Earth Science 1.3 The San Andreas Presearch SA2: Earthquake Faults in California I was awakened by a tremendous earthquake, and though I hadn t ever before enjoyed a storm of this sort, the strange thrilling motion could not be mistaken, and I ran out of my cabin, both glad and frightened, shouting, A noble earthquake! A noble earthquake! feeling sure I was going to learn something. - John Muir "Are these the end times? Yes. And they have been this way since the beginning. Welcome to planet Earth, a wonderful but not entirely stable place to live." - Craig Childs Themes: Cause & Effect, Stability & Change Objectives: 1) Identify the primary earthquake faults in California. 2) Explain how earthquakes are measured. Primary Questions: 1) How are earthquakes measured? 2) Where are California s major earthquake faults? 3) How is California s landscape affected by earthquakes?
Reading: California Faults and Earthquakes by Ted Konigsmark (attached) Read the adaptation actively. Highlight important concepts, circle unfamiliar vocabulary. Compose written responses to the prompts, and attach to the reading for collection. Illustration: California s Earthquake Faults Draw an outline map of California, showing locations of the following faults: San Andreas Owens Valley San Jacinto Garlock Banning Hayward Calaveras
California Faults and Earthquakes adapted from Geologic Trips, by Ted Konigsmark, 1998 The San Andreas Fault System Most earthquakes occur along the boundaries between the huge plates that cover the surface of the Earth. As the plates move, they gouge, scrape, tear, buckle, and fracture the rocks along the edges of the plates. When the rocks break, shock waves travel through the rocks and cause earthquakes. The San Andreas fault is a transform fault that forms the boundary between the Pacific plate, which is moving north, and the North American plate, which is moving west. California is slowly being torn apart along the San Andreas. The parts of California to the west of the fault, including San Diego, Los Angeles, Monterey, Santa Cruz, and the Point Reyes Peninsula, are moving northward at a rate of one to two inches per year relative to the remainder of California. California is, in fact, realizing its life-log dream of dividing into two separate parts, although not in the manner envisioned by many politicians. Although most of the plate movement takes place along the San Andreas fault, some movement also occurs along a number of closely-related faults that trend northwest, and in most the west side of the fault is moving north, just like the San Andreas. This group of faults is commonly referred to as the San Andreas fault system, and includes such local faults as the Hayward, Calaveras, Healdsburg-Rogers Creek, Pilarcitos, and San Gregorio. Most all of the major earthquakes in California are caused by the San Andreas fault and nearby faults that are part of the San Andreas fault system. The San Andreas fault formed about 25 million years ago when the east Pacific Rise spreading center reached the Franciscan subduction zone and began to be subducted under the North American plate. As the spreading center was subducted, the Farallon plate continued to be carried deep below the North American plate. However, the Pacific plate was moving in the opposite direction from the Farallon plate, and this caused problems in the subduction zone. Suddenly the rocks on the laeding edge or the North American plate were no longer colliding with the Farallon plate, but instead were being pulled in the
opposite direction by the Pacific plate. The portion of the North American plate that overlay the Pacific plate began to move northwest with the Pacific plate The San Andreas fault formed to accommodate this movement. There have been many attempts to determine the total amount of movement along the San Andreas fault. Ideally, to make this determination one would like to have a unique geologic feature displaced so that one part, like the head of a dinosaur, was on one side of the fault and the other part, the tail, was on the other side. By matching the head and the tail it would be possible to document the exact amount of movement that had taken place. There have been several correlations of this type using similar rocks found on opposite sides of the fault, rather than pieces of a dinosaur. Most of these correlations suggest horizontal movement of at least 200 miles for the San Andreas fault in the San Francisco area. This would represent an average rate of movement of about onehalf inch per year. It appears likely that significant additional movement took place along several of the other faults in the San Andreas system. The total movement between the plates is believed to be in the range of two inches per year. Indications are that there was even more movement on the San Andreas fault in Southern California, perhaps 400 miles or more. Earthquakes Earthquakes occur when the rocks along a fault suddenly break and move. The sudden movement is transmitted through the Earth as sharp vibrations that shake the rocks they travel through. A small break will result in minor shaking, and a large break will result in violent shaking. The amount of shaking is greatest near the break and rapidly decreases away from the break. Two types of waves are formed when the rocks break compressional waves and shear waves. The compressional waves, also known as primary or P waves, travel through the rocks like a sound wave through air, compressing the rocks ahead and stretching the rocks behind. They travel at a speed of 3.1 to 3.7 miles per second. The shear waves, also known as secondary or S waves, travel through the Earth by moving the rocks laterally, like shaking a long rope. S waves travel at about 2.0 miles per second, so they reach an observer after the P waves.
