San Andreas Fault in California
Since 19th century naming by geologist Andrew Lawson (Suppe and Crouch, 2004), San Andreas Fault remains a fascinating geological feature in today’s world. The events that lead to naming of the fault resulted from Lawson’s observation that fault features were typified by a straight valley on the San Francisco Peninsula that was occupied in part by San Andreas Lake. Specifically, geologist Andrew Lawson prior to naming he noticed indicators of geological movements evident by linear arrangement of geological features of San Francisco Bay valleys, Santa Clara counties and San Benito.
Geologically, San Andreas Fault is the principal feature of an intricate network of faults that extends from NW (Northwest) California to the Gulf of California in about 600 miles or an equivalent of 965 km in length. The significant of this fault is that, it is important geological feature in the sense that it demarcates the boundary between North American and Pacific tectonic plates. These two tectonic plates are amongst seven major shard-like fragments of the earth’s crust recognized in geological world. According to latest drilling activities, the fault penetrates as deep as 20 miles or 30 km from the surface of the earth crust to the viscous rock material of the outer mantle (Gordon 2192).
Additionally, these drilling activities has lead to founding out that the Pacific Plate is moving northwest relative to the North American Plate at approximately estimated at an average rate of 5 cm/yr which is actually the same rate that your fingernails grow (Gordon, 2003). As a result of the drilling activities, it has revealed that the Pacific plate is moving northwest in relation to the North American plate that has a total displacement 350 miles or 560 km since 30 million years (Suppe and Crouch, 2004) ago from initial of its formation. As a consequence of this movement of the Pacific plate is moving northwest in relation North American plate, frequent earthquakes usually occurs. For instance, the destructive San Francisco earthquake of 1906 the either sides of the fault were displaced horizontally up to 6.4 meters an equivalent of 21 feet (Stock and Atwater 385).
Segments of the San Andreas Fault
According to Stock and Atwater (1998), they state that there are three major identified segments of San Andreas Fault. These segments are southern segment, central segment and the northern segment. The northern segment of the San Andreas Fault starts from Hollister passing through the San Francisco Peninsula, and then it follows the coast of California and finally, ends up on offshore near Eureka (Stock and Atwater, 1998). The central segment of the San Andreas Fault runs from Parkfield to Hollister in northwest direction. The central segment’s Parkfield areas and southern section usually experiences earthquakes, while other parts only experience aseismic creep. In this regard, aseismic creep refers to the geological process that some parts of San Andreas Fault being able to move laterally without the need for earthquakes to release seismic pressure (Snyder and Dickinson 610).
The southern segment starts from near the Salton Sea and runs northward then bends to west on reaching the San Bernardino Mountains. From the San Bernardino Mountains it crosses the Tejon Pass that continues to the northern base of the San Gabriel Mountains. It is evident that, San Gabriel Mountains geologically referred as Transverse Range have been uplifted San Andreas Fault movements. This segment continues and bends northeast after crossing California’s Frazier Park. According to Gordon (2003), he states that there is high speculation among geologist that at California’s Frazier Park might be the region where San Andreas Fault locks up as the tectonic plates strain mightily to move past one another.
San Andreas Fault System
The complex processes of formation of the San Andreas Fault in California which had been a convergent plate boundary, started about 65 million years (Suppe and Crouch, 2004) ago. During this period, the plate tectonics of the Mexico, Northern California, Baja and Northern California at a varied rate began to undergo radical changes. During this era, Farallon plate was completely subducted under the North America plate that is currently overrunning a seafloor spreading center. As a consequence, the plate boundary between the Farallon plate and North America turned into a transform boundary which separated North American and Pacific plates.
In the next 35 to 40 million years ago (Suppe and Crouch, 2004) the resultant transform plate boundary developed into a SAFS (San Andreas Fault System). It is within this San Andreas Fault System that San Andreas Fault is a major fault. The fault is a right- lateral transform fault, which in simple language means that if standing on the opposite side of the fault, the people and objects appear moving rightwards. This San Andreas Fault System separates two halves of the continental crust embedded in North America plate moving southwards and other embedded in Pacific plate moving northwards.
Geological timescale and formation processes of San Andreas Fault
The past 38 million years ago, an offshore spreading center for ocean crust to form developed by East Pacific Rise. The East Pacific rise as a new crust that has formed was pushed away from the center in both west and east direction. The resultant ocean crust that moved eastwards was the Farallon Plate that collided with North American plate and sunk into the sebduction zone of Franciscan (Atwater, 1970). During the collision and subduction processes, the islands and sediments which were on the subducting plate were accreted and scraped off creating assemblage of rocks referred to as Franciscan Assemblage Terranes (Atwater, 3513). In the recent studies, it has been observed that the entire spreading center is getting closer to the North America plate which may result to the entire center to subduct (Gordon, 2003).
These events were followed by series of other concurrent events in the 26 million years ago (Atwater, 1970). During this era, the East Pacific Rise spreading center reached the Franciscan subduction zone that made the subduction to stop because the Pacific plate was moving away from the zone of subduction. At this point, the North American plate which is attached to the Pacific plate began to move northwest, making the subduction zone to change to the transform plate boundary in order to accommodate north and south offset motions of these spreading centers. It is at this point that the San Andreas Fault formed (Snyder and Dickinson, 2005) to become the boundary between these spreading plates. The western edge of the fault moves northwards with Pacific plate relative to North America plate (Gordon, 2003, p. 2193).
The follow up event in about 17 million years ago (Suppe and Crouch, 2004), the East Pacific Rise began to open up the Sea of Cortez Step as a result of breaking off of Baja, California in Mexico from the mainland that implicated that it was transferred to the Pacific plate. Then within the last 3 million years ago, the California migrated towards the north along with the Los Angeles Basin as a consequential result of the East Pacific Rise opening up the Baja and Sea of Cortez (Gordon, 2003).
The paper has exclusively described San Andreas Fault in California and the formational processes including its geological timescale. However, the presence of the fault within the San Andreas Fault System and the surrounding environment has both benefits and short comes. The good side of it is the aesthetic value of nature gained by both the locals and the foreigner visitors manifested through it wonderful and unique features. To mention, some of the features are linear arrangement of bays, lakes and valleys.
Furthermore, the fault zone has distinct landforms ranging from narrow ridges, escarpments and small undrained pond. All of these features have both socio- economic advantages and disadvantages. The dark side of the San Andreas Fault presence within the region is its association to severe earthquakes that are disastrous to the socio- economic domain of the region and US at large. Therefore, more work should be directed towards not only the study of the San Andreas Fault, but also to the San Andreas Fault system for knowledge and to inform earthquake mitigation programs through mapping.
Atwater, T. “Implications of plate tectonics for the Cenozoic tectonic evolution of western North America”: Geol. Soc. Amer. Bull., vol. 81, p. 3513; 1970.
Gordon, R. G., Effects of recent revisions to the geomagnetic-reversal time scale on estimates of current plate motions: Geophysics. Res. Lett: vol. 21, p. 2191-2194; 2004.
Stock, J. & Atwater, T. “Pacific-North America plate tectonics of the Neogene southwestern United States”: International Geology Review, vol. 40, p. 375-402; 1998
Suppe, J. & Crouch, J. “Late Cenozoic tectonic evolution of the Los Angeles basin and the Californian Borderland”: Geol. Soc. Amer. Bull., vol. 105, p. 1415; 2003
Snyder, W. & Dickinson, R. “Geometry of subducted slabs related to San Andreas transform”: Journal of Geology, vol. 87, p. 609-627; 2005.