The San Andreas Fault and its role in Plate Tectonics and Earthquake Prediction The San Andreas Fault is one of the most widely studied faults in the world. Scientists use an array of methods in collecting data and providing analysis of fault characteristics both past and present. Presently there are many differing hypothesis and models used to describe crustal movements and deformation within the Pacific and North American plate boundary. Historical earthquakes along this fault have proven to be rather large and devastating.
This is important since the San Andreas Fault runs along many highly populated areas throughout Northern and Southern California. Through further research and analysis of this fault system scientists hope to solve the some of the unknowns of plate tectonics and better predict when and where an earthquake will take place. Introduction The San Andreas Fault is a strike-slip fault located on the boundary of two tectonic plates in the Earth’s lithosphere; the Pacific plate on the West and North American plate on the East. This intricate fault system contains a network of faults extending from Northern to Southern California.
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It is one of the most accessible and widely studied faults in the world. One of the distinct characteristics of the fault is the contrast of rock types on either side, brought together from long distances by millions years of movement. This movement generates several thousand earthquakes annually. By analyzing past and present day movements and configurations of the San Andreas Fault scientists hope to better understand the faults role in plate tectonics and unlock the key to earthquake prediction. Geographic and Physical Location
The San Andreas Fault extends some 1,100 kilometers almost the entire length of California from Eureka to Brawley (Figure 1). The fault is the boundary between the North American and Pacific lithospheric plates. The fault is broken into the following segments: * Northern Segment – Extends approximately 300km from Shelter Cove to San Juan Batista, this entire segment ruptured in 1906, known as the San Francisco earthquake. * Creeping Segment ??? Extends from San Juan Batista south to Parkfield, unlike other segments which are “locked” this segment has a steady movement of approximately 3 centimeters per year (aseismic creep). Parkfield Segment ??? Extends only 30 km, this is the center of the San Andreas Fault, characteristics include lack of human disturbance, and it is the site of the SAFOD Drilling Project. * Central Segment ??? Extends 350 km from Cholame to the Cajon Pass through the Mojave Desert. Traverses the San Emigdio and San Gabriel Mountains. * Southern Segment ??? Extends approximately 300 km from the Cajon Pass to the Salton Sea, this segment has not ruptured since some time before 1700, and it is widely considered that this segment is overdue for a magnitude “8” earthquake.
The San Andreas Fault is a transform or strike-slip fault. It is the sliding boundary between the Pacific Plate and the North American Plate. Over the past 5 million years the North American Plate has been moving southeast while the Pacific Pate moves northwest. The plates are sliding slowly past each other at a rate of about 2-3 inches per year, however this no a steady movement but an average movement, the plates may remain “locked” relatively motionless as they push against one another.
Then suddenly the built up pressure is released and the plates along the fault slip and move. This slippage and release of energy sends seismic waves in all directions, which we feel as earthquakes. Historical Earthquakes along the San Andreas Fault At 5:12 a. m. on April 8, 1906 the initial foreshock of the Great San Francisco earthquake shook the San Francisco Bay Area, approximately 20 to 25 seconds later the actual earthquake was felt, with an epicenter near San Francisco the violent shaking lasted 45 to 60 seconds. Buildings were destroyed and fires broke out across the city.
Approximately 700 deaths were attributed the earthquake and fire, but it is widely believed that those numbers have been underestimated by a factor of 3 or 4 (Ellsworth, 1990). The Loma Prieta earthquake occurred on October 17, 1989 at 5:04 p. m. It had a moment magnitude of 6. 9, and lasted 15 to 20 seconds. The epicenter was near Loma Prieta, and this earthquake again caused severe damage to the San Francisco Bay Area, this time claiming 63 lives but almost 4,000 people were injured as a result of the earthquake. The earthquakes above give a brief limpse into the importance of analyzing past and present fault configurations. If scientists could make a breakthrough in this area perhaps we could better predict earthquake activity and better prepare ourselves if it is deemed a potential disaster is looming. Methods of Analysis Scientist are struggling to come up with a widely accepted model that will explain the role of non-vertical strike-slip fault segments, crustal movement and deformation within Pacific ???North American plate boundary. Multiple methods of analysis are being used; some of which are described below. Measurements of Geologic uplift and slip rates ??? in many areas of the San Andreas Fault a detailed record of strike-slip activity is provided. By studying the sediments deposited in a given area and comparing them to an established age control it is possible to determine the rates and timing of changes of tectonic deformation. * Velocity Fields and Pole of Rotation ??? Using the Southern California Earthquake Center’s Velocity Field, which contains 840 velocity vectors covering the entire Southern California region scientists, are able to identify important relations between the velocities and locations of active fault segments.
