Lesson 14: San Andreas Fault System
This week we'll explore what is, without a doubt, one of California's best known geologic features: the San Andreas fault. During the past 28 million years this strike-slip fault system has grown to a length of 1,350 kilometers and now stretches from Cape Mendocino to the Gulf of California. It is the principal boundary between the North American and Pacific plates, and differs markedly in structure and earthquake potential (see Fig. 13-22) along its length . Some segments are marked by narrow valleys (as at Crystal Reservoir, right) whereas others are broad zones in which slip is distributed across several parallel strands. Despite these differences, however, we'll see that much of the fault's behavior can be understood in terms of how a particular segment is oriented relative to the direction of plate motion.
In chapter 14 Harden begins with a brief overview of the tectonic setting, age, average slip rate and topographic expression of the whole San Andreas fault system. Then she describes the geologic features that characterize each of the fault's six major segments (and the closely-related Garlock fault and Eastern California Shear Zone) and recounts the major historic earthquakes that have occurred along each segment. As you study the San Andreas I would like you to focus on "big picture" concepts—such as, "Why is the Big Bend segment of the fault associated with thrust faults beneath the LA Basin whereas the Salton Trough segment is associated with crustal extension?—rather than the names and dates of individual earthquakes. Finally, this week's exercise asks you to read and write an outline of one of the articles I've chosen for you about California's geologic hazards or resources.
As you read about the San Andreas fault be sure to take careful notes on the topics covered by the learning objectives below. As during previous weeks, you'll find that writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Don't hesitate to post any questions you have to the Discussion Board so that your classmates or I can help you figure them out. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Predict which type of stress and which types of geologic structures one would expect to find along the San Andreas fault where it: lies parallel to the direction of plate motion; jogs to the right at a "releasing bend"; or jogs to the left at a "restraining bend".
- Infer the location and estimate the offset of a transform fault shown on a photo or topographic map by recognizing the presence of surface features such as: sag ponds, offset streams, linear valleys, shutter ridges, and scarps.
- Compare the average displacement rate of the San Andreas fault to the overall rate of motion between the Pacific and North American plates and indicate where most of the offset that is not accomodated by the San Andreas is thought to be taking place.
- Indicate whether, for the sake of seismic safety, one would prefer to live near a section of the San Andreas fault that is "creeping" or "locked" and explain your rationale.
- Briefly describe the unique circumstances that have enabled geologists to estimate the average recurrence interval of quakes along the San Andreas fault where it crosses Pallett Creek north of Los Angeles, and comment on how this interval compares with the time that has elapsed since the last quake in this area at Fort Tejon.
- Contrast the orientation and sense of shear along the Garlock fault with those of the San Andreas fault. Note that strike-slip fault faults commonly develop in conjugate sets, and the San Andreas and Garlock faults may be conjugates.
Reading and Browsing Assignment
- Read Chapter 14, focusing on the topics outlined in the learning objectives above.
- First, to see again how the San Andreas got started watch Tanya Atwater's animation of plate motions in the northeastern Pacific during the past 40 million years. You can see how the fault developed at the point of contact between the North American and Pacific plates, how it has grown during the past 28 Ma, and how subsequent shear motion has fragmented California and led to extension farther inland.
- Browse through one or both of these introductory websites on the San Andreas. The first is an introduction from the U.S. Geological Survey. The second is longer and more detailed; it is based on U.S. Geological Survey Professional Paper 1515. (Note that navigation on this second site is a little difficult, as it seems to "turn off" my browser's navigation controls.)
- To see the difference between the subvertical strands of the San Andreas fault and the gentler dips of the thrust faults that occur near the Big Bend in southern California check out this animation of two fault types from the Southern California Earthquake Center.
- Finally, to zoom in and see what the surface expression of any segment of the San Andreas fault is like check out this interactive map from Thule Scientific.
Exercise 14: Outline of Article 2 (Due by 9:00 AM on 25-Apr-2011)
This week's exercise is to complete the third of our four writing assignments. To learn about the assignment read through the pointers below and then click on the "Resources" link on the left side of this page, scroll down, and click on "Outline 2" under "Writing Assignment" near the bottom of the page. This week we'll be completing steps one and two and you'll be turning in the outline of the article you choose from the second set.
- Your first task is to choose one of the two articles on geologic hazards or resources in Set 2 and to read that article carefully. You'll find PDFs of the articles we'll be using under the "Resources" tab on our class Etudes site, not on this website. Just choose the article that interests you most, download it, and read it carefully. (Remember, if you were in Environmental Geology last semester and read one of the articles, please choose the other.)
- After you've read through the article once to get the jist of it, go back, read through it a second time, and write down notes on your key observations and conclusions. Do not hestitate to post a note to the discussion board or contact me via private message if you have questions about an unfamiliar term or idea. If you don't understand it there's a good chance other people will have questions too.
- Next, draft an outline of the article's major conclusions and supporting evidence that follows the format of the "sample outline" linked to the Writing Assignment page. In order to organize your outline imagine that you are telling a friend who hasn't read the article what its three or four key conclusions are and what data or observations are presented to support each of them. You will want to do your best to focus on the main points of the article and not get sidetracked by peripheral details.
- Your outline does not need to follow the style of the sample outline exactly (e.g., you don't need to use the same font and bullets), but be sure that it includes the complete article citation, your "PIN" (last 5 digits of your Etudes user id), and the course name and date at the top. Also, be sure that your text is written in complete sentences (not single words or phrases) and that it is well organized and coherent so that a reader can easily follow the main themes of the article.
- Finally, before you submit your outline you should check out the outline grading rubric that is linked to the Writing Assignment page. It will show you what criteria I will use to evaluate your outline. There is no set length for your outline, but if it's longer than about a page and a half it won't be as helpful when you go to write your abstract. (Abstracts are very short, so the more you can winnow the article's main themes from its peripheral details as you prepare your outline the less work you'll have to do later). Also, if you print a copy of the article be sure to save that copy so that you'll have it to refer to again in a couple of weeks when you'll be writing the corresponding abstract.
- When you have completed your outline go to the Etudes site for our class, go to Exercise 14 in the Etudes "Assignments, Tasks, and Tests" tool and submit your outline as an attached file. Outlines are due by 9:00 AM next Monday (25-Apr-2011.) I ask that you to send it as an attachment rather than pasting it into the text box because this will preserve your formatting. You should send it as an MS Word document or, if you write it in another program, as a Word-compatible file. If you have any problems just let me know. I will return your outline with comments and a completed rubric by 2-May-2011 in time for you to get started on the second abstract.
Quiz 14: San Andreas fault (Due by 9:00 AM on 25-Apr-2011.)
After you feel you have met the learning outcomes outlined above please complete Quiz 14 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions about the San Andreas fault and each is worth one point. If you can answer all of them correctly it means that you know your way around the San Andreas pretty well and are ready to start learning about California's coast and the processes that operate there next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.