When the P and S waves reach the surface of the Earth, two new types of waves are formed, Raleigh waves and Love waves, and these waves travel along the Earth s surface. The Raleigh waves travel like ripples on the surface of a pond, moving the surface of the Earth up and down. The Love waves shake the ground laterally. It is these two sets of waves that cause most of the shaking and damage in an earthquake. Where the rocks are solid and hard, the earthquake waves travel fast and shaking is minimized. In soft, unconsolidated rocks the waves travel more slowly and the shaking is maximized, like shaking a bowl of Jell-O. Most earthquake damage occurs in areas of soft, unconsolidated sediments. Three things can happen when earthquake waves reach these areas and all are bad. First, the intensity of the shaking is magnified, perhaps by an order of magnitude or more. Secondly, the shaking settles the loose grains in the unconsolidated sediments, so that the surface of the ground can settle from several inches to over a foot, like cornflakes settling in a box. Lastly, where fine, unconsolidated sediments are saturated with water, some of the water is squeezed out from between the sand grains as the grains settle. This water rises to the top of the unconsolidated sediments and forms a lubrication layer that allows the overlying structures to settle or slip downslope, as if on a giant waterslide. This process is called liquefaction. During liquefaction water may flow up through cracks in pavement or asphalt and form small craters. Major damage can be done far from an earthquake epicenter in areas susceptible to liquefaction. The size of an earthquake is usually measured by the Richter scale (M) or the Modified Mercalli Intensity scale (MMI). The Richter scale measures the intensity of the earthquake based on seismograph records. The scale ranges from small magnitude M1.0 earthquakes, to great M8.0 earthquakes. The scale is logarithmic: for a one-unit increase in the scale there is a ten-fold increase in ground motion and a thirty-fold increase in energy released. An earthquake of M7.0 is ten times more violent than an earthquake of M6.0. This scale is used in many scientific studies of earthquakes. The Modified Mercalli Intensity scale measures the intensity of an earthquake based on damage to structures. This scale is used for detailed
analysis of earthquake damage and also where good seismograph records are not available. The MMI scale used Roman numerals I through XII and thus can be readily distinguished from the Richter scale. Major California Earthquakes It takes a huge break, probably through the entire crust, to create an earthquake of M8.0 or greater. A break of this magnitude would normally extend from tens to hundreds of miles along the fault, and the rocks along the fault would be offset tens of feet. Earthquakes of this size are very effective in releasing stress and it takes a long time for stress to build up again that would cause another M8.0 earthquake. Fortunately, these earthquakes are infrequent. Earthquakes of M7.0 are typically accompanied by offsets of several feet over a distance of several tens of miles along the fault. These earthquakes can cause severe damage over wide areas. Earthquakes of M6.0 may or may not have breakage along the surface of the ground. However, if an earthquake of this size occurs in a metropolitan are a, like the 1994 Northridge Earthquake (M6.7) in Los Angeles, it could still provide quite a jolt. After a large earthquake, there are normally a large number of aftershocks, smaller adjustments of the rocks along the fault plane to the new stresses caused by the initial rupture. The largest aftershock is normally about one magnitude smaller than the main shock and the shocks become smaller and more numerous with time until they die out. In California, there have been 66 earthquakes greater than M6.0 over the last 200 years. Of these, 47 were from M6.0 to M6.9; 16 were from M7.0 to M7.9; and three were greater than M8.0. The most recent earthquake in California greater than M8.0 was the great San Francisco Earthquake of 1906 (M8.3). The other two great quakes were the Owens Valley Earthquake of 1872 (M8.3) and the 1857 Fort Tejon Earthquake (M8.3) over 100 years ago. Konigsmark, Ted. Geologic Trips. Gualala, CA: Geopress, 1998.
Earth Science Response to Non-fiction California s Faults and Earthquakes from Geologic Trips by Ted Konigsmark Compose written responses to the following prompts. Your work should be done on loose-leaf paper, and attached to this cover. 1) Explain earthquakes in terms of cause and effect. 2) What is a transform fault? Explain in text and diagram. 3) Contrast faults and fault systems. 4) Describe the tectonic events responsible for creating the San Andreas fault. 5) Identify and describe the four types of waves produced by earthquakes. Which causes the most damage, and why? 6) Define and describe the phenomenon of liquefaction. 7) Explain the reasons for aftershocks. 8) Describe the two scales by which earthquakes are typically measured. What is the essential difference between the two?