Although it is difficult to imagine this great amount of shifting of the Earth's crust, the rate represented by these ancient offsets is consistent with the rate measured in historical time. Surveying shows a drift at the rate of as much as 2 inches per year.
What Is an Earthquake? The crustal plates of the Earth are being deformed by stresses from deep within the Earth. The ground first bends, then, upon reaching a certain limit, breaks and "snaps" to a new position.
In the process of breaking or "faulting," vibrations are set up that are the earthquakes. Some of the vibrations are of very low frequency, with many seconds between waves, whereas other vibrations are of high enough frequency to be in the audible range. The vibrations are of two basic types, compression waves and transverse or shear waves.
Since the compression waves travel faster through the Earth, they arrive first at a distant point; they are known as primary or "P" waves. The transverse waves arriving later are referred to as shear or "S" waves. In an earthquake, people may note first a sharp thud, or blast-like shock, that marks the arrival of the P wave. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S wave.
Magnitude is a measure of the size of an earthquake. The Richter Scale, named after Charles F. Richter of the California Institute of Technology, is the best known scale for the measuring of magnitude M of earthquakes.
The scale is logarithmic; a recording of 7, for example, signifies a disturbance with ground motion 10 times as large as a recording of 6. The energy released by an earthquake of M 7, however, is approximately 30 times that released by an earthquake of M 6; an earthquake of M 8 releases times 30x30 the energy of an earthquake of M 6. An earthquake of magnitude 2 is the smallest earthquake normally felt by humans.
Earthquakes with a Richter value of 5 or higher are potentially damaging. Some of the world's largest recorded earthquakes--on January 31, , off the coast of Colombia and Ecuador, and on March 2, , off the east coast of Honshu, Japan--had magnitudes of 8.
As the Richter scale does not adequately differentiate between the largest earthquakes, a new "moment magnitude" scale is being used by seismologists to provide a better measure.
On the moment magnitude scale, the San Francisco earthquake is estimated at magnitude 7. Intensity is a measure of the strength of shaking experienced in an earthquake. The Modified Mercalli Scale represents the local effect or damage caused by an earthquake; the "intensity" reported at different points generally decreases away from the earthquake epicenter. For example, an earthquake of intensity II barely would be felt by people favorably situated, while intensity X would produce heavy damage, especially to unreinforced masonry.
Local geologic conditions strongly influence the intensity of an earthquake. Commonly, sites on soft ground or alluvium have intensities 2 to 3 units higher than sites on bedrock. Earthquakes Along the Fault Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth's crust.
The largest historical earthquakes that occurred along the San Andreas fault were those in and The earthquake of January 9, , in southern California apparently was about the same magnitude as the San Francisco earthquake of According to newspaper accounts, ground movement in both cases was roughly the same type. An account of the earthquake describes a sheep corral cut by the fault that was changed from a circle to an "S"-shape--movement clearly representative of right-lateral strike-slip.
Studies of offset stream channels indicate that as much as 29 feet of movement occurred in The San Francisco earthquake and fire of April 18, , took about lives and caused millions of dollars worth of damage in California from Eureka southward to Salinas and beyond. The earthquake was felt as far away as Oregon and central Nevada.
The earthquake, which has been estimated at a magnitude 8. With about 45 years between the historic earthquakes but about years since the last one, it is clear that the fault does not behave like a clock with a regular beat. Along the southernmost San Andreas, from Palm Springs to the Salton Sea , earthquakes happen infrequently, about every years. The most recent earthquake occurred during the time of Spanish exploration, about years ago, but there is no historic record of the event.
Instead, radiocarbon dating provides the age of the most recent earthquake and six more that occurred since about A. A paleoseismology site in Wrightwood, CA has been studied by several scientists, and recently in the detailed data from multiple studies were joined together to create a single timeline. The resulting year record includes 29 surface-rupturing earthquakes. Careful analysis of the age of the earthquakes, including the uncertainties in radiocarbon dating see Determining the Age of a Paleoearthquake in Introduction to Paleoseismology , showed that the average time between earthquakes is about years.
The recurrence intervals times between earthquakes at Wrightwood are more regular than clustered determined by a mathematical analysis , and only four times in the past has the interval between two major earthquakes been longer than the current interval since The results of this study indicate that this section of the San Andreas Fault is likely to have a large earthquake in the not-too-distant future.
About km to the northwest along the fault another site at Frazier Mountain has been investigated. At that location, the record is about years long, and in that time period, there are about 9 large earthquakes recorded in the sediments, including the rupture. Comparing the data from sites like Wrightwood and Frazier Mountain, earthquake scientists are working to understand the pattern of large earthquakes — asking questions such as how typical was the large M7.
Note that because the magnitude scale is a log scale, there is about a fold difference in the energy released by these different earthquakes. Map of faults in northern California. The Hayward fault in the San Francisco Bay area runs through a densely-populated area, so it has been studied quite a bit. The most recent major earthquake on this fault was approximately M6.
The fault has been creeping about 4. The average time interval between the 5 most recent earthquakes is a little shorter, about years. The Maacama fault is the northward continuation of the Hayward-Rodgers Creek fault system in northern California. In , a paleoseismology site at Hael Creek on the Maacama fault reiterated the results found on the Hayward fault to the south — creeping with infrequent large earthquakes, and a large one expected in the not-too-distant future.
Observations in the trenches along with radiocarbon dating of charcoal, wood chips, and small plant remains, combined with a reevaluation of three previously-studied nearby paleoseismic sites revealed a variation in seismic activity in the past.
Three earthquakes occurred within a year period between and , but there were no earthquakes during the years before that, and there have been no earthquakes in the years since This shows that the average time between earthquakes includes some intervals that are short and some intervals that are long. There is abundant camping, bird watching, wild flowers and wildlife, rock collecting and natural beauty along the way. State and National parks are strung along the fault like beads on a string.
David K. When not hanging around the fault or using the large telescopes on Mauna Kea, he plays fiddle, collects rattlesnakes, gives public lectures on rainbows and writes books Color and Light in Nature, Cambridge University Press and essays.
The book contains twelve one-day driving trips along different parts of the fault, and includes mile-by-mile road logs and GPS coordinates for hundreds of fault features. As it happens, Dave's house was destroyed in by the magnitude 6. Find Other Topics on Geology. Maps Volcanoes World Maps. Largest Earthquake. California Earthquake Maps. What Causes a Tsunami? New Madrid Seismic Zone.
What is the San Andreas Fault? Earthquake Lessons. The Japan Earthquake. Rocks: Galleries of igneous, sedimentary and metamorphic rock photos with descriptions. Minerals: Information about ore minerals, gem materials and rock-forming minerals.
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