History Channel's How the Earth Was Made is a wonderful earth science documentary series. In the episode titled "San Andreas Fault," the program describes the history of how the fault was identified and linked to earth’s moving plates. The episode is set up as a type of detective story, with evidence shown to back up a hypothesis. The episode is available on YouTube
The video worksheet is a one-page, double sided handout consisting of 38 multiple choice questions that track the progress of the video. This format enables the students to pay attention to the video while quickly recording their answers. In this way, the students are not bogged down in writing long responses, and they can better enjoy watching the video. A key is included, and the files are provided in both MS Word and PDF formats.
History Channel’s How the Earth Was Made: San Andreas Fault Overview
The discovery of the San Andreas Fault occurred after the destructive 1906 San Francisco earthquake. Andrew Lawson, the first scientist to study the earthquake and its possible cause, amassed evidence such as the apparent 8-foot offset of a picket fence in Salinas, CA. When plotted on a map, this feature and others were found to run along a line extending from the ocean onto land and progressing parallel to the coast. In this way, the fault was first identified.
The discovery of earth’s moving plates in the 1950s led to a coherent explanation for the San Andreas Fault as a boundary between the North American and Pacific Plates. Originally, the Pacific Plate was forced, or subducted, under the North American Plate about 200 million years ago. Gradually the motions changed course, and about 20 million years ago the two plates began sliding side-by-side in opposite directions. This type of motion, termed strike-slip, creates situations where energy is stored in the crust and released suddenly as earthquakes.
The video also documents the efforts of today’s scientists to study the fault.
Rocks located near the coast at Mussel Rock, CA were found to present visible disparities in type indicating a fault line.
An earlier earthquake at Littlerock, CA was dated to 3,500 years by the use of carbon-14 (radiocarbon) dating.
The fault line was found to have caused the odd position of Wallace Creek. While flowing towards the fault, the creek makes a sudden turn right, runs for 300 yards, and then makes a sudden turn left creating an unusual right-angle configuration. Through radiocarbon dating, the rate of motion of the fault running through Wallace Creek was determined to be 1 inch per year.
The strange situation at Hollister, CA was also documented. This city is unique in that, despite lying on the San Andreas Fault, it has never had a documented earthquake. Despite this, manmade structures such as walls and curbs in the town show a perpetual tendency to creep out of alignment.
Parkfield, CA is a particularly useful environment for earthquake studies. Lying on the San Andreas Fault, the town features a bridge that crosses the fault and, in doing so, also extends from the North American Plate to the Pacific Plate (it even features road signs that indicate each plate!). Earthquakes at Parkfield occur at fairly predictable intervals, so scientists have set up monitoring stations to observe earthquake activity. After an earthquake in 2004, scientists were able to precisely identify the location of the San Andreas Fault below ground. A drilling apparatus was set up to extract core samples from a depth of two miles. These rocks display serpentinite, a rock that easily converts to the slippery mineral talc. The presence of talc along an earthquake fault helps to facilitate smooth movement.
The devastation occurring with some earthquakes was found to be inconsistent with recorded earthquake magnitudes. For example, much more damage occurred during the 1906 San Francisco earthquake than was warranted by its magnitude of 7.8. An explanation for such earthquakes has been provided by laboratory research featuring disks of glass that, when subjected to stress, create cracks analogous to faults. Special photographic techniques are able to reveal the type of waves emitted in these experiments. In particular, triangular shaped disturbances were observed to coincide with the direction of the experimentally induced faults. These disturbances are very similar to the sonic booms that occur when airplanes break the sound barrier. This observation led scientists to postulate the existence of “supershear” forces analogous to sonic booms. These supershear effects are augmented by particularly straight sections of the San Andreas Fault. Both San Francisco and Los Angeles are located along such fault lines, and earthquakes occurring in these regions, as exemplified by the 1906 event, may be enhanced by supershear forces to create a heightened potential for earthquake destruction.
In the fall of 2008, the results of a large study of earthquake danger and preparedness were unveiled in California. Scientists working in the study predicted that Los Angeles has a 99% likelihood of experiencing a massive earthquake in the next thirty years (stated in 2009). Such an earthquake is predicted to cause 2000 deaths, 50,000 injuries, and $200 billion dollars of damage. The state also held the Great California ShakeOut, the largest earthquake drill ever performed.