These finding enable changes to be made to existing models that are used to attempt to predict outcomes of deformation and movements in specific areas. Scientists are able to input variable data, such as direction, velocity and location, etc. , into the model in order to simulate earthquakes having a certain set of characteristics and the specific outcomes to the predetermined areas (Figure 2). * Three-Dimensional Kinematic models ??? Researchers use vertical fault geometry to model the kinematics of the San Andreas Fault. Kinematic models attempt to describe the motion of objects without consideration to the causes leading to the motion.
Methods of analysis in this field are constantly changing, much of that driven by the advantage of quickly increasing computing capacity and the development of new models, such as the Southern California Earthquake Center Community Fault Model (Suess and Shaw. , 2003). This model is a compressional wave velocity model based on tens of thousands of direct velocity and seismic reflection data. The SCEC was recently awarded $10 million dollars from the National Science Foundation to better develop computing capabilities that will lead to the ability to better forecast when earthquakes are likely to occur in Southern
California. Use of Analysis Research that is being conducted concerning the San Andreas Fault is necessary in order to provide scientist with a chance at better predicting the next large earthquake. It is estimated that a magnitude “7” earthquake has a 62% chance of hitting San Francisco in the next 30 years. If an earthquake of this magnitude were to strike Southern California on a workday, with an epicenter under Los Angeles it is estimated that between 7,000 and 18,000 would be killed (U. S Geological Survey).
Virtually the entire Southern California freeway system and railways would be destroyed, cutting off supplies and needed help. While billions of dollars are being spent retro-fitting hospitals and overpasses in California, it is estimated that 2,100 of 9,600 schools are not guaranteed to hold up in future earthquakes, so billions more dollars are still necessary. The Federal Emergency Management Agency (FEMA) has spent most of its resources to grant funds for anti-terrorism efforts, leaving little for earthquake preparedness.
This lack of federal funding has lead to community and state-wide programs such as “The Great California ShakeOut” to connect communities and provide California residents the information and knowledge to prepare and respond in the event of an earthquake disaster. This event featured the largest earthquake drill in U. S history in November of 2008, with some 5. 3 million individuals participating. The importance of awareness about earthquake preparedness in Southern California is not lost on the area residents. Conclusion The evolution of the San Andreas Fault dates back to the Mid-Cenozoic period about 30 million years ago.
Scientists have only recently begun to investigate the deformations and crustal movements associated with this fault. By further analysis and interpretation of the surface and sub-surface data scientists may one day be left with a reliable earthquake prediction system. Currently there are just too many variables and unknowns. It seems each day researchers are uncovering more advanced and accurate ways of measuring fault strength and stresses in the San Andreas Fault, often dispelling prior theories and hypothesis. With the only recent introduction of computer models and simulations this trend is sure to continue.
This is an important area of research, as many of the faults surrounding areas are densely populated, and a large earthquake would obviously be devastating to the state of California and whichever region was involved. Types of damage would be unlimited, ranging from contamination to the water supply, a halt to the transportation system, and infinite economic and health considerations. As scientists continue to gather data and make hypothesis through three-dimensional and computer modeling a breakthrough is eagerly awaited.
Figure 1: Map of California showing the location of the San Andreas Fault (David K. Lynch, Thule Scientific) Figure 2: Southern California Earthquake Center Community Fault Model, allows variable data to be input to the model showing outcomes to pre-selected areas (SCEC, 2009). References Cited Cohen, Philip, Inside the San Andreas. (San Andreas Fault, California), New Scientist, February 17, 1996, pp 24 (4) Dair, Laura, San Andreas Fault geometry through the San Gregorio Pass, California, Geology, (37)2, 119, 2009 Powell, Robert E. , R. J.
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