April 18, 1906. Fence on the E.R. Strain farm displaced 8.5 feet. [Source: USGS] Henry Fielding Reid, Professor of Geology at Johns Hopkins University, came up with the "elastic rebound theory" after studying he effects of the San Francisco 1906 earthquake. He theorized that most earthquakes are the result of the sudden elastic rebound of previously stored energy. | |
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Methods Earthquake data was acquired from the Northern California Earthquake Data Center (NCEDC) in conjunction with the Northern California Seismic Network, U.S. Geological Survey, Menlo Park and the Berkeley Seismological Laboratory, University of California, Berkeley. The online catalog allowed easy access to years of detailed information. Data used in this project was broken down into year starting with the year 2000 and ending in November of 2005. All earthquake data acquired for analysis was of magnitude 3.0 or greater. The earthquake data from the NCEDC has latitude and longitude coordinates and were imported into ArcMap for analysis. (NCEDC Catalog Metadata) The output data from the NCEDC was in a plain text form. This data was then saved by year into separate text files. These files were then imported into Microsoft Excel. After imported into Excel the data was then resaved as dBASE IV files. Excel and dBASE IV data did not maintain the information in the same way so data had to be first extracted from Excel, reformatted, and then resaved in dBASE IV for significant digits to be saved as well as date and time data. Fault data was acquired from the Fault and Fold Database maintained by the USGS in the form of shapefiles. The fault data is incomplete and projected to be completed in October of 2006 but it does give a very good sense of the earthquake faults in California. (USGS Fault and Fold Database) Background map data was acquired from the US Census from the 2000 Census in the form of shapefiles. (Census 2000: State and State Equivalent Areas Cartographic Boundary Files) | |
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Results Once all the data was imported into ArcMap by year the symbology was changed to standardize the size and color of earthquake magnitudes. Sizes ranged from 3.0 to 10.0. Each whole number range received a different color to better distinguish them from the others since a lot of earthquakes take place in the same areas along fault line zones as seen in the figures below.
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Figures and Maps
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Analysis/Conclusions Going through this project it became clear that with just using earthquake frequency it is hard to predict when a big earthquake will take place. There is some talk about how many small earthquakes might help a big earthquake from taking place but since the earthquake magnitudes are on logarithmic scale it doesn't seem to hold true to this. Previous predictions have been made relating to earthquakes but they more of probabilities and when they were made the earthquakes happened after the predicted time window. This makes these predictions somewhat unreliable. It is said that another big earthquake will take place within the next 25 to 30 years (2030 - 2035) with roughly a 60-67% chance. California has the second highest rate of earthquakes in the United States with Alaska being number one.Earthquake understanding is still in its infancy. The only way we will be able to better understand earthquakes and one day be able to predict them with a higher rate of probability is if we look at the areas where faults are active. One such endeavor is in Southern California in Parkfield. It is called the San Andreas Fault Observatory at Depth (SAFOD) (SAFOD) SAFOD is a deep borehole observatory that will directly measure changes in the physical conditions of the active level of the San Andreas fault zone at depth. It will be at a 3.2km depth and will monitor temperature, pressure, and strain during earthquake activity. With such instruments scientists will be able to better understand earthquakes. Another project currently happening is the B4 Project. (B4 Project Overview) Scientists took LIDAR (Light Detection And Ranging) of the San Andreas Fault. They want to be able to look at the fault before the next big earthquake and then take imaging again after the next big earthquake. | |
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References B4 Project http://researchnews.osu.edu/archive/faultgps.htm Hawaii Natural History Association: Teacher's Guide to the Geology of Hawaii Volcanoes National Park http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html San Andreas Fault Observatory at Depth (SAFOD) http://www.earthscope.org/safod/index.shtml This Dynamic Earth: The Story of Plate Tectonics, USGS 1996 by W. Jacquelyne Kious and Robert I. Tilling ISBN 0-16-048220-8 http://pubs.usgs.gov/publications/text/dynamic.html USGS Earthquake FAQ http://earthquake.usgs.gov/faq/ | |
